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SURGEON GENERAL'S: OFFICE 

LIBRARY. 




Section, 



A HAND BOOK 



OF THE 



REFRACTION OF THE EYE 



ITS ANOMALIES AND THEIR CORRECTION 



BY 



CHARLES D'A. WRIGHT, M. D., 

Demonstrator of Ophthalmology and Otology in the University 
of Michigan. 




Sheehan & Co., Ann Arbor, Mich. 

AND 

John V. Sheehan & Co., Detroit, Mich. 

Publishers and Booksellers. 



Copyright, 1896, 
By C. D'A. WRIGHT, M. D. 



The Inland Press, Printers. 



Page 45, line 20, thes should read the. 

Page 19, lines 6 aud 9, anterio -posterior should read 
antero -posterior . 

Page 53, line 23, slip should read slit. 

Some places dioptra appears which should read dioptre. 

Page 117, line 26, plain should read plane. 

Page 125, line 18, plain should read plane. 

Index, page ii, column 2, line 16, faculative should read 
facultative. 

Index, page iii, column 2, line 10, -pseudo -my ophia should 
read pseudo-myopia. 

Index, page iv, column 1, line 1, pilocarpine should read 
pilocarpine. 

Page 10, line 3, comma after A should be out. 
Page 19, line 5, comma after / should be after g. 
Page 62, line 26, if should read of. 

Page 72, in line 3 after the should be inserted the words 
previously existing. 

Page 10, liue 16, abberation should read aberration. 

Page 42, the paragraph beginning with line 18 should end 
at quantity, line 21. 

Page 22, line 17 after the word concave should be the 
words it has not. 



TO 

FLEMMING CARROW, M. D., 

PROFESSOR OF OPHTHALMOLOGY AND OTOLOGY IN THK 
UNIVERSITY OF MICHIGAN, WHOSE UNCEASING 
AND ENERGETIC WORK HAS DONE SO MUCH 
TO ELEVATE THE STANDARD OF OPH- 
THALMOLOGICAL INSTRUCTION AND 
TO WHOM I AM INDEBTED FOR 
MY ADVANCEMENT IN 
THE SCIENCE, I 
DEDICATE THIS 
VOLUME. 



PREFACE. 



The object of this little volume is to present, 
in a plain and practical form, an explanation of the 
anomalies of refraction and their correction. 

It substantially embodies the instruction given 
to the demonstration classes in the University of 
Michigan, and while it is intended, primarily, to 
facilitate the work of Junior students in preparing 
the subject, the perusal of its pages may prove of 
benefit to other readers. 

It has been thought unnecessary to go into the 
more intricate and scientific demonstrations of the 
refraction of light, which are, happily, not of an 
extremely practical nature. 

The chief aim has been to go explicitly into 
every practical detail so that the beginner may not 
be lost in the labyrinth, and it is hoped that those 
more advanced may not tire of the minutia. 

It has been compiled during the hours which 
could be spared from active duties, and though the 
writer has sought to be exact in all his assertions, 
an occasional inaccuracy may have inadvertently 
crept in. Any emendations which my co-laborers 
may propose will be gratefully received and noted 
in a subsequent issue. 

I am under many obligations to my congener, 
Dr. C. B. Bliss, who has assisted me very materially 
in compiling and arranging the contents. 



ERRATA. 



Page 45, line 20, thes should read the. 

Page 19, lines 6 and 9, anterio -posterior should read 
antero-posterior. 

Page 53, line 23, slip should read slit. 

Some places dioptra appears which should read dioptre. 

Page 117, line 26, plain should read plane. 

Page 125, line 18, plain should read plane. 

Index, page ii, column 2, line 16, faculative should read 
facultative. 

Index, page iii, column 2, line 10, pseudo-myophia should 
read pseudo-myopia. 

Index, page iv, column 1, line 1, pilocarkine should read 
pilocarpine. 



CONTENTS. 



Chapter I. 

Chapter II. 

Chapter III. 

Chapter IV. 

Chapter V. 

Chapter VI. 

Chapter VII. 

Chapter VIII. 

Chapter IX. 

Chapter X. 

Chapter XI. 

Chapter XII. 



Lenses — Refraction of Light and 
the Formation of Images. 

Emmetropic Eye. 

Visual Acuity — Accommodation 
— Convergence. 

Hyperopia. 

Myopia. 

Astigmatism. 

Presbyopia. 

Skiascopy. 

The Ophthalmoscope Relative to 

Refraction. 
Eye Muscles and their Anomalies. 
Fitting of Frames. 
General Remarks. 



CHAPTER I. 



Lenses, Refraction of Light and the Formation of 

Images. 



REFRACTION OF LIGHT. 

Rays of light are refracted when they pass 
from one transparent medium into another of dif- 
ferent density, unless fchey fall perpendicularly, 
to the surface of the second medium. The devi- 
ation of rays passing from a vacuum into air 
is represented as 1. The ordinary spectacle or 
crown glass lens is represented by 1.50; the pebble 
or so-called optician's glass by 1.66. When a ray of 
light passes from a less to a more refractive medium 
the deviation is toward the perpendicular. When a 
ray of light passes from a more to a less refractive 
medium the deviation is away from the perpendic- 
ular, and to the same extent providing the first and 
third media are equal. 

In Fig. 1 — A, equals air; A' equals glass; An- 
gle O equals angle 0'. 

From this it is seen that rays are restored to 
their original direction, if the sides of the medium 
of greater refraction are parallel; but if the sides of 



8 



A HA.ND BOOK OF THE 



this medium form an angle (the angle of incidence 
and emergence being equal), there must be an an- 
gle formed by the intersection of the ray of inci- 
dence and the ray of emergence if continued. 

In Fig. 2 — The deviation shown by the angle 0 
is equal to about one-half the refraction angle x in 
a crown glass prism. B. C. B'. is an angle which 




would not occur if the side a, d and d, e were par- 
allel. For lenses of small determination the refrac- 
tion of rays is the same at all angles of incidence, 
if at the same distance from the principal axis. 

In Fig. 3 angle a b c in A equals angle a b c 
in B, and angle a b c in A equals angle a b c in 6\ 



REFRACTION OF THE EYE. 



9 



and the angles ah c are equal in each case. Hence 
the nearer the object the farther the image from the 
lens. Each transparent medium refracts light but 
not equally. Opticians call those of higher refrac- 
tive media, denser media, and those of lower refrac- 
tive media, rarer media. 

In Fig. 4 — A equals air: A' equals water. 
The angle A , B, D, is the angle of incidence, and 
(7, B, JZ, is the angle of refraction. The A, B, D, 

A 




is to the angle C, B, E, as the propagation of 
light in A is to the propagation of light in A'. 

Rays passing through the principal focus or 
optical centre of the lens, Fig. 5, are not refracted. 

Every incident ray refracted by the first sur- 
face in such a way as to pass through the optical 
centre of the lens emerges from the system in a 



10 



A HAND BOOK OF THE 



direction parallel to the path of the primitive or 
incident ray. 

In Fig. 6 — A, B and C D, are parallel lines. 

From the foregoing it will be seen that the 
image formed at or behind the principal focus of a 
convex lens, on the opposite side to the object will 
be a positive, real and inverted image. See Fig. 7. 




Hence the nearer the object is to the lens the 
farther the image recedes from the lens and the 
larger the image. As a matter of fact when the 
image is at a distance of double the focal distance 
of the lens the image is the same in size as the 
object. In a positive meniscus the relation of dis- 
tance, position, and size are determined by the same 
method as in a biconvex lens, but there is no 
spherical abberation, parallel rays of light being 



REFRACTION OF THE EYE. 



11 



brought to a focus at exactly a common center. All 
lenses of low power are ground in menisci; the 
only disadvantage being that in high power they 
are too heavy for practical use. 

MEASURING LENSES. 

The stronger a lens is the more is light refract- 
ed by it. The greater the deviation given to par- 




allel rays by a lens the nearer is its focus to the lens 
and the less its focal distance. The focal distance, 
of a lens is determined by dividing one hundred, 
(which is the number of centimeters in a meter), 
by the number of dioptres of the lens. A one 
dioptre lens has a focal distance of one meter 
or one hundred centimeters. A two dioptre lens 
lens has a focal distance of one-half a meter or fifty 
centimeters. A ten dioptre lens has a focal dis- 



12 A HAND BOOK OF THE 

tance of one -tenth of a meter or ten centimeters. 
Hence the refractive power of a lens and its focal 
distance are inversely proportionate to each other. 
The number of dioptres of a lens may be found by 
using the one meter as a numerator and the focal 
distance expressed in meters as a denominator. 
Suppose a lens to have a focal distance of forty 
centimeters : 

1 100 
~ = — = dioptres. 

A prism optically considered may be said to 
be a transparent body having two large flat surfaces 




Fife. 5 



which are not parallel, with two equal parallel trian- 
gular ends, a base and an apex. An object seen 
through a prism is seen displaced toward the apex 
of the prism. The amount of displacement varies 
directly as the size of the refracting angle of the 
prism, and the object seen, appears to come from 
the position indicated by the rays of light, after 



REFRACTION OF THE EYE. 



13 



they leave the prism. This fact is utilized to 
remove the point of fixation, or in other words, to 
lesson the amount of convergence demanded to 
produce binocular vision ; to relieve diplopia when 
caused by insufficiency of the ocular muscles; to 
test the strength of the extrinsic eye muscles, and 
to test malingerers. 




In Fig. 8 — A and B parallel as incident rays 
are parallel as emergent rays. 

The relative direction of a number of rays is 
not changed by a prism. All rays are deflected 



14 



A HAND BOOK OF THE 



toward the base by a prism. If parallel as incident 
rays they are parallel as emergent rays. 

Ri.7 




In Fig. 9 — A represents the object and B the 
apparent object after the light has passed through 
a pair of prisms which are placed in front of the 
eyes, base in. 

In the eye the objects are seen to be in the 
direction from which the rays of light enter the eye. 




Of the positive and negative lenses the follow- 
ing varieties are common: 

In Fig. 10 — A is a biconvex lens. B is a 



REFRACTION OF THE EYE. 



15 



plano-convex lens. G is a convergent meniscus or 
a convergent periscopic lens. I) is a biconcave 
lens. E is a plano-concave lens, Eis a, divergent 
meniscus or a divergent periscopic lens. 

A, B and C render rays of light falling on 
their surface at different sides of the principal axis 
more convergent and are called positive or plus 




lenses. And D, E and F render rays of light fall- 
ing on their surface at different sides of the princi- 
pal axis less convergent and are called diminishing, 
minus or negative lenses. 

The second or posterior focus of a convex lens 
is the point at which rays of light are focused, 



16 A HAND BOOK OF THE 

which, when incident were parallel to its principal 
axis. As a matter of fact this focus varies some- 
what, as rays entering the lens farther from its 
principal axis are refracted more than those enter- 
ing nearer the principal axis, and from this fact 
results spherical aberration. 

In Fig. 11 — a is the focus of rays d, falling on 
the periphery of the lens, b is the focus of rays 
e y falling on the lens nearer its principal axis. 




The refracting power of a lens varies inversely 
as its focal length. In the human eye the iris is 
hung in such a manner as to shut off rays of light 
that would otherwise pass through the periphery 
of the lens. Again, the centre of the nucleus of 
the lens is denser than the peripheral part of the 
nucleus. By these two phenomena the spherical 



REFRACTION OF THE EYE. 



17 



aberration in the eye is partly overcome, but when 
by the use of a mydriatic the pupil is well open, 
and rays of light pass through the periphery of the 
lens, some spherical aberration results and the pa- 
tient complains of a hazy appearance of the letters 
on the test card, notwithstanding the fact that 
normal visual acuity is obtained. 

Landolt calls attention to an interesting fact. 
At 5 metres distance a myope of .40 of a dioptre 




is regarded as an emmetrope, whereas a myope of 
.09 of a dioptre is considered as a hyperope. 

The myopia must exceed .40 of a dioptre before 
he will see better with a — .25 dioptre glass than 
with his naked eye, because with this glass, circles 
of diffusion are smaller than those produced by 
the eye alone; again as soon as the myopia falls 
below .09 of a dioptre, vision will be improved by 
a +.25 of a dioptre glass, because the circle of dif- 
fusion becomes smaller than without the glass. 



CHAPTER II. 



The Emmetropic Eye. 

The camera is often used to represent the 
human eye, and the two are similar in many re- 
spects. By the system of refractive media, an 
inverted image of objects that appear within the 
visual field of the eye, is produced on the retina. 
So perfect is the construction of this human cam- 
era, that an iris is found hung in front of the crys- 
talline lens to exclude the peripheral rays of light, 
which on passing through the periphery of the 
lens, would produce spherical aberration as ex- 
plained in the preceding chapter. (See Fig. 11). 

We find the sensitive plate or retina provided 
with a pigmentary layer, to lessen the reflection 
of light, by the absorption of rays that would 
otherwise be reflected, and being reflected would 
produce a certain amount of dazzling under all 
circumstances. 

With the camera the operator must move the 
sensitive plate nearer the lens the farther the object 
is placed from the lens; and the farther from the 
lens the sensitive plate is situated, the nearer must 
be the object to be photographed. 



REFRACTION OF THE EYE. 



19 



This adjustment is made in the eye by the con- 
traction of the ciliary muscle and corresponding 
thickening of the lens when near objects are fixed. 




A represents the ciliary muscle in a state of 
rest and that part of the line/", g of the line e,f, 
g indicates the antero- posterior diameter of the 
lens c. The dotted line b represents the anterior 
boundary of the lens c when the ciliary muscle (a) 
shall contract. The antero-posterior diameter of 
the lens c would then be indicated by the line /, 
g and the refracting power of the lens would be 
increased. 



20 A HAND BOOK OF THE 

This phenomena of changing the focus of the 
eye is called accommodation and is treated of more 
fully under Chapter III. It is also necessary in 
order to have a single image perceived, when both 
eyes are used, that the image be formed in corres- 
ponding parts of each retina. The region of the 
macula lutea is, as we know, the most sensitive 
field in the retina, and hence the most desirable for 
use when the best vision is required. When fixing 
distant objects the position of each retina is such 
that the image falls on both maculae at the same 
time; but when a near object is fixed, unless the 
eyes could be rendered more convergent, it is evi- 
dent that neither image would fall directly on the 
macula. 

The turning of the eyes inward, to that extent 
that the image may fall on both maculae, when a 
near object is fixed, is called convergence. As one 
must both accommodate and converge to fix near 
objects and obtain a single image, it would seem 
that a co-ordination between these two functions 
would be imperative. It is, and whenever from 
causes later considered this co-ordination is inter- 
fered with, much annoyance results. From anatomy 
we have learned that the centers for accommodation 
and convergence are very intimately connected and 
situated upon the floor of the fourth ventricle. 
The more anterior nuclei govern the ciliary and 
Bphincter pupillse muscles and just posterior to 



REFRACTION OF THE EYE. 



•21 



them are the nuclei governing convergence. Indeed 
so closely are they placed that they are simultan- 
eously set in action by any stimulus, yet the action 
of either center may be partly or quite inhibited. 

When the emmetropic eye is in a state of rest 
(that is when the accommodation is suspended), 
parallel rays of light falling on the cornea are 
focused on the retina. Now if the refraction of the 
eye, when the accommodation is at rest, is such that 
parallel rays of light are not brought to a focus 
until after they reach the retina (in other words if 
the static refraction of the eye is weaker than 
normal), the eye is called hypermetropic. If, on 
the other hand, the refraction of the eye, when the 
accommodation is at rest, is such that parallel rays 
of light falling on the cornea are brought to a 
focus, too soon, or before they reach the retina (in 
other words if the refraction is relatively too strong), 
the eye is called myopic. If one meridian refracts 
light more strongly than another meridian in the 
same eye, the eye is called astigmatic. 

As physicians we have also to deal with many 
troubles growing out of deficiencies of the extrin- 
sic ocular muscles, both when they are doing their 
work in helping produce binocular vision under 
protest (insufficiency), and when having worked 
along at an overtask they abandon all hope of 
doing their work and succumb to the strength of 
their antagonist allowing an actual deviation 



22 



A HAND BOOK OF THE 



(strabismus). In either the refractive or muscular 
phenomena a large per cent, of all eyes possess some 
anomaly. 

By the refraction of the eye is meant the 
amount of deviation expressed in dioptres that rays 
of light undergo when passing through the eye. 
By use it has been commonly accepted to mean the 
abnormality of the refraction existing in a given 
eye, as + 1.00 dioptre is given as the refraction of 
an eye, that is one dioptre hypermetropic. 

Let us hastily consider the refracting surfaces 
and media of the eye. The first refracting surface 
met with is the cornea, and the first refracting 
medium, the aqueous. The cornea is the most impor- 
tant of the surfaces to be considered, not because 
its convex surface has a shorter radius than its con- 
cave, but because it separates two media (air and 
aqueous), the refractive indices of which differ more 
than any other two contiguous media to be con- 
sidered. 

The posterior surface of the cornea is more 
curved than its anterior surface, and hence the 
cornea forms a divergent meniscus, but because it 
separates these two greatly different media, this may 
be practically overlooked. Helmholz gives as the 
average radius of the cornea 7.829 millimeters; 
Donders 7.6 millimeters. 

Granting the index of refraction of air as 1 .00, 
the index of both aqueous and vitreous may be 



REFRACTION OF THE EYE. 



23 



considered as 1.34. The crystalline lens is about 
3.75 millimeters thick and has a radius on its 
anterior surface of about 7.5 millimeters; its pos- 
terior surface presents a radius of about six milli- 
meters. The cortical substance of the crystalline, 
forms many divergent menisci — (their concave sur- 
faces having a shorter radius than their convex sur- 
faces), they also are more strongly divergent in 
their action near the circumference of the nucleus 
of the lens which is a convex lens of very high 
refractive power. This greater divergence given to 
the rays (by the cortical divergent menisci), farther 
from the principal axis of the eye, beautifully 
assists in correcting what spherical aberration 
remains uncorrected. This is especially of service 
when rays enter the eye at a considerable angle to 
the optic axis. The construction of the crystal- 
line is then infinitely superior to an ordinary convex 
lens, in that it produces a more distinct image. 
Taken as a system it is evident that the dioptric 
system of the eye is a convex lens. 

The second or posterior focus is about 23.00 
millimeters back of the cornea and represents the 
length of the emmetropic eye. The first principal 
focus is situated about 14.00 millimeters in front of 
the eye. 

The conjugate foci may be defined as two 
points situated one in front of the eye and the other 
behind the lens on the retina. The first point 



24 



A HAND BOOK OF THE 



being in such a position that rays of light emanat- 
ing from it are brought to a focus at the second 
point. 

If an object from which an image is to be 
formed be situated on the principal axis of the eye 
the rays given off will be focused on the principal 
axis behind the crystalline. If the object be situ- 
ated at a given distance from the principal axis of 
the eye, the image will be formed upon a secondary 
axis or on a straight line drawn from the object 
through the nodal point. All lines of secondary 
axes cross at nearly the same place, about 7 milli- 
meters back of the cornea. Hence images will be 
inverted relative to their object, and many images 
may be formed at the same time. In the emme- 
tropic eye the retina is found at the focus of the 
dioptric system of the eye. The point for which 
the eye is focused when at rest is called the Punc- 
tum Remotum. The point for which the eye is 
focused during maximum accommodation is called 
the Punctum Proximum. The Punctum Remotum 
for the emmetrope is infinity and for the hyperope 
beyond infinity. For myopes the Punctum Re- 
motum is a finite distance. 

In the study of refraction we have the optic 
apparatus of the eye to consider in a state of 
repose and in all degrees of variation of accommo- 
dation, depending upon the position of the object 
fixed. An eye, whose refraction when in a state of 



REFRACTION OF THE EYE. 



25 



rest does not bring parallel rays of light to a focus 
on the retina, is called ametropic. Hence the term 
ametropia is applied to the eye where the retina is 
not situated at the focus of the dioptric system of 
the eye. 



CHAPTER III. 



Visual Acuity, Accommodation and Convergence. 



VISUAL ACUITY. 

The visual angle is formed by the rays of light 
which pass from the terminal points of the object 
in view, through the nodal point of the eye, to the 
retina. It is the measure of visual acuity. In 
forms of ametropia and in diseases of the fundus, 
as well as in old age, the visual acuity is decreased. 
Visual acuity, as spoken of, is used relative to the 
central vision, as the macula is the most highly 
sensitive part of the retina. Many persons have 
visual acuity above normal, while some emmetropes 
have visual acuity somewhat below the normal. 
Visual acuity is measured by measuring the angle 
formed by the rays which pass from the extremities 
of the object fixed through the nodal point to the 
retina; and by long experiment it was determined 
that a normal eye should see the object forming an 
angle of one minute as above stated. Hence it is 
said that a person, who cannot receive a distinct 
visual impression by an object of proper size and 
at proper distance to form such an angle, has not 



REFRACTION OF THE EYE. 



27 



normal visual acuity. The visibility of a small 
object depends greatly on its luminosity. Hence 
figures with regular or parallel parts, or a number 
of dots printed black (which remains always of 
equal luminosity), on a white back ground should 
be used in testing visual acuity, and the distance at 
which the separation of the dots or the peculiar 
construction of the figures can be determined, will 
give a correct measure of visual acuity. 

Mr. Snellen has constructed a card of letters 
arranged in different rows, each row of a different 
size, and the number of feet at which a five minute 
angle is formed written over each row. If the 
patient at twenty feet sees the line which at that 
distance gives a one minute angle by each extremity 
of the letter, or a five minute angle with the whole 
letter, the visual acuity is normal or If he sees 
only at twenty feet a line that forms a five minute 
angle at forty feet, the visual acuity is %. 

The testing of near vision is made by using 
very small print, and here the smallest type that 
can be read is determined; also tne distance of 
space through which it can be read. 

The nearer the object is approximated to the 
eye the larger is the retinal image. The ratio is 
direct and therefore the smaller the object the 
nearer we bring it to the eye; however, persons 
with very greatly diminished visual acuity always 
bring objects very near to the eye in order to get a 



28 



A HAND BOOK OF THE 



large retinal image, as for instance, the myope and 
the absolute hyperope (the former produces rays 
of light divergent enough from the object to adapt 
them to the refraction of the eye, while the latter 
has the sole object of increasing the size of the 
retinal image). Of course the nearer the object 
the greater must be the accommodation used to get 
a distinct retinal image. Hence we determine the 
amount of presbyopia by finding the amount of 
accommodation yet available. 

The myope, whose near point is moved in 
toward the eye, can bring objects very close and 
get large retinal images and they are accorded to 
have excellent eyesight, until a chance for compar- 
ing distant vision occurs. 

In testing the visual acuity when the large 
letters on the card cannot be told, the patient should 
be requested to count the fingers held in front of 
the eye; if he is unable to do this the hand or any 
large object may be used for the test; if nothing 
can be seen and there still remains a perception of 
light, the patient is said to have quantitative vision. 
Proper illumination must be preserved in all tests 
for visual acuity, and in cases where reports are 
required for the navy, railroad service, etc., visual 
acuity should be taken when the accommodation is 
at rest and the correcting lenses are on. When the 
pinhole disc helps the visual acuity it is a strong 
indication that the trouble is a refractive anomaly; 



REFRACTION OF THE EYE. 



29 



when it does not improve the visual acuity of a 
person that cannot read % it strongly points to 
some trouble other than a refractive anomal}^. 

ACCOMMODATION. 

Every one must have observed that it is easier 
to look at an object five feet distant than to fix 
one five inches away. Why is this ? Because in 
order to get a distinct image of the near object in 
one eye, the work of accommodation must be called 
into play. When both eyes participate, converg- 
ence and accommodation must act together. This 
work of accommodation requires a continuous con- 
traction of the ciliary muscle in order to increase 
the index of refraction of the dioptric system of 
the eye. This continual effort on the part of the 
ciliary muscle causes many troubles which are 
summed up under the head of accommodative asthe- 
nopia. When an object five feet distant is fixed 
we cannot distinctly see the one five inches distant. 
Why ? Because the eye being in focus for the far 
object has not at that time, enough refractive power 
to bring the rays of light from the near object to a 
focus when they reach the retina. 

These rays not being completely focused form a 
blurred or diffusion circle image, which image imi. 
tates the contour of the pupil. (See Fig. 12.) 

If the emmetrope wishes to see anything 40 
centimeters from the eye he must use 2.50 dioptres 



30 



A HAND BOOK OF THE 



of accommodation if he obtain a clear image. If 
the object be 10 centimeters from the eye he must 
use 10. dioptres of accommodation. When the 
image approaches so near the eye that the entire 
accommodation cannot bring the focus, of the rays 
of light, from the image, on the retina; or when 
the eye is hyperopic to such a degree that the 
error cannot be corrected by the action of the cil- 
iary muscle, or when the distant object is viewed 
by the myope, indistinct vision results. 

All indistinct vision from ametropia is due to 
diffusion circles, and the diffusion images are 
larger as the retina is farther from this point of 
focus and as the pupil is larger. When at rest 
the emmetropic eye is adjusted so that parallel 
rays of light falling on the cornea will come to a 
focus on the retina. Accommodation depends on 
the elasticity of the lens substance owing to which 
the latter always tends to form a sphere. The 
capsule is attached by Zinn's zonula to the ciliary 
body. The zonula is placed continually on the 
stretch, exerting a uniform traction and keeping 
the lens flattened. When the annular la) r er of 
fibres of the ciliary muscle contracts the circle pro- 
tecting the tension of the zonula is lessened, and 
the tension thereby taken off the capsule, the lens 
at once becomes thicker, and consequently refracts 
light more strongly. Hence the more this process 
goes on the more divergent are the rays of light 



REFRACTION OF THE EYE. 



31 



that can be focused on the retina. The longitud- 
inal fibres of the ciliary muscles are attached in 
the sclera near the corneal margin, and to the 
movable choroid; by the contraction of these fibres 
the ciliary body is drawn forward and the action 
of the annular fibres assisted. As the lens becomes 
more convex the equatorial diameter decreases and 
a way is made for the advancing ciliary body. 

The anterior surface of the lens is more 
affected by the accommodation than the poste- 
rior surface, as the anterior surface can more 
easily invade the aqueous than can the posterior 
surface lying in the fossae patelliformis invade 
the vitreous. The sphincter pupillse and the in- 
ternal recti generally contract simultaneously with 
the ciliary muscle, their centres being very closely 
connected in the anterior part of the oculo- motor 
tract. When the ciliary muscle is at rest and 
the lens has its minimum convexity, the eye is 
adjusted for its far point. When the greatest pos- 
sible contraction of the ciliary muscle has taken 
place and the lens has assumed its maximum con- 
vexity, the eye is adapted for its near point. The 
far point or Punctum Remotum of all emmetropic 
eyes is at infinity. The near point or Punctum 
Proximum will depend largely on the strength of 
of the accommodation. This Punctam Proximum 
may be determined by noticing the distance at 
which an eye can read the smallest letters, and by 



32 



A HAND BOOK OF THE 



expressing this distance in centimeters or better 
millimeters. The space between the Punctum Re- 
motum and the Punctum Proximum is the range 
of accommodation and is no criterion of the amount 
of work done by the eye in adapting itself to the 
Punctum Proximum from the Punctum Remotum. 
The region of accommodation is a term used to 
express the distance of the entire range of accom- 
modation from the eye, thus the region of accom- 
modation of a myope is said to be closer to the eye 
than that of an emmetrope, as his Punctum Proxi- 
mum and Punctum Remotum are closer to the eye. 
The difference of the amount of the index of re- 
fraction of the eye between its Punctum Proximum 
and its Punctum Remotum equals the amplitude of 
accommodation. If we look at infinity and then at 
an object ten centimeters away, it requires some 
effort to fix the near object. Now, if we look from 
the object ten centimeters distant to one five centi- 
meters distant, we find that the latter change of 
fixation caused a greater exertion than the former, 
whereas in the former instance the refraction of 
the eye was changed from infinity to fixation at 
ten centimeters, while in the latter the refraction 
was only changed from a fixation at ten centime- 
ters to a fixation of five centimeters. It is evi- 
dent then that the range of the accommodation 
cannot form a basis for the determination of the 
the amplitude of accommodation. The amplitude 



REFRACTION OF THE EYE. 



33 



equals the Punctum Remotum minus the Punctum 
Proximum expressed in dioptres. To determine 
the amplitude of accommodation in emmetropia 
divide the number of centimeters in a meter by the 
Punctum Proximum expressed in centimeters (the 
Punctum Proximum of an emmetrope expressed in 
centimeters, we will suppose in this given case to 
be ten centimeters). Divide one hundred, or the 
number of centimeters in a meter, by ten, which is 
the near point expressed in centimeters, and the 
quotient equals ten, or the amplitude of accommo- 
dation expressed in dioptres, or the amplitude of 
accommodation of an emmetropic eye with a Punc- 
tum Proximum of ten centimeters. 

The range of accommodation shows the avail- 
ability of the eye, and the amplitude of accommoda- 
tion, the change in the index of refraction of the eye. 

Hyperopes require some of their accommo- 
dation to produce distinct, distant vision. There- 
fore, in order to determine the amplitude of accom- 
odation, we must either have such a glass as 
corrects their hyperopia, before taking their near 
point, or add to the near point expressed in diop- 
tres the correction for distance. Given a case 
hyperopic by 5 dioptres, when the near point was 
33.3 centimeters, ^ equals 3 + 5, (or the number 
of dioptres of hyperopia present) equals 8 dioptres 
or the amplitude of accommodation. 

In myopia the reverse is true. Find a glass 



34 



A HAND BOOK OF THE 



that has a focal distance equal to the Punctual 
Proximum and deduct the correction for distance. 
A myope of four dioptres with a Punctum Prox- 
imum of 8 centimeters equals 12.5 — 4, (or 
the number of dioptres of myopia) equals 8.50 
dioptres or the amplitude of accommodation of a 
myope of 4 dioptres having a Punctum Proximum 
of 8 centimeters. With the same ciliary muscle 
the Punctum Proximum is nearer in myopia than 
in emmetropia and nearer in emmetropia than in 
hyperopia. We speak of the absolute accommoda- 
tion as the amount of accommodation that can 
be used when one eye works alone, and of 
the relative accommodation as the amount that 
can be used when both eyes act together. As 
we know, the elasticity of the lens substance changes 
with age and with it gradually decreases the pos- 
sibility of accommodative changes. Accommoda- 
tion is greatest at about ten years of age, at which 
time it represents about 14 dioptres, and approx- 
imately it may be said to decrease .3 of one dioptra 
each year until forty years are reached, after which 
each year may be said to decrease .2 of one diop- 
tra, until at 65 years of age very little, if any, 
accommodation remains. 

CONVERGENCE. 

By convergence we maintain binocular vision 
when the object of fixation is brought nearer the 
eye than infinity. Convergence is then the rotating 



REFRACTION OF THE EYE. 



35 



inward of the eyes in a regular manner so that the 
visual axes may meet at any point between infinity 
and a few centimeters from the face. Or, converg- 
ence may be said to be the power of directing the 
visual axes of the two eyes to any given point, pro- 
vided this point is somewhere inside of infinity. 
The range of convergence may be called the dif- 
ference between the Punctum Remotum and the 
Punctum Proximum of binocular vision. The 
meter angle is used as the unit of measurement of 
convergence. When the eyes converge to an object 
one meter distant, we say this convergence equals 
one meter angle or the unit of measurement of con- 
vergence. If for 50 centimeters, the convergence 
equals 2 meter angles. If for 25 centimeters, the 
convergence equals 4 meter angles and the meas- 
urement of the meter angle is the inverse ratio of 
the distance as is the amplitude of accommodation 
in dioptres. 

The exact value in work done, or the ampli- 
tude of convergence, can only be determined accur- 
ately when taken in consideration with the inter - 
pupillary distance which is constant in each given 
person, but not constant as between different per- 
sons. There is a co-ordination between accommo- 
dation and convergence which produces harmonious 
combined action, and yet they are capable of co- 
ordinate action when their equilibrium is disturbed; 
as will be shown by the following: 



36 



A HOD BOOK OF THE 



An emmetrope fixing an object one meter dis- 
tant uses one dioptra of accommodation and one 
meter angle of convergence. Disturb their equi- 
librium by adding a — 2.00 dioptra glass in front 
of the eyes, or by substituting for the emmetrope a 
hyperope of 2 dioptres, and 3 dioptres of accom- 
modation are used, and only one meter angle of con- 
vergence, yet there is harmonious action of the two 
faculties. Again substitute a myope of one dioptra 
for the emmetrope and no accommodation is made, 
yet one meter angle of convergence acts and vision 
is binocular and perfect. 

The maximum convergence minus the mini- 
mum of convergence equals the amplitude of con- 
vergence. The maximum of convergence equals 
about 9.5 meter angles. And the minimum of con- 
vergence will be seen to be equal to minus one 
meter angle. Subtracting the minimum from the 
maximum the remainder will be 10.5 meter 
angles or about the average amplitude of conver- 
gence. The power of convergence depends on the 
relative strength of the internal rectus muscles. 
When the eyes are adjusted for infinity the optic 
axes are generally a little divergent. Therefore an 
angle results between the optic and visual axes. 

In emmetropia this angle generally equals 
about +4 degrees, in hyperopia about +7 degrees, in 
myopia about — 2 degrees. The Punctum Remo- 
tum of convergence is therefore often beyond 



REFRACTION OF THE EYE. 



37 



infinity. To test the convergence when we have 
no instrument made for that purpose, we place 
prisms base outward and generally 30 degrees to 40 
degrees are overcome. Prisms as you know are 
numbered according to the angle of the prism and 
produce about one -half the deviation of light that 
their mark indicates. If a prism is placed before 
one eye, it is in effect equally divided between the 
two. When the base line is of an average length 
or about 62 millimeters, a meter angle equals 
about 1.75 degrees of a circle and requires a 3.5 
degree prism, according to the old numbering, to 
equal in effect, one meter angle of convergence. 
To diminish the convergence by one meter angle a 
3.5 degree prism is placed in front of each eye, 
base in. Prisms are now being numbered by many 
manufacturers according to the deviation they pro- 
duce and are marked with a d following the num- 
ber designating their strength, as 2°^, meaning that 
a 2° deviation is produced by the prism. 

If you will call the base line the distance be- 
tween the centers of rotation of the two eyes, which 
for practical purposes may be called the pupillary 
distance, and erect a perpendicular to this line at 
its midpoint, any object situated on this perpendic- 
ular line demands equal convergence of both eyes 
when fixed. Suppose a case with an average pu- 
pillary distance or 62 m. m. to be fixing an object 
1000 millimeters (1 meter) away. One-half the pu- 



38 



A HAND BOOK OF THE 



pillary distance, or 31, divided by the distance of 
the object of fixation in millimeters being in 
this case 1000 = .031 which is the sine of 1 meter 
angle. The sine and the arc for these angles are 
practically equal. .031 is then the sine of the 
angle formed by 1 meter angle of convergence used 
by a person having a pupillary distance of 62 mil- 
limeters (or about 2.48 of an inch), which equals 
1° 46'+. If an object is \ of a meter distant and is 
fixed by the same person .062 is the sine of the angle 
of convergence. 

The value of the meter angles in degrees is 
approximately obtained by multiplying 1° 46' by 
the number of meter angles. The deviation of the 
meter angle is measured on the sine. 

A prism dioptre is a prism which shall deflect 
light 1 c. m. at 1 m. distance, or one hundredth 
part of the radius measured on the tangent. 

Dennett's Centrad is a prism that will produce 
a hundredth part of the deviation of a prism whose 
length equals the radius of its curvature. This 
arc equals 57.295°. This will give uniform devi- 
ation, ten centrads having ten times the deviation 
of one centrad. 

Should either of these ideas become universal, 
as Mr. Hardy urges, when a lens is decentered one 
centimeter, the prismatic deviation of the lens will 
be equal to as many prism dioptres as the number 
of dioptres of the lens. 



REFRACTION OF THE EYE. 



39 



When the eye fixes any object and an image is 
formed on the macula lutea, a line drawn from the 
macula to the object of fixation is called the line 
of visual axis. 

The optical axis of the eye is a line drawn 
exactly through the center of the cornea and lens 
to the posterior boundary of the eye. Now if the 
posterior end of this optical axis falls in the center 
of the macula, the optical axis and the line of vis- 
ual axis and the line of fixation (which is an imag- 
inary line drawn from the center of rotation of the 
eye to the object fixed) coincide. 

With the hyperope and, generally, with the em- 
metrope the optical axis passes to the inner side of 
the macula, and either the line of visual axis or the 
line of fixation forms an angle with the optical axis. 
The angle formed by the latter, forms the angle 
gamma, while the angle formed by the visual axis 
with the optical axis is often considered as the 
angle gamma, being easier to determine clinically. 
A line intersecting the cornea at right angles to its 
summit, continued back, may form an angle with 
the line of visual axis and is known as the angle 
alpha. The angle gamma is called + when it lies 
on the nasal side of the optical axis and — when 
on the temporal side. 

In high myopia the angle gamma is often 
— and an apparent convergent squint is pro- 
duced when fixing a near object. Likewise when 



40 



A HAND BOOK OF THE 



the angle gamma is + there is an apparent diver- 
gent squint and before operating, for strabismus, the 
determination of the angle gamma should be made 
and allowed for in the correction given. 



CHAPTER IV. 



Hyperopia. 

Hyperopia is that refractive condition of the 
eye in which parallel rays of light falling on the 
cornea come to a focus behind the retina. The 
retina then is situated between the lens and the 
posterior principal focus of the eye, and only when 
rays are convergent before they fall on the cornea 
can they be focused on the retina without the aid 
of accommodation. In hyperopia, when parallel 
rays fall on the cornea, they reach the retina and 
are cut off by it before a focus is obtained. 




Therefore it is evident that the hyperope, even 
when looking at distinct objects, requires the help 
of accommodation to produce a clear image on the 
retina. In order that rays of light coming from a 
distant object should be convergent when they fall 



42 



A HAND BOOK OF THE 



on the cornea they must start back of infinity, or 
at a point far enough away so that, if parallel lines 
were produced from it, and were on their way to 
infinity, they would begin to converge. Hence 
the Punctum Remotum of the hyperope is beyond 
infinity. 

In old age the cornea flattens and the eye be- 
comes hyperopic so that often an emmetrope at 30 
requires a correction for distance when reaching 70 
years of age. Nothing can be plainer than that 
the more hyperopic the eye, the more convergent 
must be the rays of light when falling on the cor- 
nea, in order to meet when they reach the retina. 

Hyperopia may result from either the axis of 
the eye being relatively too short or the refraction 
of the eye being relatively too weak; the former is 
generally the case. 

The amount of hyperopia is known by the dis- 
tance of the far point from the eye. The Punc- 
tum Remotum of a hyperope may be expressed by 
a minus quantity, or the distance between the pos- 
terior principal focus of the eye and the retina, as 
in Fig. 13. The distance from a to b which we will 
suppose to be 25 centimeters expressed as a minus 
quantity is the far point of this eye four dioptres 
hyperopic. 

The correction in this case would require a 
lens whose focal distance is 25 centimeters which 
we know is (-^) 4.00 dioptres. 



REFRACTION OF THE EYE. 



43 



Therefore the correction would be a +4.00 
dioptra lens which would give clear vision to a 
hyperope whose Punctum Eemotum equals a — 25 
centimeters. Without this correction, clear images 
are only obtained at the expense of 4.00 dioptres 
of accommodation. 

Hyperopia is called Manifest where it is not 
masked by the accommodation and accepts of cor- 
rection without a mydriatic, in contradistinction to 
Latent Hyperopia which refers to the hyperopia 
which is hidden by the action of the ciliary muscle. 
The Hyperopia Total of some writers refers of 
course to the Hyperopia Manifest plus the Hyper- 
opia Latent. To determine the total hyperopia, 
the action of the ciliary muscle must be suspended 
by the thorough use of a mydriatic. In youth, 
while the accommodation is very strong, less of the 
hyperopia is manifest, but as age gradually advances, 
more and more of it becomes so, until after 50 
years at latest, practically all becomes manifest. 

This, of course, should be remembered in de- 
termining the per cent, of the correction to be pre- 
scribed, that is, the younger the person, the less of 
his correction to be given, other things being equal, 
or, in other words, persons under 18 years are sel- 
dom given a full correction for hyperopia, unless 
the convergence is evidently overstimulated. While 
at 40 years the total correction, unless very great, 
would be at once extended. However, in young 



44 



A HAND BOOK OF THE 



persons, where the lens is to be worn for near work 
only, a full correction is often accepted immedi- 
ately. When a determination of hyperopia with 
insufficiency of the internal recti is made, both or 
neither should be corrected. To correct the hyper- 
opia and leave the insufficiency uncorrected would 
be to lessen the stimulus aiding the already falter- 
ing convergence. 

When a person has a large amount of hyper- 
opia (three to ten dioptres), it should not all be given 
at once, but at first a little more than the eye will 
accept with clear distinct vision, gradually in- 
creased until the desired amount is accepted. The 
desirable ultimatum depends on the amount and 
effect of the ametropia. From this standpoint 
hyperopia may be classified in three divisions. 

To the first class belong people with a small 
amount of hyperopia .50 to 1.50 dioptres, who have 
little or no trouble unless continued near work is 
indulged in. For these a full correction for read- 
ing, or a partial correction for distance, if required, 
may be given. These are called by some Faculta- 
tive Hyperopes. 

To the second class belong persons suffering 
from so great a degree of hyperopia as to have pro- 
duced insufficiency of the external rectus muscles or 
even convergent strabismus. For this class, often 
called Relative Hyperopes, a full correction is desir- 
able and generally imperative. 



REFRACTION OF THE EYE. 



45 



To the third class belong those persons afflicted 
with a very great degree of hyperopia where the 
visual acuity is extremely low, and where the ob- 
ject is drawn up close to the eye to produce a large 
retinal image. These people are often mistaken 
for myopes, and are called by some Absolute 
Hyperopes. 

Visual acuity is so affected that the usual test 
is quite unsatisfactory, little difference being noted 
by a change of two or three dioptres in the test 
lens. In these cases the lens should be given which 
gives the best general vision to the patient, and by 
other instruments of precision the total error re- 
corded, and the strength of the lenses increased as 
it will be accepted, within the limit of total correc- 
tion. A person who has normal visual acuity and 
yet presents himself for examination is suspected of 
hyperopia, especially if the usual symptoms are 
complained of, that is, headache and pain after near 
work, feeling of sand in thes eye and congestion of 
bulbar conjunctiva. In using the subjective method 
for determining hyperopia, the first step is to take 
the visual acuity of each eye separately. This is 
done by placing the patient 20 feet from Snel- 
len's test card, and requesting him to read the line 
marked 20 feet. This is generally read in simple 
hyperopia and is recorded on a card as follows: 

Optica Dextra V. A. = f 0 . 

Optica Sinistra V. A. = 



46 



A HAND BOOK OF THE 



In this fraction the numerator expresses the 
number of feet the patient is from the card and the 
denominator is the number of feet at which the 
line the patient reads should be read. If a line 
that should be read at 40 feet is the best that the 
patient can read when 20 feet from the card the 
record would show: 

Optica Dextra V. A. = %. 

Optica Sinistra V. A. = 

After this record has been carefully made the 
eyes are put under the influence of a mydriatic. 
Various are the opinions of the proper one to use, 
but it is generally agreed, except in cases of spasm, 
that eight instillations (of a solution containing one 
grain, to the dram, of both Cocaine Hydrochlorate 
and Homatropine Hydrobromate) made five min- 
utes apart with a ten minute rest, after the last ap- 
plication, is entirely sufficient to completely suspend 
the accommodation. 

Gelatine discs of the same ingredients are often 
used having the advantage of self-preservation 
which is much wanting in the solution. The sol- 
ution is far preferable when it can be used, and 
should be dropped over the upper margin of the 
cornea while directing the patient to look down. 
The annoyance of the bitter taste in the throat, 
when using the solution, is much lessened by hold- 
ing the finger tip over the punctse a short time after 
each application. After this procedure the patient 



REFRACTION OF THE EYE. 



47 



is again placed in the chair 20 feet from the test 
card and the reading of each eye separately taken 
again. This is recorded as before and shows the 
vision without the aid of accommodation. The 
trial frame is now placed on the face with an opaque 
disc over the left eye and the total hyperopia 
of the right eye determined by beginning with the 
weakest plus glass and increasing until the strong- 
est plus glass is found with which the best or nor- 
mal visual acuity is obtained. 

This is the measure of hyperopia. The amount 
is recorded and the opaque discs placed over the 
right eye and the procedure repeated with the left 
eye. After the measure of hyperopia has been 
ascertained in both eyes the record presents this 
appearance : 

John Doe aet. 20 years. 

Optica Dextra V. A. = g, after the V. A. = g 
with +2.50 D.-V. A. = g. 

Optica Sinistra V. A. = g, mydriatic V. A. = 
| with + 1.75 D.-V. A. = g. 

In hyperopia when prescribing lenses where 
one eye presents by far a greater degree of hyper- 
opia than the other, as Optica Dextra + 3.00 diop- 
tres; Optica Sinistra + 10.00 dioptres; the less 
ametropic eye is given its correction and the more 
ametropic eye is given but slightly more than the 
good eye. Should the full 10.00 dioptres be given 
to the left eye, the retinal image would be so differ- 



48 



A HAND BOOK OF THE 



ent in size as to produce disgust and disuse of the 
lenses. This is equally true in myopia and more 
practical as the myopic eye is more sensitive. 

The strongest plus glass is the correction be- 
cause there may not be full mydriasis. And the 
action of accommodation assisting a + 2.00 diop- 
tra glass would correct 3.00 dioptres of hyperopia, 
if 1.00 dioptra of accommodation were available. 

In using instillations of sulphate of atropine 
where the instillations are repeated daily for some 
time, as in cases of spasm, many advise that \ to 
1.00 dioptra be subtracted from the total finding. 
I cannot appreciate this advice myself, having often 
eventually given the full correction taken under 
atropine sulphate, which was happily accepted. 
When the hyperopia is considerable, three dioptres 
or more, it is always best to begin with a weak 
glass that corrects a little more than the manifest 
hyperopia, which glass is changed and increased in 
strength as soon as clear distant vision is obtained 
until the total amount to be worn is accepted. 

The glass that corrects must be of such a 
strength that it will render parallel rays of light 
falling on its surface convergent enough to meet at 
the Punctum Remotum of the hyperopic eye, no 
matter at what distance the glass is placed in front 
of the eye. The farther from the eye the glass is 
placed the greater must be the focal distance of the 
glass (and hence the weaker the glass), to make 



REFRACTION OF THE EYE. 



49 



parallel rays of light falling on the glass meet ex- 
actly at the Punctum Remotum of the hyperopic 
eye. An eye with a Punctum Remotum expressed 
by a — 100 millimeters requires a + 10.00 D 
in contact with the cornea to correct the ame- 
tropia. Place the plus glass 13 millimeters in front 
of the eye, and so situated, the correcting glass 
must have a focal distance of 113 millimeters, which 
we know is about a nine dioptra lens. Hence the 
effect of a plus glass is increased as it is moved far ■ 
ther from the eye. One exception to this rule must 
be noted. When the object of fixation is less than 
twice the focal distance of the glass from the eye, 
the effect of the plus glass decreases as the glass 
recedes. 

A person who obtains a clear image of a dis- 
tant object by moving the lenses away from the 
eyes should have a stronger glass to wear for dis- 
tance. A person who reads better when the lenses 
are farther from the eyes, should have weaker 
lenses for reading, provided the reading is done 
within twice the focal distance of the glass worn. 
The application of this rule often proves of benefit 
in advising persons, at a distance, about lenses, 
of which they write. 

It exceptionally occurs that a hyperope of ex- 
treme degree, as 8.00 dioptres, obtains best visual 
acuity with a fraction of his total correction, say in 
the case cited + 3.00 dioptres; this is because the 



50 A HAND BOOK OF THE 

visual acuity is lowered by the existing hyperopia; 
the eye becoming somewhat amblyopic. In cor- 
recting these cases the most acceptable glass is 
given and the total hyperopia determined by the 
skiascope and ophthalmoscope and recorded. From 
time to time a more full correction should be offered 
as often the visual acuity improves after wear- 
ing a correcting lens. 

In prescribing correcting lenses for a hyperope 
it should be remembered that the greatest visual 
acuity is obtained when the focus of rays falls ex- 
actly on the layer of rods and cones. Rays from 
objects four to six meters distant are not exactly 
parallel, though often considered so. From this 
fact it follows that the strongest plus glass very 
slightly over corrects the hyperope when exactly 
parallel rays are considered. Hence it is that the 
idea of subtracting \ of 1 dioptre from the total 
correction becomes worthy of consideration, pro- 
vided the test was made with the test card not 
over 6 meters distant. After a hyperope is cor- 
rected and still under the mydriatic he should read 
print held the focal distance of any glass which is 
added to the correction. If the correcting lens is a 
+ 2.00 D. and a + 5.00 is added, he should read 
print at 20 c. m. and if he reads fine print farther 
than this distance, while both lenses are on, he is 
not fully corrected. If much nearer, he is over cor- 
rected. When under the mydriatic, the hyperopia 



REFRACTION OF THE EYE. 



51 



may be determined, when dealing with young or 
illiterate people, by noting with what glass they can 
count small dots placed 25 centimeters in front of 
the cornea. This as we know should demand a + 
4.00 D. lens, and if it requires a + 7.00 there exists 
3.00 D. of hyperopia. In testing for hyperopia with- 
out a mydriatic, as is often done between 35 and 50 
years of age, it is of advantage to begin with a 
strong lens and run down the scale. Some advise 
that both eyes be tested at once to avoid exercise 
of the accommodative power, but this will not 
prove generally satisfactory, as in many people the 
two eyes vary much in the degree of ametropia. 

After the use of sulphate of atropine 
the correction desired is often given at once and 
ordered worn as the effect of the drug leaves the 
eye, and the changing and strengthening of the 
lenses thus avoided. In cases of high degrees of 
hyperopia and spasm, reporting at the clinic, after 
the homatropine solution had been used and the 
hyperopia determined, the patient has been treated 
with atropine sulphate until the next clinic day, 
but never so far as observed has the sulphate re- 
vealed more hyperopia than was evinced after the 
homatropine solution was used. 



CHAPTER V. 



Myopia. 

Myopia is that refractive condition of the eye 
in which the parallel rays of light falling on the cor- 
nea come to a focus before they reach the retina. It 
is readily seen that this may result from too strong 
a refractive media or from an increase of distance 
between the retina and cornea. When rays of light 
entering the eye become divergent enough to re- 
quire this extra amount of refraction (possessed by 
the myopic eye), to produce a clear image on the 
retina, then is the visual image clear. This clear- 
ing of the image is accomplished in two ways. By 
bringing the object near enough to produce a suf- 
ficient divergence of rays emanating from the object 
to atone for the extra refraction of the eye, or by 
the use of proper concave glasses. The former is 
the method made use of by the unskilled and con- 
stitutes nature's myopic panacea. 

Myopia is always present when the image is 
formed in front of the retina, if the accommodation 
be at rest. 

The cornea, far from being more convex in 
myopia, is generally less so, and the index of refrac- 
tion of the lens in the myopic eye does not mater- 



REFRACTION OF THE EYE. 



53 



ially differ from the lens in the emmetropic eye. 
Myopia, as a matter of fact, therefore, is generally 
due to elongation of the visual axis of the eye, and 
hence the myope presents, relative to the position 
of the retina, an excess of refraction. When my- 
opia is not so great as to bring the far point within 
the place where work is held, it does not present 
a formidable trouble, unless progressive. It allows 
near work with less exertion of the accommodation 
and is often called the anomaly of culture, espe- 
cially as it is found in more highly educated classes 
and only in the human eye, hyperopia being the 
rule with the lower animals. An eye myopic by 
£ to 3.00 dioptres is often a much better eye than 
an eye hyperopic in the same degree, and yet the 
hyperope can use accommodation for far and near 
work and attain good vision for both distances, 
while there is no reverse accommodation that the 
myope can use for his distant work. The myope 
sees better in advanced age on account of presby- 
opia, also because the pupil becomes smaller and 
this lessens the size of the diffusion circles formed 
on the retina. The use of the stenopaic slip helps 
the myopes vision because it cuts off part of the 
rays of light. One trouble often presents itself in 
medium myopia — disproportionate amount of 
convergence and accommodation used. The con- 
vergence demanded to sustain binocular vision in 
the myope is practically the same at any given dis- 



54 



A HAND BOOK OF THE 



tance as in the emmetrope. Physiological co-ordi- 
nation of these two functions exists in emmetropia, 
and accommodation always stimulates convergence, 
as we have seen before; hence it is not wonderful 
if the myope experiences great difficulty in main- 
taining convergence as he uses little or no accom- 
modation. As the hyperope produces too much 
convergence by the abnormally large amount of 
accommodation used, so the myope produces too 
little convergence by the abnormally small amount 
of accommodation used. From this anomaly diver- 
gence often results, and it affords the most frequent 
cause of asthenopia in typical and medium myopia. 
When once the effort to sustain binocular vision 
has been given up, the eye persists in its wander- 
ing, the retinal image is suppressed and the result 
is divergent strabismus with amblyopia exanopsia of 
the diverging eye, and the effort of accommodation 
and convergence which informs the emmetrope of 
the proximity of the object is gone. Insufficiency 
of the internal recti is most common in myopes, 
and when the myope does near work he is quickly 
fatigued, and suffers from pain in the head and 
even dizziness which gradually increase in intensity, 
provided this insufficiency exists, until the near 
work is discontinued or the proper correction 
given. 

This, then, is the trouble dependent on me- 



REFRACTION OF THE EYE. 



55 



dium myopia. Visual troubles are much more 
pronounced in high degrees of myopia. 

Binocular vision for near work becomes im- 
possible, muscae volitantes form a source of great 
annoyance and monocular diplopia appears with 
hyperesthesia of the retina, and hemorrhagic sco- 
tomata. The eye is elongated, large, voluminous, 
with an enormous antero-posterior diameter. 

The movements of the eyeball are limited me- 
chanically because it loses its spherical shape except 
at its anterior and posterior aspect; and it requires 
much more force to perform its excursion or to con- 
verge. The field of possible fixation is very lim- 
ited. The muscles lose their elasticity from dis- 
tension and the ophthalmoscope shows, in many of 
these cases, an unusual crescent, and characteristic 
choroidal changes. 

The term malignant myopia is applied to a 
myopia when it is persistently progressive. Malig- 
nant myopia is accompanied with many intra- ocu- 
lar changes and fraught with the most serious 
dangers. 

It is somewhat hard to diagnose in its inci- 
piency; generally the hyperemia of the retina, the 
red disc, rapid increase of the myopia, coupled per- 
chance with hereditary history will enable us to 
avoid error in diagnosis. Scintillation, photopsia, 
and metamorphopsia, which are due to the retinal 
hyperemia also act as pilot fish and give warning 



56 



A HAND BOOK OF THE 



of the progressive character of the disease. Irreg- 
ular pigmentation and the crescent can soon be ob- 
served at the external edge of the pajDilla and the 
vessels seem to enter the disc nearer the temporal 
side of the nerve. The disc even becomes indis- 
tinct and later the hypersemic inflammation sub- 
sides and leaves an atrophic condition of nerve and 
retina, the crescent is generally increased, the exu- 
date resorbs and leaves the white sclera in patches 
showing through. The posterior region becomes 
ectasic because the region of the lamina cribrosa 
offers the least resistance and posterior staphyloma 
follows. The whole visual axis of the eye elon- 
gates, the choroid continues chronically inflamed, 
the vitreous liquefies and carries about in its sub- 
stance parts of the retinal and chorodial coats, and, 
as if to draw a curtain on the deplorable scene, de- 
tachment of the retina often ends the functional 
life of the organ. 

The chief causes of myopia are first heredity; 
second, a long continued over amount of near work; 
third, a poor position in which the work is done; 
fourth, everything that tends to impair the general 
health. 

Of the first, many attribute a hereditary weak- 
ness of the sclerotic coat as a great factor, especially 
in malignant myopia, and all admit that an over 
amount of near work, between the years of 4 and 
18 is very productive of the malady. Young 



REFRACTION OF THE EYE. 



57 



children who become so intent over their kinder- 
garten pictures and work, their books and toys, as 
to continually work at them at a distance of 10 or 
12 centimeters produce more myopia than is pro- 
duced in ail other ways. In reading, the head 
should be kept erect, as bending the neck shuts off 
the free return of blood from the head, and con- 
gests the choroidal vessels. This bent position is 
the one naturally assumed, however, by the child 
elated with a new picture book or kindergarten 
structure. The full development of the eye is 
reached between the ages of 12 and 18 years, and 
if the child is carefully guided past this period one 
need have little fear of myopia. 

The treatment of myopia will be considered 
under four heads. 

First. Prophylactic — Which consists in insist- 
ing on work being done at a proper distance, and 
in proper amount only; in seeing that the position 
is correct; in having side or rear illumination of 
equal intensity, and in building up and maintain- 
ing the general health. One instance only will 
suffice to show the extreme benefit of this part of 
the treatment. A child, age 4 years — mother my- 
opic by four dioptres — child had taken greatly to 
picture papers, kindergarten work and toys. After 
one year's work of this kind child was pale, deli- 
cate, with capricious appetite, and skiascopy showed 
a myopia of 1.00 dioptre. All books and toys 



58 



A HAND BOOK OF THE 



were discontinued, child turned out doors to play, 
and whenever looking at near work (which she 
persisted in holding about 12 centimeters dis- 
tant) was instantly treated to a pair of 1.50 diop- 
tres plus lenses. The result was that one year and 
four months later the eyes were normal, the body 
Laving caught up to them. This then should be our 
aim in treating cases of myopia beginning in young 
children. 

Second. Age — Static refraction diminishes 
with age and if slight, will be entirely rectified in 
time. Unfortunately this self -remedy is not applied 
until 50, and reaches but 2.50 dioptres at about 80 
years of age. 

Third. The operative method — The removal 
of the crystalline lens will relieve quite a degree of 
myopia. Unfortunately, however, the lens in a 
normal state is too seldom successfully extracted to 
warrant the procedure. When opacity and age 
favor absorption or if the lens is ripe enough to 
allow extraction, it may, in severe cases, be admissible. 

Pseudo-myopia due to the spasm of accommo- 
dation is of course relieved by the instillation of 
atropine or some other mydriatic, which must often 
be continued for a considerable length of time. 

Fourth. The optical treatment — Since it is 
impossible to cure myopia or even diminish it, we 
must content ourselves with adjusting concave 
glasses that will give clear vision and yet allow 



REFRACTION OF THE EYE. 



59 



near work. Having found the weakest glass that 
gives the best or normal visual acuity (which is the 
measure of the myopia), if it be two dioptres or 
under, it will generally suffice to give a distance 
glass only, as near work within fifty centimetres, 
can be carried on easily without the aid of a lens. 
If the finding be over two and under four dioptres 
and the accommodation be very good, the one glass 
will answer for distance and reading both, provided 
the internal rectus muscles have previously been 
somewhat insufficient in performing their work of 
convergence. This having been the case the myope 
complains of having experienced trouble in read- 
ing without correction. On the other hand if the 
myopia equals four dioptres and the accommoda- 
tion be poor, this person will experience great diffi- 
culty in reading through the correcting glass, if it 
be even possible. It is therefore important in 
treating myopia to determine the amplitude of 
accommodation. This will be done in the follow- 
ing manner. Take the near point in millimeters 
(which we will suppose to be 111 millimeters), 
divide the number of millimeters in a meter or 
1,000 by the near point expressed in millimeters, or 
111, and the quotient will equal 9. Subtract the 
degree of myopia in dioptres, or in this case 4.00, 
and you have the amplitude of accommodation 
which equals 5 dioptres. Now you know from the 
chapter on accommodation that 5 dioptres of accom- 



60 



A HAND BOOK OF THE 



modation fill all requirements made on this myope 
in reading through the correction. Again you 
take a myope of 4 dioptres, who shows a near 
point of 200 millimeters. (1,000 -s- 200 = 5), — 
4 = 1.00 dioptre, amplitude of accommodation. 
If this person myopic by 4.00 dioptres were to be 
presented with a minus 4 dioptre glass for near 
work, possessing only 1.00 dioptre amplitude of 
accommodation, the result would hardly be gratify- 
ing, as one cannot work with ease when over three - 
fourths of their accommodation is constantly ex- 
erted. 

Now, in the first instance, having 5 dioptres of 
accommodation at command, after the correction of 
the myopia, if the glass is not used for reading, 
asthenopia will likely follow continued near work, 
as a result of the want of co-ordination be- 
tween convergence and accommodation. In the 
case last mentioned, asthenopia will also result 
without or with the minus four dioptre glass given 
as a correction, as there is practically no accommo- 
tion. Then, in the case lacking the accommodation, 
we have yet to relieve the asthenopia due to the 
effort of convergence. This can be done in two 
ways. First, by carrying the point of fixation 
further away, by such a part of the correction as 
the accommodation permits of, coupled with prisms 
to still more moderate the demand on the internal 
recti; or the prisms may be used alone. 



REFRACTION OF THE EYE. 



61 



Suppose a case demands 10 meter angles of con- 
vergence to do near work and has 4 dioptres of myo- 
pia, one and one-half dioptres amplitude of accommo- 
dation and eight meter angles of convergence. 
One dioptre of correction will be accepted and will 
remove the point of fixation by one meter angle 
and the myope will work at one -third instead of 
one-fourth of a meter. This removes the near point 
so expressed one meter angle and the demand for the 
still remaining meter angle deficiency, is supplied 
by three and one-half degree prisms base in. With 
this correction the patient works at over 25 centi- 
metres with ease, and, relieved of this muscular 
asthenopia, can pursue his near work any reasona- 
ble length of time. 

A myope of 2.00 dioptres should do work at 
50 centimeters without trouble, and this distance 
includes the performance of the practical duties of 
life of the carpenter, the blacksmith, the piano 
player and most others. If this work is impossible 
in such a myope it is because of spasm of accom- 
modation, which is to be treated with atropine. If 
the work be nearer than 50 centimeters, say 33 
centimetres, there will be found in nearly all cases, 
at least one dioptre of accommodation present 
which will enable him to do such work. If, how- 
ever, the patient be old or the accommodation 
nothing, one dioptre plus glass would be indi- 
cated for this work. In each case compatibil- 



62 



A HAND BOOK OF THE 



ity of accommodation and convergence mast be re- 
membered and consideration given whenever de- 
manded. While three-fourths of the accommoda- 
tion may continually be used, only one-third of the 
convergent power is available for prolonged labor. 
Hence we should determine the amplitude of con- 
vergence and compare it with the amount required 
by the distance of the work to be followed. 

Often in low degrees of myopia, choice must 
be made between a minus glass and a prism for 
near work. In high degrees of myopia the dimin- 
ishing influence of the glass places it open to much 
criticism for near work, especially among laborers, 
as so many dioptres are required to gain so few 
centimeters in distance. A myope of 20 dioptres 
must give 8 dioptres for the 3 centimeters increase 
in distance, and on this account prisms are more often 
prescribed. Again the prisms are heavy and often 
rejected, and this pushes us on to attempt the sur- 
gical help to convergence, or the partial or com- 
plete tenotomy of one of the external rectus muscles. 
This procedure is practical and is destined to hold 
a very much higher place in the treatment of my- 
opia coupled with insufficiency of the internal recti. 
Complete tenotomy of one external rectus muscle 
seldom over corrects the myope having over 20° if 
insufficiency of the interni. 

The lorgnette is used for the occasional help 
of myopes who do not accept a constant correction, 



REFRACTION OF THE EYE. 



63 



and some have even attempted to give the myope 
a large inverted image by placing a strong plus 
glass at a little distance from the eye, which would 
throw the inverted image of the landscape just in 
front of the face. This would seem impracticable 
however. The diminishing effect of a concave glass 
is increased as it is removed from the eye. Hence 
it is desirable to place the glass as near as possible 
to the myopic eye. The concave glass always in- 
creases the range of accommodation. In extending 
the correction to myopes of high degree, they should 
be given a partial correction, which is gradually 
increased until the desired lens is worn. 

Should a full correction be at once given in 
cases of 6.00 to 12.00 dioptres of myopia, the 
patient would often be unable to continue wearing 
the lenses on account of the sudden change pro- 
duced in the focus of the rays of light, dizziness 
and headache resulting. 



CHAPTER VI. 



Astigmatism. 

In the preceding cases rays of light coming from 
a given point fell on the cornea and were brought 
to a common focus, the light being equally re- 
fracted in all meridians of the refracting surface. 
We have now to consider that refractive condition 
in which different meridians refract light in differ- 
ent degrees. 

ASTIGMATISM. 

From what has been already studied on refrac- 
tion it is readily seen that astigmatism may be 
either myopic or hyperopic. When in myopic 
astigmatism one meridian is normal and one my- 
opic it is called Simple Myopic Astigmatism. 
When both are myopic and one is in excess of the 
other, it is called Compound Myopic Astigmatism. 
When one meridian is myopic and the other hyper- 
opic it is termed Mixed Astigmatism. 

When in hyperopic astigmatism one meridian 
is normal and the other hyperopic it is called- 
Simple Hyperopic Astigmatism. When both are 
hyperopic and one is hyperopic in excess of the 
other it is termed Compound Hyperopic Astig 
matism. 



REFRACTION OF THE EYE. 



65 



Astigmatism is regular when the refraction is 
equal in the two radii of any given meridian, 
irregular when the refraction is not the same in 
the two radii of any given meridian. In regular 
astigmatism the meridians of greatest and least cur- 
vature are always at right angles to each other. 
Of the two meridians the one of greatest curvature, 
having the greatest refraction — the one with the 
shorter focal length — is as a rule vertical, and the 
meridian of least refraction, therefore, horizontal. 
The vertical meridian is the meridians of 90°. The 
horizontal meridian is the meridian of 180°. This 
is called astigmatism ivith the rule. 

When the greatest curvature of the astigma- 
tism is at 180°, the astigmatism is said to be 
against the rule. When the meridians of greatest 
and least curvature do not fall on either axis, 90° or 
180°, astigmatism is said to be of irregular 
axes. This must not be confounded with 
irregular astigmatism. Nearly every eye has some 
corneal astigmatism. The cause of this may be 
found in the position of the lids, tending by their 
attachment to preserve a greater curvature of the 
cornea at 90°, also by the oblique position of the 
refracting surfaces to the visual line. Other causes 
that produce astigmatism are, progressive myopia, 
corneal scars, operations in the cornea, as cataract 
operation or iridectomy. 

The vision is not simply indistinct as in hyper- 



66 



A HAND BOOK OF THE 



opia and myopia, but presents irregular images due 
to the diffusion circles formed on the retina in one 
or both meridians. Let us take the case of Simple 
Hyperopic Astigmatism with the rule, and the cornea 
over the meridian of 90 degrees, that is over the 
vertical meridian, has the greatest curvature and 
focuses light on the retina. At 180° or over the 
horizontal meridian there is less curvature, less 
refractive power, and the focus of this meridian 
is behind the retina. 

At first sight one would say, that when look- 
ing at the cross- lines, 

10° 

I$0° 



the 90° line would be plainer and 180° line dimmer. 
Now carefully consider the line at 90°. It gives 
off many divergent planes of light to the right and 
left of the central plane, all of which are plainly 
dealt with by the 180° meridian. So the 180 de- 
gree meridian being of too small a refractive index 
in Simple Hyperopic Astigmatism, the 90 degree 
line will be not distinct but blurred, and from the 
same reasoning the 180° line will be distinct, as the 
90° plane is correctly adapted for the retina. 



REFRACTION OF THE EYE. 67 

Hence it is that the line seen plainly on the 
astigmatic chart is at right angles to the correct or 
most nearly correct meridian. 

Diagram to show that the horizontal line de- 
pends on the emmetropia of the vertical meridian 
for a distinct image: 




Let A. B. represent the meridian of 90° in the 
eye with Simple Hyperopic Astigmatism. This 
meridian is therefore correctly adapted to the posi- 
tion of the retina. Let c be the end of a horizon- 
tal line, g d, c e, c /', c g, c h, c i, c j and c h 
represent rays or planes of light given off from the 
horizontal line, which are evidently dealt with by 
the cornea A. B. at axis 90. 

The degree of astigmatism is expressed by the 
difference between the meridian of greatest and 
least curvature. 



68 



A HAND BOOK OF THE 



Whenever normal visual acuity cannot be 
attained with spheres, we must undertake to deter- 
mine the astigmatism. This is most easily accom- 
plished by placing the stenopaic slit over the eye 
in either principal meridian and finding the sphere 
that most nearly or quite gives normal visual 
acuity. Then removing the lens turn the stenopaic 
slit at right angles to the meridian first used and 
find the sphere to correct this meridian, from 
which two results a deduction can easily be made. 
To determine at what meridian to place the steno- 
paic slit, the astigmatic chart is brought into use 
and the slit may be placed first at right angles to 
the plainer lines seen on the astigmatic chart, which 
is the meridian of emmetropia or the least ame- 
tropia. 

From the amount of ametropia in each merid- 
ian the determination of the difference in the two 
meridians is easily made, and should be corrected 
by use of a cylinder added to the sphere which cor- 
rects the meridian of least ametropia. In supplying 
this cylinder it must be remembered that a cylinder 
refracts light only with the meridian at right angles 
to its axis, and when a cylinder is prescribed to cor- 
rect the meridian of 180° (as in Simple Hyperopic 
Astigmatism) it is put on at axis 90°. 

In Compound Hyperopic Astigmatism with 
the rule the sphere is used to bring up the meridian 
of 90° to the retina, and also to bring the image 



REFRACTION OF THE EYE. 



69 



formed by the meridian of greater error (or 180°) 
part way up to the retina. The cylinder is then 
added at axis 90° to finish the correction of the 
180° meridian, and the prescription reads, plus 
1.00 sphere combined with plus 1.00 cylinder axis 
90°, and this corrects the Compound Hyperopic 
Astigmatism with the rule. 

MIXED ASTIGMATISM. 

In using the stenopaic slit when one meridian 
is '-orrected by a minus sphere and the other by a 
plus sphere, mixed astigmatism is present and to 
combine this in one lens a Stokes' lens is pro- 
duced. 

CASE. 

Stenopaic slit at 180° plus 1.00 sphere ob- 
tains normal visual acuity. 

Stenopaic slit at 90° minus 1.00 sphere ob- 
tains normal visual acuity. 

The Stokes' lens correcting would be plus 1.00 
cylinder axis 90° combined with minus 1.00 cylin- 
der axis 180°, (remembering that a cylinder refracts 
light only at right angles to its axis). This Stokes' 
lens may be reduced to a sphero- cylinder, using a 
+ 1.00 sphere which corrects the meridian of 90° 
and renders the meridian of 180°, one dioptre 
more myopic then demanding a minus 2.00 
dioptre cylinder axis 180° (to correct the 1.00 
dioptre of myopia of the eye and the one addi- 
tional dioptre of myopia produced by the plus 1.00 



70 



A HAND BOOK OF THE 



sphere), the correction would read, plus 1.00 
sphere combined with minus 2.00 dioptre cylinder 
axis 180°. 

After the student has experimented in the cor- 
rection of astigmatism with the stenopaic slit it is 
desirable to determine it also by the use of the 
sphere and cylinder. The sphere is increased as 
long as visual acuity is bettered, the cylinders 
being used when the sphere fails. In a case of 
hyperopic astigmatism after the strongest plus glass 
is used that betters visual acuity a plus cylinder is 
added (first at axis 90°) which is strengthened if of 
benefit, until normal visual acuity is reached. If 
the cylinder at axis 90° does not prove of benefit 
the axis of the cylinder is slowly turned until it is ro- 
tated through an arc of 180°, as the astigmatism may 
be of irregular axes or against the rule. When this 
is not of benefit at any axis a minus cylinder is 
placed at axis 180° over the sphere, because the plus 
sphere may already over correct the meridian of 
least ametropia. This cylinder is also rotated that 
it may rest on every possible axis and when this 
combination proves best it is reduced to a plus 
sphere and a plus cylinder having a different axis 
but producing the same optical effect (+ 3.00 sph. 
3 — 1.00 cyl. axis 180° = + 2.00 sph. O + 1.00 
cyl. axis 90°). In determining myopic astigmatism 
the same procedure is observed with minus spheres 
first combined with minus cylinders. It is also 



REFRACTION OF THE EYE. 



71 



expedient to notice the axis of greatest and least 
ametropia before starting to determine astigmatism 
by the use of spheres and cylinders. In determin- 
ing visual acuity during refraction the attention of 
the patient should be directed to some one particu- 
lar letter and this letter should be one of the more 
complicated in structure. 

In irregular astigmatism visual acuity is poor, 
and often double images are seen by one or both 
eyes, and images appear distorted. 

Lenticular astigmatism, when irregular, pre- 
sents a deplorable picture, especially when myopic. 
No correction is accepted; images are reproduced 
in various parts of the field of vision, and often to 
read large type j)art of each eye must be excluded 
when receiving the image. This exclusion of part 
of the pupil may shut off some of the many images 
formed from the one object. Such cases of ame- 
tropia are, however, happily very rare. 

Put a weak cylinder before the eye and the 
astigmatism produced is corrected and often over- 
corrected by the ciliary muscles. Again, after the 
instillation of atropine the meridian of greatest 
curvature often changes, showing that the corneal 
astigmatism had been over- corrected by the irregu- 
lar contraction of the ciliary muscle. Hence astig- 
matism, if uncorrected, will seem to increase as 
accommodation fails, because it will not be so fully 
corrected by the action of accommodation. 



CHAPTER VII. 



Presbyopia. 



THE VISION OF OLD AGE. 

Presbyopia is that condition of the eye pro- 
duced by age, in which the accommodation, and the 
vision at the punctum proximum is diminished. 
A person is called presbyopic when the accommo- 
dation is affected to such an extent that the punc- 
tum proximum recedes beyond 22 centimeters, or 
in other words, when the dynamic refraction at its 
utmost does not exceed 4.50 dioptres. This occurs 
at about 40 years of age in the emmetrope, who 
corrects the grievance for three or four years by mov- 
ing the object of fixation a little farther away. The 
time arrives, however, when the farther removal 
of the object renders the visual acuity so poor, that 
common print cannot readily be seen and at this 
point the patient applies for help, or at about the 
44th year. 

The hyperope becomes presbyopic before the 
emmetrope, in proportion to his hyperopia. Below, 
is given a table denoting the amount of available 
accommodation or the dynamic refraction that it is 



REFRACTION OF THE EYE. 



73 



possible to use at the different ages after 40 years: 

40 years 4.50 dioptres 

45 years 3.50 dioptres 

50 years 2.50 dioptres 

55 years 1.75 dioptres 

60 years 1.00 dioptres 

65 years 75 dioptres 

70 years 25 dioptres 

To determine the age at which a hyperope will 
become presbyopic, it is only necessary to add to 
the 4.50 dioptres of dynamic refraction at 40 years, 
the amount of hyperopia existing and comparing 
this sum with the table above. A hyperopia of 2.50 
dioptres plus 4.50 equals 7.00 dioptres or the dy- 
namic refraction at 30 years of age. Hence the 
hyperope of 2.50 dioptres is at the presbyopic 
verge at 30 years. A few people possess a dynamic 
refraction stronger than the normal, preferably 
hyperopes, with whom the beginning of presbyopia 
will be relatively later. With the myope, the static 
refraction being stronger, it is evident that presby- 
opia will appear later, if indeed at all. 

An emmetrope, as we have seen, will become 
presbyopic at 40 years of age. In order to deter- 
mine the age at which the myope will become pres- 
byopic, the amount of myopia (which is the 
increase of static refraction) must be added to the 
dynamic refraction remaining at any given age. 



74 



A HAND BOOK OF THE 



When this sum equals 4.50 dioptres the myope is 
at the entrance of presbyopia. 

A myope of 4.00 dioptres at 40 years has there- 
fore (if his dynamic refraction is normal), a com- 
bined available amount over the normal static 
refraction of 8.50 dioptres, and at 65 when his 
dynamic refraction equals plus .75 dioptres his 
extra amount of static refraction added to this, 
leaves him plus 4.50 dioptres and he is about to 
enter the presbyopic field; while the myope of 7.00 
dioptres will become presbyopic after 80 years. 
Many claim that hyperopes generally have more 
than normal dynamic refraction and also that 
myopes often have somewhat less than normal 
dynamic refraction, hence the fitting of presbyopia 
by rule would lead to serious error. Again, a 
hyperope of any amount, if corrected, will be dis- 
pensed from presbyopia until forty. 

From the phenomena produced by presbyopic 
changes the myope receives the credit of receiving 
second sight as he gets sufficiently presbyopic to 
overcome his excess of static refraction. The cor- 
rection given the presbyope will depend on the 
amount of presbyopia and the work for which the 
glasses are intended. In testing for presbyopia 
(after the correction for distance has been given in 
both eyes, if required) they should both be tested 
at once by using lenses of the same denomination 
in each change, as the amount of accommodation 



REFRACTION OF THE EYE. 75 

used must be the same in every emmetropic eye 
(with unimpaired accommodation) in fixing a cer- 
tain point. In each case the work to be done must 
be considered and the near point brought a little 
nearer than the working distance. 

If a person requires a plus 2.00 dioptre lens 
to bring his near point to 12 inches, and is a piano- 
player, it is evident that dissatisfaction will result 
if this lens is given, as his work is done much 
farther away than is his near point placed by the 
used of a +2.00 dioptre lens. The correction 
must, therefore, be according to the use intended, 
as unhappily no means of accommodation is obtain- 
able in glass. It is well to remember that tall peo- 
ple generally hold their reading and work farther 
from the eye than do small people. 

In testing for presbyopia a hand test card is 
used which is brought to any desired proximity. 
After testing for presbyopia and making a deter- 
mination, it must be borne in mind that the eye 
will accept a stronger glass than is necessary, which 
glass would prevent the healthful exercise of the 
remaining part of accommodation when near work 
is done. The measure of presbyopia is the weakest 
plus glass that brings the near point to the desired 
location. 

When a myope begins to be presbyopic the 
correction for reading that has been used must be 
gradually decreased, (which equals in emmetropia 



76 



A HAND BOOK OF THE 



the adding of a plus glass) as the testing indicates. 
If a cylinder is worn it must be left on, or ground 
in with the plus glass, when such a glass is de- 
manded, until way along toward 70 years of age, 
when the flattening of the cornea through the 90 
degree meridian, corrects the astigmatism, if of 
low degree, and with the rule. 

It will be apparent from following the cycle 
of changes that go on as the person with myopia 
or myopic astigmatism becomes presbyopic, that 
the determination of the glass required for near 
work will be a source of great annoyance, both to 
the physician and patient. Neither, will the result 
in all cases be fortunate. 



CHAPTER VIII. 



Skiascopy. 



BY C. B. BLISS, M. D. , ASSISTANT TO THE PROFESSOR OP 
OPHTHALMOLOGY AND OTOLOGY. 



As a means of supplementing the subjective 
determination of errors of refraction, skiascopy 
holds a prominent position, and is steadily growing 
in favor. 

In some cases, as with very young children, 
foreigners, malingerers and insane people, where 
the subjective tests may be unreliable, it affords 
the most convenient method of determining ame- 
tropia. 

The principle of the test is as follows: If rays 
of light be thrown into an eye from an ophthal- 
moscopic mirror held at a little more than a meter 
from the observed eye, the observer, looking through 
the sight hole, sees the fundus reflex. If now the 
mirror be rotated on its axis, a shadow is seen to 
move across the red reflex. The direction of the 
movement of this shadow, whether "with" or 
"against" the movement of the mirror on its axis, 
determines the character of the ametropia, though 



78 



A HAND BOOK OF THE 



not its amount. In low degrees the movement is 
quick and sharp and in high degrees it is slow and 
indistinct. 

In making the test either the plane or concave 
mirror may be used, the plane mirror being more 
generally preferred. In either case it should have 
a diameter of 30 to 50 millimeters; if concave, a 
focal distance of 20 to 25 centimeters. The sight 
hole should be 3 or 4 millimeters in diameter, and 
to avoid reflections from around it, which are very 
annoying, its margin must be free from dust and 
drippings and should be blackened with a dead 
black. If the patient be over 40 years of age, suf- 
ficient dilatation of the pupil can be obtained by 
the use of cocaine; but if the patient be young, a 
mydriatic must be used, otherwise the refraction 
will vary with changes in the accommodation. 

The room should be perfectly dark and the 
light used in the test come from through a small 
aperture opposite the brightest part of a flame situ- 
ated above and back of the patient's head. 

With these few general remarks we will take 
up first the practical use of the plane mirror in 

MYOPIA. 

The myope's far point, unlike the far point in 
emmetropia and hyperopia, is situated at a defi- 
nite distance from the eye, and this distance corres- 
ponds with the focal length of the lens required to 
correct the myopia. 



REFRACTION OF THE EYE. 



79 



In skiascopy this far point is called the " point 
of reversal," and beyond this point the observer 
sees the movement of the shadow "against 11 the 
movement of the mirror on its axis, because he 
views an inverted image ; but if the observer views 
the image from a point within the point of reversal, 
he gets an upright image, and the movement of the 
shadow will be " with" the mirror, though in both 
cases the real movement of light is "with" the 
mirror. 

Therefore in practice the observer seats himself 
at one or two meters from the patient and finds that 
the shadow moves "against" the mirror; as he 
approaches the patient the movement becomes 
more rapid till the far point is reached, when the 
shadow's movement becomes indistinct, but ap- 
proaching still closer the movement is sharply 
"with" the mirror. The distance from this point 
of reversal to the patient's eye is the focal distance 
of the lens required to correct the myopia. 

In emmetropia, as rays emerging from the eye 
are parallel there is no far point so no point of re- 
versal, and in myopia of low grade the point of 
reversal will be situated back of the observer, 
seated at one or two meters distance from the 
eye, consequently the shadow will move "with" 
the mirror. But by placing a convex lens be- 
fore the observed eye it is rendered myopic in 
refraction. The distance of the point of reversal 



80 



A HAND BOOK OF THE 



is then found, and if it equals the focal length of 
the lens placed before the eye, the eye is emme- 
tropic. If it exceeds it, the difference is the meas- 
ure of the myopia present. 

For example: The operator finds the shadow 
move sharply "with" the mirror even when he is 
three meters distant from the eye; he places a plus 
1.00 dioptre lens before the observed eye and 
approaches till he finds the point of reversal at one 
meter distance from the eye, the case is emme- 
tropia. If the point of reversal is found at 75 
centimetres the entire myopia will be 1.50 dioptres; 
subtracting the 1 D. of myopia caused by the plus 
1 D. lens, leaves minus .50 dioptres as the real 
measure of the myopia. 

HYPEROPIA. 

In hyperopia the emergent rays are divergent, 
so the far point is not a real one but is back of the 
retina. In applying the test a convex lens is placed 
before the eye to render it myopic. The point of 
reversal is then found, and its distance from the eye 
measured. This represents the amount of myopia 
produced by the convex lens and must be subtracted 
from the strength of the lens to give the amount of 
hyperopia. 

ASTIGMATISM. 

From what has preceded, it will be seen that 
the point of reversal of the principal meridians 



REFRACTION OF THE EYE. 



81 



may be found separately, the difference giving the 
amount of astigmatism present. 

CONCAVE MIRROR. 

In using the concave mirror the operator seats 
himself a little more than one meter from the 
patient. 

With the plane mirror the apparent or imme- 
diate source of light thrown into the eye was an 
image of the real flame, situated as far back of the 
mirror as the mirror was distant from the light. 
Therefore rotation of the mirror on its axis caused 
a movement in the same direction of the retinal 
reflex. 

When the concave mirror is used, however, 
an inverted aerial image of the original source of 
light is formed at the focus of the mirror. This 
forms the immediate source of illumination, and 
the rays diverge from this focus to the plane of the 
observed eye, to be brought to a focus again back 
of the retina. So any movement of the mirror on 
its axis causes an opposite movement of light on 
the retina. 

Moreover, in using the concave mirror it must 
be remembered that the operator is constantly 
seated at one meter from the patient and does not 
change his position, and as in all tests the point of 
reversal is brought, by the aid of lenses, to this 
point, so in all cases minus 1.00 dioptre must be 



82 



A HAND BOOK OF THE 



added to the glass which brings the point of 
reversal to this distance from the eye. 

MYOPIA. 

The shadow moves "with" the mirror, the 
myopia therefore is of more than 1.00 dioptre. So 
concave glasses are placed before the eye till the 
point of reversal is brought forward to one meter's 
distance from the eye. Minus 1.00 dioptre being 
added gives the correction, or entire amount of 
myopia. 

If the myopia present be of just one dioptre, 
there will be no movement of the shadow. If the 
myopia be less than one dioptre the movement of 
the shadow will be " against," in which cases con- 
vex glasses are placed before the eye till the point 
of reversal is at one meter from the observed eye. 

HYPEROPIA. 

The shadow moves "against " the mirror, con- 
vex lenses are added till the point of reversal is at 
one meter, then a minus 1.00 dioptre is added as 
before. 

ASTIGMATISM. 

The two principal meridians are tested sepa- 
rately and a minus 1.00 dioptre added to each 
result, and the difference of refraction in the two 
meridians equals the amount of astigmatism. 



CHAPTER IX. 



The Ophthalmoscope Relative to Refraction. 

Ophthalmoscopy furnishes a second mode of 
supplementing the subjective determination of ame- 
tropia. It demands years of work to acquire this 
science, yet with those who have mastered it, it is 
reasonably reliable. Though very little informa- 
tion can be imparted through a book, as to the prac- 
tical part of ophthalmoscopy, a few words on the 
technique may be of value. 

As to illumination, the argand burner and 
tubular chimney are much in favor with some, 
while the condenser used for laryngoscopic exam- 
ination placed over the tubular chimney seems to 
to be displacing the plain lamp, as>it gives a clearer 
fundus. 

Pure kerosene oil gives a good light, but now, 
happily, gas may be obtained in nearly every city 
of any size, and it will be found more convenient. 

The lamp should be at the side of the patient's 
head on a level with the eye, and the nearer the 
lamp, eye and ophthalmoscope form a straight 
line, the less will be the angle of incident and 
reflected light; and consequently less rotation of 
the ophthalmoscope on its vertical axis will be 



84 



A HAND BOOK OF THE 



demanded. The ophthalmoscopic mirror should 
be concave and have a focal length of about nine 
inches. Before the examination of the fundus is 
begun it is desirable to examine the eye (by oblique 
illumination and by sunlight) to satisfy ourselves as 
to the normal condition of the cornea, iris and lens. 

In the direct examination with the ophthalmo- 
scope, the accommodation of the examiner should 
be relaxed and the pupil of the patient opened by 
a mydriatic. As skill is acquired, the use of the 
mydriatic is less important. The room of course 
must be darkened. When the examiner throws 
the reflected light into the eye and looks through 
the hole in the ophthalmoscope there is seen a red 
reflex due to the reflection of light by the choroidal 
vessels. This reflex must be studied until it re- 
solves itself into a picture of the fundus, and this 
is best done by moving the head to and from the 
patient and examining at different distances and 
angles, to get familiar with the process. 

The inverted image is obtained by the indirect 
method of examination, in which the observer, 
placed much farther back from the patient, obtains 
a reflex the same as in skiascopy, and then places 
a strong plus lens directly in front of the eye. 
This lens is drawn toward the ophthalmoscope 
until it is about its focal distance from the eye, 
when a small, aerial, inverted image, of the fundus, 
is seen in the air between the ophthalmoscope and 



REFRACTION OF THE EYE. 



85 



the plus lens, a short distance in front of the 
latter. 

When the eye of the patient is emmetropic the 
fundus will be seen plainly in all meridians with 
the direct method of examination. The entrance 
of the optic nerve will appear as a whitish -red 
circle to the nasal side of the center of the retina, 
surrounded by the red retina and crossed by veins 
and arteries as they pass from the retina to the cen- 
ter of the nerve and disappear. In some cases as 
in albinos and negroes the want or excess of pig- 
ment alters this reflex to a great extent. The latter 
often presents a fundus dark and lustrous, with 
very little reflex whatever. When the pigment is 
highly developed in the choroidal stroma, and not 
so developed in the smaller vessels, the spaces be- 
tween the vessels instead of being light, are dark 
grey or even black, and collected into masses be- 
tween the interstices of the vessels. A very black 
striped appearance is thus given the fundus, which, 
when seen for the first time, often gives rise to some 
apprehension. 

The fundus is examined a small part at a time, 
and the patient directed to turn the eye so that the 
different parts of the fundus are brought to view. 
Some few things are noticed in the fundus which, 
though not constant, are nevertheless of no patho- 
logical significance. 

The disc is often surrounded, through part of its 



86 



A HAND BOOK OF THE 



circumference by a dark crescent which is produced 
by the edge of the choroidal coat showing through 
the retina and is called the choroidal crescent. Often 
there appears a white physiological cup occupying 
a part of the disc, narrowing toward the center of 
the nerve-head, which should be studied in order 
that it be not confounded with the funnel-shaped 
cup of atrophy or the abrupt marginal cup of glau- 
coma. One should remember, the stronger the 
illumination, the redder the reflex appears. Again 
the disc or papilla is not always round or approx- 
imately so, even in the emmetrope, as it is often 
possessed of a physiological elongation due to its 
anatomical make up, which resembles closely the 
optically elongated disc of astigmatism. 

The greyish white appearance of the disc is 
produced by the mass of nerve fibres and the lamina 
cribrosa. Sometimes the medullary sheath of the 
nerve fibres does not end at the lamina cribrosa, 
but is continued on, giving a queer white striped 
appearance to the fundus. This condition is often 
seen in the rabbit, though it is rare in the human 
eye. The surface of the disc may be mottled, grey- 
ish-white, due to the fact that the medullary sheath 
is discontinued sooner on some nerve fibres than on 
others, which fact should cause no alarm. The dis- 
tribution of the capillaries also varies in each disc. 

The ring of white surface generally seen near 
the edge of the disc is formed by the connective 



REFRACTION OF THE EYE. 



87 



tissue of the sheath of the nerve as it continues 
upward to the innermost part of the sclera. 

The veins are darker and about one -third 
larger than the arteries; both show a distinct white 
line following their course which seems to lie upon 
their surface. This line is much more plainly seen 
when the artery is observed. There is no pulsation 
seen in the arteries nor in the veins except over the 
disc, and that is not frequently noticed. The yel- 
low spot or macula lutea is situated to the temporal 
side of the disc, its border is best seen with the 
indirect method, and appears, with the inverted 
image, as a dim silvery ring to the inner side of the 
nerve. The fovea centralis is the central part of 
the macula and will appear as a dark spot in the 
macula; it is better seen in some eyes than others, 
and always better with the direct method of exam- 
ination. This, then, is the picture presented by the 
emmetropic eye. 

When an emmetrope looks into the eye and 
sees a clear image over all parts of the fundus 
without using his accommodation, he is aware 
at once that he is looking at an emmetropic 
eye; and if the fundus is indistinct, but brought out 
clear, by use of the accommodation, it is plain that 
the eye must be hyperopic. If again the fundus is 
indistinct and cannot be cleared up by accommoda- 
tion, it is evidently a myopic eye or one that will 
present a pathologic condition. 



88 



A HAND BOOK OF THE 



There are other things which help in determin- 
ing the condition of the ametropia. In hyperopia 
the disc appears smaller with the direct image, and 
in cases of considerable degree the disc is red, even 
very red, and poorly denned at best. 

It is then only necessary to relax the accom- 
modation and turn on plus lenses in the ophthal- 
moscope, until the strongest plus glass is employed 
with which a distinct fundus is obtained and the 
measure of the hyperopia is taken. In order to 
measure the hyperopia with the ophthalmoscope, it 
is not necessary, in most cases, to paralyze the 
accommodation of the patient. This point, though 
much disputed, is capable of clinical demonstration, 
and is thought to be due to the fact that the atten- 
tion of the hyperope is not attracted to any point 
of fixation as in the refraction with lenses, and the 
accommodation therefore relaxes. 

In myopia the disc appears larger, when using 
the direct method, and usually whiter and clearer of 
outline, than in hyperopia. When of high de- 
gree the disc looks displaced somewhat and its 
fibres are curved or the circumference is even dis- 
torted, especially on the temporal side. The cho- 
roid is also drawn away from the disc on the 
temporal side, so that a whitish crescent appears. 
If the myopia is of very high degree, the vitreous 
often detaches itself from the retina, and even the 
retina may detach from the choroid; hemorrhages 



REFRACTION OF THE EYE. 



89 



occur, and hence the picture in high degrees of pro- 
gressive myopia may be of a great variety. 

In determining the amount of myopia with the 
ophthalmoscope we use the weakest minus glass 
that gives a clear image of the fundus and say it is 
the measure of the myopia. In using the indirect 
method, the relative size of the disc in myopia and 
hyperopia is reversed; the disc in high degrees of 
hyperopia being large, diminishing directly with 
the refractive anomaly, decreasing as emmetropia is 
reached and on down through myopia. Again, 
with the indirect method in astigmatism the image 
of the disc appears elongated in the direction of 
the meridian of least curvature when the plus 
lens is held near the eye, and it is by this means 
that the physiologically elongated disc can be 
told from the elongation of the disc due to astigma- 
tism. 

Astigmatism is determined objectively by the 
ophthalmoscope. As the refraction of the differ- 
ent meridians differs, with the direct method, the 
observer will not receive a clear image of all the 
fundus at once, neither will a plus or minus glass 
bring out both meridians clearly at the same time. 
Again, the fundus in the meridian of the greatest 
curvature appears larger or magnified and the round 
papilla assumes a vertical oval at the axis of great- 
est curvature, or (in astigmatism with the rule) at 
90° in either the myopic or the hyperopic variety. 



90 



A HAND BOOK OF THE 



If an eye were emmetropic in the 180° plane . 
and myopic by 3.00 dioptres in the 90° plane, the 
observer is adapted only to the 180° plane, and 
will see distinctly the fundus over the 90° plane. 
The enlargement is also different in the dif- 
ferent planes or meridians. The meridian of 
least refractive power is seen with a weaker or 
stronger glass (as the case is compound hyper - 
opic or compound myopic astigmatism) than the 
meridian that is at right angles to it. The enlarge- 
ment of the fundus occurs through the meridian of 
greatest refractive power. 

In hyperopic astigmatism with the rule the 
disc presents an oval longer at 90° with the di- 
rect method, yet the disc becomes wider at axis 
180° when viewed by the indirect method (when 
the plus lens is held near the eye), and as the plus 
lens is drawn out from the eye the disc becomes 
relatively smaller through the horizontal meridian. 
When the distance between the lens and the first 
principal point of the eye is equal to the focal 
distance of the lens, the disc becomes round, and 
if the lens is farther drawn out, the papilla takes 
on a vertical elongation again. 

From the foregoing it is evident that the aerial 
image of the disc contracts relatively, in the meri- 
dian of least refraction, and enlarges relatively, in 
the meridian of greatest refraction, as the plus lens 
is drawn out from the eye; therefore, in nyperopic 



REFRACTION OF THE EYE. 



91 



astigmatism the horizontal diameter, of the aerial 
image, is diminished relatively, as the lens is drawn 
out from the eye, and in myopic astigmatism the 
vertical diameter of the aerial image is relatively 
enlarged under like withdrawal of the lens, pro- 
vided in both cases the astigmatism is with the rule. 

In mixed astigmatism the aerial image en- 
larges in the direction of the myopic meridian and 
contracts in the direction of the hyperopic meridian 
as the plus lens is withdrawn. 

The reason that this phenomena is not more 
apparent clinically is because in astigmatism there 
is often an accompanying physiological elongation 
of the disk at 90°, in which case the diameter of 
the papilla at 180° is much decreased relatively 
speaking (with the indirect method), while the 
plus lens is held its focal distance from the eye, 
and instead of a round disc, one is seen that pre- 
sents an elongation at 90° with a distinct bulging 
on both sides at the axis of 180°. 

To correct astigmatism with direct opthalmo- 
scopic examination the lens which gives a clear 
image in each meridian is the measure of the 
ametropia in the opposite meridian, and the differ- 
ence is the amount of astigmatism. 

Irregular astigmatism is determined with the 
ophthalmoscope by the peculiar and twisting outlines 
of portions of the fundus and the indistinctness of 
part or all of the picture. Often one meridian may 



92 



A HAND BOOK OF THE 



be corrected and some useful vision given, and it be- 
comes our duty to attempt to do this especially 
when both eyes are affected. 

In conical cornea and some other high degrees 
of ametropia from other causes, the fundus looks 
like a shining, glistening ball of red, surrounded 
and buoyed up by a dark fluid. When the proper 
correcting lens is turned into the aperture of the oph- 
thalmoscope the peculiar picture clears up instantly. 
In determining the exact axis of astigmatism with 
the ophthalmoscope, if a match or slight rod is held 
between the flame and the mirror, the shadow of 
the rod is not equally defined in all meridians, and 
the region of the least and greatest distinctness will 
locate the axes practically well. 



CHAPTER X. 



Extrinsic Eye Muscles and Their Anomalies. 

The four recti muscles of each eye arise around 
the bony circumference of the foramen opticum 
and run forward to be inserted into the sclera. 
They form a funnel with its apex situated at the 
foramen opticum and give a broad tendonous at- 
tachment to the sclera, which is much thickened 
thereby. 

The muscle that attaches to the sclera, to the 
inner side of the cornea is called the internal 
rectus. The one that attaches to the sclera to the 
outer side of the cornea is called the external 
rectus. In the two eyes these four muscles form 
two pairs. The internal of one eye and the exter- 
nal of the other working together in the excursions 
of the eyeball, and yet the two interni work to- 
gether when convergance is required. 

The external rectus of the right eye and the 
internal rectus of the left eye turn the eyes to the 
right; and the external rectus of the left eye and 
the internal rectus of the right eye turn the eyes to 
the left. 



94 



A HAND BOOK OF THE 



The two remaining rectus muscles of each eye 
run forward and outward and are inserted into the 
sclera one above and the other below the cornea, 
and are called respectively the superior and inferior 
rectus. In excursions of the eye demanding action 
of these muscles, the two superior recti and the 
two inferior obliques act in unison; and the two 
inferior recti and the two superior obliques also. 

In all excursions of the eye Tenon's capsule 
acts as a socket for the enarthrodial joint. 

The superior rectus turns the eye up and in. 
As it runs forward to be inserted into the sclera it 
forms an angle with the sagittal axis of the eyeball 
by running outward as well as forward. It pro- 
produces adduction as well as elevation. 

The inferior rectus turns the eye down and 
also for the reason explained in speaking of the 
superior rectus, draws the corneal portion of the 
eye in, that is, acts as an adductor. 

Two other extrinsic muscles are given to each 
eye. The superior oblique which also arises from 
the margin of the foramen opticum, but its virtual 
insertion is the trochlea, at the upper and inner 
wall of the orbit. This muscle passes out over the 
eyeball to be inserted into the sclera behind the 
center of the eyeball. The trochlea is relatively 
higher than the insertion of the muscles. A con- 
traction of this muscle would then tend to throw 
the cornea down, rotate the top of the axis of 90° 



REFRACTION OF THE EYE. 



95 



over toward the bridge of the nose, and abduct the 
anterior half of the eyeball. 

The inferior oblique arises from the lower 
margin of the orbit, near its inner boundary. It 
runs out and up and is inserted into the sclera be- 
hind the equator, in about the horizontal meridan. 
The action of each of these inferior oblique muscles 
on their separate eye, is to rotate the lower ex- 
tremity of the axis of 90° toward the nose (opposite 
to the rotation produced by a contraction of the 
superior oblique), to elevate the cornea and to 
abduct or turn the anterior or corneal portion of 
the eyeball out. 

Many of the tests for paralysis of the oblique 
muscles are made dependent on the rotation of the 
retina on its sagittal axis, by the contraction of 
these muscles. In this chapter we will make the 
diagnosis on the abduction produced by the ob- 
liques as compared with the adduction produced 
by the superior and inferior rectus. 

We have then three pairs of muscles. 

First — The internal and external rectus, which 
rotate the eye about its vertical axis. 

Second — The superior and inferior rectus, 
which turn the eye principally about its horizontal 
axis and produce some convergence. 

Third — The superior and inferior oblique, 
which rotate the eyes about their sagittal axes and 
produce some divergence. 



96 



A HAND BOOK OF THE 



In considering the diagnosis and treatment of 
the anomalies of the eye muscles, we have chiefly 
two conditions to meet. 

Insufficiency — Where the muscle, though in- 
sufficient for the work, is still so adjusting the 
retina of each eye as to produce images on the 
same relative part of each retina. 

Strabismus — Where the insufficient muscle 
gives up the hopeless task of overwork and allows 
one eye to wander far out on any of its excursions 
and remain there. 

It is evident that the diagnosis of a squint and 
the muscles at fault can be easily made after a 
careful study of the origin, insertion and action of 
these several extrinsic muscles. The difficulty will 
be to diagnose the insufficiency where the muscle, 
though of inferior strength, is still by extra effort 
doing its work. This is done by placing a prism 
in front of one of the eyes (preferably the left), 
and a red glass in front of the right. The prism 
throws the object seen by the left eye toward its 
apex; therefore, the image of the left eye appears 
higher than the image of the right eye. It is now 
impossible by any effort, to fuse the images, and the 
insufficient muscle accepts the situation, ceases its 
over-exertion, and allows the eyes to swing into 
their true relative position, which position, indi- 
cates the comparative strength of the muscles. The 
position of the two images thus produced will de- 
termine the muscle at fault. 



REFRACTION OF THE EYE. 



97 



For instance, to test the internal and external 
rectus muscles, which are most frequently at fault, 
having placed an 8° prism base down over the left 
eye and the red glass over the right eye, the patient 
is directed to look at the lighted candle. Two 
candles are seen. The higher and white one by 
the left eye, and the lower and red one by the right 
eye. 

Now if the internal recti as a pair are perfectly 
balanced with reference to the external recti, one 
image will appear directly above the other; pro- 
vided the prism is placed carefully in a perpendic- 
ular position. If these two lights are not one 
directly over the other, the relative position of the 
lights refers us to the weak muscles; that is, if the 
internal recti are relatively too strong, a slight con- 
vergence is produced, we will say for simplicity, in 
the right eye alone (though both eyes are con- 
verged). 

What has this done? Well, the part of the 
retina at the left of the macula (which in the right 
eye had been placed so as to receive the retinal 
impressions of objects to the right of the center of 
the patient's field of vision), has been drawn around 
and occupies the place the macula for merly occu- 
pied. And what is the effect? The object is 
oriented subjectively out in space to the right hand 
side of the center of the field of vision and is there 
seen. Then the image of the right eye stays to 



98 



A HAND BOOK OF THE 



the right, and is, in this case, the red and lower 
image, while the white higher image of the left eye 
is of course seen to the left. This is called ho- 
monymous diplopia, and indicates an abnormal 
amount of convergence which results from an insuffi- 
ciency of the external rectus muscles. 

When on the other hand, with the prism and red 
glass remaining the same, where the internal recti are 
insufficient, we will say for simplicity, the right eye 
only is turned slightly out (though both are so 
turned), the retina to the right of the macula is 
drawn around where the macula should be, and 
this part of the retina has been receiving images 
from objects to the left of the center of the field 
of vision, and the object is oriented subjectively 
over to the left, or across the path of the image of 
the left eye. The red lower light is then seen to 
the left of the white higher light of the left eye 
and this is called crossed or heteronymous diplopia 
and always indicates divergence or insufficiency of 
the internal rectus muscles. 

In the first instance prisms are placed base out 
until the insufficient external rectus is strengthened 
up till one image is straight over the other. In the 
second case, where the diplopia is crossed, we put 
the base of the prisms over the insufficient internal 
rectus until the image is brought back to the right 
far enough to be directly under the white higher 
image of the left eye. The marking of the prism 



REFRACTION OF THE EYE. 



99 



required in this procedure is the degree of insuffi- 
ciency of the muscle corrected. Now having seen 
that these muscles are normal or are corrected up 
with the prisms, we raise the candle (which has 
hitherto rested level with the plane of the eyes) to 
the upper field, then if the muscles which raise the 
eye are normal (the superior rectus and inferior 
oblique), the images will stay at the same distance 
from each other ; but if one eye lags in this upward 
excursion the images will separate; and as the eye 
that does not follow up will receive the images 
lower down on the retina than the eye which fol- 
lows the candle up more fully; this image will be 
oriented subjectively higher than the other (with 
relation to the distance which they were apart 
when they started). Then the higher image belongs 
to the eye with the affected muscle. Again, we 
know from the first of the chapter, that of these 
two muscles now acting, while they both elevate 
the eye, the superior rectus produces convergence 
and the inferior oblique produces a corresponding 
amount of divergence. We have also learned that 
with convergence, homonymous diplopia results. 
We determine insufficiency of one of these two 
muscles, because the images (relatively) separate in 
the upper field, and if the separated images are 
homonymous we diagnose an over amount of con- 
vergence in the upper field which is produced by the 
adducing action of the superior rectus when not 



100 



A HAND BOOK OF THE 



balanced by the abducting power of the inferior 
oblique. Hence we know the inferior oblique is at 
fault when we have images separating in the upper 
field and homonymous diplopia. Contra, when the 
images separate in the upper field and heterony- 
mous diplopia is produced, we know that diverg- 
ence in the upper field is produced by the inferior 
oblique, when the superior rectus is not counter- 
balancing its diverging power. Hence we know 
that the trouble lies with the superior rectus, which 
is insufficient. 

Now the candle is again placed on a level with 
the eye and lowered into the lower field. If the 
images stay one above the other and at the same 
relative distance, the muscles which turn the eyes 
down (inferior rectus and superior oblique) are 
correct. Again, if the images separate in the 
lower field, one or both of the-e muscles are insuf- 
ficient and the lower image belongs to the lagging 
eye. 

We have already learned that the inferior rec- 
tus does the converging in the lower field, and that 
homonymous diplopia results from convergence. 
Hence, if the separating images are homonymous 
in the lower field, the trouble is from insufficiency 
of the superior oblique. If the images are heter- 
onymous and separating in the lower field, the infe- 
rior rectus is insufficient. In this simple way the 
exact condition of the muscles is easily obtained. 



REFRACTION OF THE EYE. 



101 



As to the correction of the insufficiencies, the 
case where the internal rectus is insufficient is by 
far the one more often demanding correction. 
When this muscle is insufficient for distance the 
insufficiency will be greater when any work de- 
manding much convergence is performed, as read- 
ing, sewing, etc., whereas, a few degrees of insuffi- 
ciency of the external rectus will not be a serious 
anomaly and is even acceptable in the case of the 
myope, who does not make the demands on accom- 
modation to assist convergence that the emmetrope 
does. Unhappily, however, the opposite condi- 
tion of affairs, or insufficiency of the interni is the 
rule with myopia, while the hyperope is more 
often found with insufficiency of the externi. In 
reality the insufficiency of the internal or external 
recti affects each eye equally when it occurs. 

When starting this test with the prism, base 
down over the left eye, the image of the left eye is 
thrown higher than the image of the right eye and 
if the right eye lags, in the upward excursion, the 
image of the right eye first approaches, passes, and 
then separates from the white image of the left eye 
as the candle is lifted higher. This was a virtual 
separation from the start, but the first of the move- 
ment is neutralized by the effect of the 8° prism base 
over the inferior rectus of the left eye. When in 
the lower field the left eye lags, the images first 
approach and then separate, which is virtually a 



102 



A HAND BOOK OF THE 



separation from the start on account of the prism 
8° base down in front of the left eye. 

In prescribing prisms when a considerable 
amount of insufficiency exists, the prism is divided 
and put one half over each eye, base over the 
muscles to be strengthened. In 8° insufficiency of 
the external rectus muscles, 4° is placed in each eye 
base over the external rectus. 

In prescribing prisms for the superior and infe- 
rior rectus muscles it is better to divide the prism. 
In 10° insufficiency of the superior rectus of right 
eye, 5°, base up, is placed over the superior rectus 
of the right eye, and 5°, base down, over the infe- 
rior rectus of left eye. In this way the object of 
one eye is lowered by 5° and the object of the other 
eye raised by 5°, whereupon they meet on a plane 
and the images are fused. The correction of the ob- 
lique muscles, when insufficient, has not proved 
practical. It is advisable to limit the use of 
prisms to insufficiencies. 

In strabismus where an operation will not be 
submitted to, a correction of the ametropia will be 
of great service and is generally considered com- 
plete without the addition of prisms, and in para- 
lytic strabismus nothing could be more absurd than 
the wearing of a prism. 

Paralytic squint may be easily tested without 
the prism in front of the eye by remembering that 
diplopia occurs only when the eye is rotated in a 



REFRACTION OF THE EYE. 



103 



direction demanding the co-operation of the affect- 
ed muscle; that the images will separate when the 
object is moved in the direction of the action of 
the paralyzed muscle and the image of the affected 
eye travels further away from the image of the 
sound eye when the object is so moved. 

The secondary deviation (when the sound eye 
is covered and any object demanding the action of 
the affected muscle is fixed by the affected eye) is 
greater than the primary. 

In testing the superior and inferior rectus 
muscles in the lateral plane, a prism of 20° is 
placed base over one internal rectus muscle and if 
this does not produce diplopia another prism is 
placed base over the other internal rectus muscle 
and these prisms are strengthened until two candles 
are seen. Now if the images stand equally high 
the superior and inferior recti muscles are consid- 
ered in balance. 

The test in the upper and lower fields are, 
however, more reliable. 

Insufficiency of the external rectus muscles is 
termed esophoria. Insufficiency of the internal 
rectus muscles is termed exophoria. Insufficiency 
of the inferior rectus muscles is termed hyperphoria. 
Insufficiency of the superior rectus muscles is 
termed cataphoria. 

Often the superior and inferior recti and the 
superior and inferior oblique muscles are tested as 
herein described, except that the prism used to pro- 



104 



A HAND BOOK OF THE 



duce diplopia is placed base in over the internal 
rectus muscle and lateral images are produced; 
(after having placed in the frames the prism cor- 
recting the insufficiency of the lateral muscles if 
any exists). The images then separate in the upper 
or lower field from the instant either eye starts to 
move more slowly than its congener. 

When the external rectus muscles are insuffi- 
cient a mydriatic should be used in every case of 
refraction, whether the patient is hyperopic or my- 
opic; in hyperopia that we may determine the full 
amount of ametropia (a full correction of which 
will remedy much of the muscular trouble), and in 
myopia that we may avoid over- correction. 

When the internal rectus muscles are insuffi- 
cient, a mydriatic is also imperative, as this insuffi- 
ciency often causes spasm of the accommodation, 
and emmetropic eyes appear myopic, myopic eyes ap- 
pear more myopic, and hyperopia becomes all latent. 

For a patient with insufficiency of the interni: 
if hyperopic, we correct both muscles and ametro- 
pia, or the muscles only; if myopic, a full cor- 
rection is extended for distance and reading, if 
accommodation permits. 

When the external rectus muscles are insuffi- 
cient, hyperopia is always fully corrected and 
myopia only partly corrected for reading, though 
often a full correction for distance is accepted. 

Nothing is more gratifying than the effect of a 
caref ully graduated tenotomy in many of these cases. 



CHAPTER XI. 



Measuring of Frames. 

The importance of measuring frames correctly 
is evident when we consider that by them the lenses 
are held in front of the eye in either a proper or 
an improper position. As an example of the harm 
done by the giving of an incorrect pupillary dis- 
tance, we will instance the case of a myope with a 
pupillary width of two and four-sixteenths who 
has been given a frame carrying a pupillary dis- 
tance of two and three-sixteenths, the lenses are 
thereby decentered with relation to the eye so that 
a prism base out is placed over the external rectus 
muscle, which is already, generally, oversufficient 
for near work in myopia. 

The first thing to be considered in measuring 
the face for a spectacle is the pupillary width. 
When the glass is to be used for near work only, 
it is well to request the patient to look at the cen- 
tral portion of the forehead of the measurer and 
the pupillary distance taken while this fixation is 
maintained. When the glass is to be worn for dis- 
tance only, the patient may be requested to look at 
a distant object over the right or left shoulder of 
the measurer, and the pupillary distance then 
taken, which will generally be two -sixteenths of 
an inch greater than in the foregoing case. 



106 



A HAND BOOK OF THE 



In dealing with an eye with no muscular 
anomaly, many request the patient to fix his view 
as first mentioned in this chapter, and add one- 
sixteenth of an inch to that measure if the frames 
are for constant wear, or one and one-half sixteenths 
of an inch for distance only. In adding one -six- 
teenth of an inch to the pupillary distance, taken 
while the patient fixes the forehead of the meas- 
urer, the mean for general work will be found 
approximately accurate. The exact center of the 
pupil should be carefully sought and measure taken 
with a steel rule, which is divided into sixteenths 
or thirty seconds of an inch. 




The next thing that will invite our attention is 
the height of the bridge. By this is meant the 

distance above or below the pupillary line A B 

(Fig. 16), at which the bridge is wished to rest 
on the patient's nose, or the distance of the line 
c a (Fig. 16). 

The lower we wish the lenses to drop to- 
ward the malar bone, the greater must be the 
height above, of the bridge, and the higher the 



REFRACTION OF THE EYE. 



107 



bridge of the nose the greater is the height above 
of the bridge. When glasses are to be used for 
reading only (the height above of the bridge being 
sufficient to drop the lenses) the patient is thereby 
relieved of the great inconvenience of bending the 
head forward or of elevating the book. In case of 
a very flat nose the bridge may be set below the 
line of the pupils in order to lift the pupillary line 
of the lenses up to the pupillary line of the eyes. 

The position of the crest is to be considered 
and it is evident that when the crest of the bridge 
is thrown back, the lenses must be thrown forward 
and vice versa. In case of long lashes and promi- 
nent eyes, the crest of the bridge must be set back 
far enough to throw the lenses forward sufficiently 
to enable the lashes to pass without interference, 
otherwise the lenses are continually covered by the 
moisture from the cilia, and the patient very great- 
ly annoyed by this interference. 

When the eye is deep set and the cilia short, 
if the bridge were prescribed on plane there would 
be too much space between the cornea and the 
lens. In this case, therefore, the bridge is thrown 
forward and the lenses thus pushed back nearer the 
eye. The width at the base of the bridge (b to g, 
Fig. 16) must be great enough to just allow the 
nose to set between its sides, neither being tight 
nor loose. This is measured by holding the steel 
rule on top of the bridge of the nose and estimat- 



108 



A HAND BOOK OF THE 



ing the width at the base of the nose bridge, where 
the frames are to come. 

By the angle of the crest is meant the angle 
which the crest of the bridge makes with the plane 
of the lenses. The angle of the crest in Fig. 17 

Fig] 7 




<j0° 

shows a nose demanding a crest, at an angle of 45°, 
and this angle will fit a large majority of cases, 
and no other need be prescribed, unless the nose 
protrudes directly forward so as to demand an 
angle of less than 25°, or drops so flat as to require 
an angle of more than 60°; in these extreme cases 
it is well to specify the angle counting from the 
horizontal axis at zero to the perpendicular axis at 
90°. The large variation in the angle of the crest 
is received, happily, by so many differently shaped 



REFRACTION OF THE EYE. 



109 



noses, because of the round under surface of the 
bridge as generally made. 

The width of the temples is measured back 
where the temples will first touch the head, or near- 
ly an inch back from their junction with the 
frames. 

As to the length of the temples, it is only 
when a very long or very short rider is needed that 
any particular instruction is required, as the temple, 
in a riding bow at least, is so easily changed in 
length by a gentle bending. 

The straight temple is to be prescribed where 
the frames are to be often removed, as for reading 
glasses; the riding bows, where more continued 
wear is demanded. A skeleton frame is one where 
the binding does not surround the lens and it gives 
in beauty what it lacks in stability. For those un- 
able to wear eye-glasses it presents an available 
alternative to the plain riding bow. As to the 
style of bridge, the saddle bridge is a general fa- 
vorite and is usually furnished where none is speci- 
fied. The C bridge may be prescribed for straight 
temples and especially for cataract glasses, as it is 
not so easily bent in the folding of the frames. 
The X bridge is of use in some cases, the chief 
indication being a flat nose. 

The angle that the temples make with the 
frames should be determined when fitting the 
glasses to the face of the patient, at which time the 



110 



A HAND BOOK OF THE 



beak of the temple may be gradually filed until a 
proper spread is produced. As to the size of the 
eye, a number one eye is generally used with a 
bound lens, except in cases of bifocal lenses or 
cataract glasses, where a naught eye is preferable. 

In skeleton lenses a naught eye is in general 
use and the long oval is a favorite, especially for 
tall or slender people. The larger sizes, double 
naught, etc., are considered more beautiful, but it 
is seldom that the features permit of their use with- 
out just objection. The small numbers, two and 
three, are not much used of late. 

In measuring for eye-glasses the pupillary dis- 
tance is taken the same as for spectacles. The 
thickness of the nose where the top of the guard 
and where the bottom of the guard will come is 
carefully measured. The average guard may be 
regarded as about ten- sixteenths to twelve -six- 
teenths of an inch long, and in giving the width 
above and below, one should specify whether the 
measure is intended to be when on, or off, the nose. 
When prescribing width for guards when not on, 
two-sixteenths to three -sixteenths of an inch is 
allowed for spring, according to the firmness of the 
nose bridge. These guards will be easily changed 
by a pair of parallel bar forceps at the time of fit- 
ting them to the face, so that they will exactly 
meet the contour of the sides of the nose. The 
spring may be easily loosened or tightened with 



REFRACTION OF THE EYE. 



Ill 



the bending forceps. The bending forceps present 
one lip convex and the other concave, so that when 
shut, these lips form the arc of a small circle. 

Many devices are made to enable those with 
an unfavorable nose, to wear eye-glasses, and one 
must choose as their experience teaches them. 

The skeleton eye-glass again sacrifices stability 
for beauty, but possesses one admirable quality ? 
that is, the holes may be drilled above the center 
of the lenses when to be used only for reading, and 
thus the lenses are lowered, as when in a riding 
bow the bridge is placed high above the pupillary 
line. When the brow is very prominent, the crest 
of the spring is set forward a corresponding dis- 
tance, and when very receding the crest of the 
spring should be set back so as to rest on the bow, 
as the stability of an eye-glass greatly depends on 
this one point. 

Any acceptable shaped spring may be used, 
the round one being in great demand at present. 
Again steel is more preferable for young people 
using eye-glasses, while gold enjoys first rank for 
the old. In riding bows the metal is largely deter- 
mined by the Louis D'Or. Probably the neatest 
combination is the steel skeleton eye-glass, at least 
for those under 40 years of age. Gold and steel 
are easily mended when broken, while aluminum, 
though light, cannot be repaired. This should be 
considered in ordering frames for rough use. 



CHAPTER XII. 



General Remarks with Decentering of Lenses. 

When it becomes necessary to put a prism 
over an eye muscle or a pair of eye muscles, instead 
of grinding a prism in the lens, the lens is often 
decentered so that it produces the prismatic effect 
desired. It bas often been laid down as a rule that 
one maydecenter one-half of one degree to the diop- 
tre, and that the ratio increases as the dioptres, 
provided that the lens is kept small. The smaller 
the lens is in contour the greater is the prismatic 
effect that can be produced by decentering it. 

Annexed is a table used by some of our most 
reliable opticians, which differs very slightly from 
Maddox' rule. 

Table showing number of millimeters to de- 
center spherical lens in order to add a prism of 
from £° to 2°: 



REFRACTION OF THE EYE. 



113 



DIOPTRES 


i ° 

¥ 


1 o 


3° 


1° 


2° 


0.25 












0.50 


4.6 










0.75 


3. 










1.00 


2.3 


4.6 








1.25 


1.8 


3.7 


5.5 






1.50 


1.5 


3.1 


4.5 






1.75 


1.3 


2.6 


3.9 


5.3 




2.00 


1.1 


2.3 


3.4 


4.6 




2.25 


1. 


2.0 


3.0 


4.1 




2.50 


.9 


1.8 


2.7 


3.7 




2.75 


.8 


1.7 


2.5 


3.4 




3.00 


.75 


1.5 


2.3 


3.1 




3.25 


.7 


1.4 


2.1 


2.8 


5.7 


3.50 


.65 


1.3 


1.9 


2.6 


5.3 


4.00 


.6 


1.15 


1.7 


2.3 


4.6 


4.50 


.5 


1. 


1.5 


2.1 


4.1 


5.00 


.45 


.9 


1.3 


1.8 


3.7 


5.50 


.4 


.8 


1.2 


1.7 


3.4 


6.00 


.35 


.7 


1.1 


1.5 


3.1 



114 



A HAND BOOK OF THE 



In giving a pupillary distance of too great or 
too small an amount, the lenses are unintentionally 
decentered. Take a myope, given too narrow a 
pupillary distance, where the rays of light entering 
the eye must pass externally to the principal axis 
of the lens and the lenses act as prisms base out or 
over the external recti, (which muscles are generally, 
relatively too strong in myopia to allow of free 
convergence), here the trouble caused by the prisms 
is more than the benefit derived by the correction 
of the ametropia. 

If, however, the error was made in the oppo- 
site direction, and the pupillary distance given was 
a little too great in myopia, so the prisms would be 
set base in, convergence would be rendered easier 
and the greatest satisfaction possible would be ob- 
tained. In hyperopia, where the internal recti are 
generally overstrong, the error is less serious if too 
narrow a pupillary distance is given and the exter- 
nal recti strengthened; yet it is better exact, un- 
less insufficiencies require correction. 

In reasoning these results one has only to re- 
member that in a minus glass the apex of the prism 
is in the center of the lens, and in a plus glass it is 
at the periphery. 

In prescribing prisms, if one has a reliable 
optician, it is preferable to state the amount of 
prism required and in what direction the base 
should be placed and let the optician grind it in 
with the lens according to his own rule. 



REFRACTION OF THE EYE. 



115 



In considering prisms there is a question, 
which is an important one, and that is that the 
prisms now ground by all the large factories in this 
country are ground on the prism dioptre scale, 
which is based on the rule that a one dioptre lens 
should be decentered ten millimeters in order to 
produce the effect of a 1° prism. 

A two -dioptre lens would require a decenter- 
ing of five millimeters for each degree of prism, 
and so on. 

Now the American trial cases in the market 
contain prisms based on this system, while the for- 
eign trial cases are made on the old plan. 

Physicians ought to take some action on this 
matter so that their set of prisms should be replaced 
with others based on the prism dioptre system, and 
a universal rule of decentering, in accordance with 
it, be adopted by all opticians when filling pre- 
scriptions. 

As to the preferable mydriatic to use in general 
refraction the one already referred to, (one grain 
each of cocaine hydrochlorate and homatropine 
hydrobromate to the dram of water,) is serviceable 
and efficient in nearly all cases of refraction, but 
when it is determined from the use of other instru- 
ments of precision that the ametropia is yet un- 
masked, sulphate of atropine (four grains to the 
ounce) dropped in the eye three times a day for 



116 



A HAND BOOK OF THE 



three or four days will prove satisfactory and cer- 
tain in its results. A tablet of cocaine and homa- 
tropine has been placed on the market in the form 
of a refraction disc. It has not as sure an action as 
the homatropine solution, but will keep longer, and 
claims attention from those doing a limited amount 
of refraction, in whose hands the solution would 
spoil before being used. The solution will keep 
well, forty days at a temperature of 65° Fahrenheit. 

The visual acuity for distance is not changed 
by instillation of a mydriatic in emmetropia or 
myopia, but is lowered in hypermetropia and astig- 
matism. Visual acuity is often increased to normal, 
in spasm of accommodation, during the use of 
atropine, when, if hyperopia be present, it will 
decrease proportionately to the amount of the 
hyperopia. 

By anisometropia is meant a difference in the 
refraction of the eyes to such an extent as to give 
perceptibly different sized images. 

Anti-metropia is an anisometropia where one 
eye is hyperopic and the other myopic. 

Congenital anisometropia is often of high de- 
gree, six to ten dioptres, and is due to faulty devel 
opment of one or both eyes. It often occurs with 
imperfect development of one side of the head. 

Vision is binocular in proportion as the ani- 
sometropia is not of too high a degree. Vision may 
be accomplished by the better eye, the poorer one 



REFRACTION OF THE EYE. 



117 



rapidly becoming amblyopic. Some few use each 
eye alternately. 

In considering anisometropes, the prism may 
be used to see if both eyes participate in vision. 
The hyperopic anisometrope cannot use the accom- 
modation in one eye more than in the other, hence 
he is unable to correct the difference of hyperopia 
in the two eyes ; neither, when accommodating does 
he get the same idea of distance from the conver- 
gence produced, though it be equal to the emme- 
trope's convergence. This would tend to show that 
convergence is not the only factor in the judging of 
distances. In correcting anisometropia, when the 
eyes are used alternately or when one eye is used 
more than the other, the better eye is to be correc- 
ted as the requirements of work and ametropia de- 
mand, and the same correction placed over the 
other eye, so that their relative refraction is not 
changed. 

As an exception to this rule may be cited the 
case that is emmetropic in one eye and myopic in 
the other. To this person a correction is made of 
the myopic eye and a plain glass given to the em- 
metropic eye. When one eye is myopic and the 
other hyperopic, the myopic eye may be corrected 
and the hyperopic eye given a plain glass. This 
will evidently be accepted for distance and may or 
may not be used for near work. The inclination 
might be to correct the hyperopia of the one eye or 



118 



A HAND BOOK OF THE 



to give the emmetropic eye in the first instance a 
plus 3.00 dioptres glass for reading, to adapt it to 
the same near point as its myopic partner, but un- 
happy results of such a correction are certain to 
follow, provided both eyes participate in the vis- 
ual act. When only one eye is used, or has vision, 
it is the only one inviting attention. 

Many of these cases of anisometropia admit of 
no rule and must be given the correction indicated 
by experiment. 

We often have trouble with the accommodation 
not due to the diminution, in the possible amount 
of dynamic refraction, that can be exercised at any 
given time. From accident, a rupture of the zonula 
may result, after which the lens assumes its great- 
est convexity and remains always the same, pro- 
ducing a constant blurring, of all distant objects, 
seen by the affected eye. 

Again anything which affects the working of 
the ciliary muscles, affects the usefulness of vision. 
Paresis of the accommodation results from such 
causes as contusions of the eye, and has resulted 
from the operation of tenotomy, and even the forci- 
ble dilation of a lachrymal structure. Generally 
the pupil changes with the interference of accom- 
modation, and in paralysis of the accommodation 
the amplitude of convergence is lessened. 

The principal symptoms of paresis, or paralysis 
are, recession of the punctum proximum, and 



REFRACTION OF THE EYE. 



119 



micropsia. Outside of contusion and injuries, 
paresis or paralysis occurs as a precursor of sympa- 
thetic ophthalmia, and they are often concurrent 
with syphilis, diabetes, severe affections of the 
central nervous system, (as Tabes Dorsalis in 
which the Argyle Robinson pupil is noticed), and 
poisoning from putrid meats, especially imperfectly 
salted pork, and fish, as well as from the use 
of cocaine, atropine, etc. The accommodation 
weakens after exhaustive diseases, profuse hemor- 
rhages or any great excesses. Clonic spasm of accom- 
modation occurs during fixation and convergence, 
and yields when an ophthalmoscopic examination 
is being made. 

It is not a pathologic process, though if volun- 
tarily kept up for a long time will give much 
annoyance, as testifies any person during his first 
month of energetic work with the microscope. 

Tonic spasm may be called pathologic spasm 
and may affect one or both eyes or one eye more 
than the other. The principal symptoms are, ap- 
proximation of the punctum remotum, micropsia, 
and the reduction of the amplitude of accommoda- 
tion (which is a common symptom with paresis or 
paralysis). The patient complains of pain over 
the eyes and temples, and of a feeling of constric- 
tion through the forehead. The pupil is generally 
small, as compared with the pupil of paresis. 

When the visual acuity is improved by the 



120 



A HAND BOOK OF THE 



milder mydriatics it strongly indicates spasm. 
Conjunctivitis, keratitis, sympathetic ophthalmia 
and many other inflammations of the eye, reflexly 
stimulate tonic spasm of the accommodation. It 
also occurs with hyperesthesia of the retina and 
with insufficiency of the internal rectus muscles. 

Meningitis and other acute diseases of the 
brain cause convergence and spasm of accommoda- 
tion, with contraction of the pupil. Among the 
drugs that produce spasm, eserine the alkaloid of 
calabar bean demands first attention. From a 
one-fourth of one per cent., to a one per cent., solu- 
tion is generally used, and the sulphate is the pre- 
ferable salt. 

Hydrochlorate of pilocarpine also may be 
used, but is not as powerful as the former and does 
not alter the static refraction of the eye. Among 
drugs that produce paralysis of the accommodation, 
sulphate of atropine is the most powerful, four 
grains to the ounce being sufficient to paralyze the 
accommodation completely. It is generally in- 
stilled three or four times a day for four or five 
days, when spasm is to be reduced. Homatropine 
and cocaine both act to paralyze the accommoda- 
tion and have been previously referred to, and when 
properly used in combination, they will very sel- 
dom be found insufficient to suspend accommoda- 
tion, even in the most troublesome cases. 



REFRACTION OF THE EYE. 



121 



The following cases are selected from the 
clinical patients presented at the University clinic 
and are of such a variety as to illustrate the more 
important principles in prescribing lenses. 

The cases here recorded were at the time of 
refraction living near the hospital, and in each case 
they were requested to report when the result 
obtained was not satisfactory. When no report 
was returned it was assumed that relief was 
obtained, and where return was made it was duly 
noted. The strength of the prism overcome by the 
external rectus muscles was taken before the use of 
the mydriatic and while the patient was fixing at 
about the near point. 

CASES. 

Case 1. — Anna B., aet 35. Occupation, Teacher. Am- 
plitude of accommodation, 6 D. Punctum Proximum, 240 
m. m. Muscles normal. Internal recti overcame 30° prism, 
base out. 
Symptoms — 

Inability to do near work, headache after trying. 
Refraction — 

O. D. + 2.00 D. sph. 

O. S. + 2.00 D. sph. 
Treatment — 

O. D. + 1.50 D. sph. 

O. S. + 1-50 D. sph. 

To be worn constantly. Patient requested to return if 
not satisfied. 
Result — 

No report ever made. 



Case 2. — Jane C, aet 8. Occupation, School girl. Am- 
plitude of accommodation, 14 D. Punctum Proximum, 72 



122 



A HAND BOOK OP THE 



m. m. Muscles. Insufficiency of external recti of 40°. 
Had been troubled with periodic squint. Internal recti over- 
came 60° prism, base out. 
Symptoms — 

Constant headache. 
Refraction — 

O. D. + 3.75 D sph. 

0. S. + 4.00 D. sph. 
Treatment — 

Patient given + 1.50 each and gradually increased to 
full correction. To be worn constantly. 
Result — 

Headache was relieved when abont one-half the correction 
was worn. 



Case 3. — John L., aet 18. Occupation, Student. Am- 
plitude of accommodation, 11 D. Punctum Proximum, 91 
m. m. Muscles normal. Internal recti overcame 35° prism, 
base out. 
Symptoms — 

Slight headache after near work. 
Refraction — 

O. D. + 1.25 D. sph. 

O. S. + 1.25 D. sph. 
Treatment — 

O. D. + 75. 

O. S. + 75. 

To be used for near work only. 
Result — 

Headaches ceased. 



Case 4. — Arthur B., aet 21. Occupation, Student. Am- 
plitude of accommodation, 10 D. Punctum Proximum, 100 
m. m. Muscles. Insufficiency of internal recti of 4°. In- 
ternal recti overcame 30° prism, base out. 
Symptoms — 

Headache after study. 
Refraction — 

O. D. + 1.00 D. sph. 

O. S. + 1.00 D. sph. 



REFRACTION OF THE EYE. 



123 



Patient was wearing -f- 1.00 in both eyes for near work. 
Treatment — 

Given 4° prism base in for near work only, and exercise 
for internal recti. 
Result — 

Patient complained some, but was better able to study. 



Case 5. — Minnie B., aet 14. Occupation, Servant girl. 
Amplitude of accommodation, 11 D. Punctum Proximum 97 
m. m. Muscles normal. Internal recti overcame 35° prism, 
base out. 
Symptoms — 

Was often dizzy and had suffered from headache, (after 
near work), which of late was nearly constant. 
Refraction — 

O. D. + 3.00 D. sph. 

O. S. + 10.00 D. sph. 
Treatment — 

Was given + 2.00 in both eyes and these lenses were in- 
creased to -(- 3.00 in both eyes, after two weeks. 
Result — 

Good. 



Case 6. — Lydia Ma, aet 30. Occupation, Housewife. 
Amplitude of accommodation, 6 D. Punctum Proximum, 110 
m. m. Muscles normal. Internal recti overcame 35° prism, 
base out. 
Symptoms — 

Indistinct distant vision, could read without any trouble. 
Refraction — 

O. D. — 3.00 D. sph. 

O. S.— 3.00 D. sph. 
Treatment — 

Given — 3.00 both eyes for distance only. 
Result — 

Good. 



Case 7.— Kate E., aet 18. Occupation, Student. Am- 
plitude of accommodation, 10. D. Punctum Proximum, 70 



124 



A HAND BOOK OF THE 



m. m. Muscles 10° insufficiency of the internal recti. In- 
ternal recti overcame 25° prism, base out. 
Symptoms — 

Severe headache continually, but worse after studying. 
Refraction — 

O. D.— 4.00 D. sph. 

O. S.— 4.00 D. sph. 
Treatment — 

Given — 4.00 in both eyes for constant wear. 
Result — 

Patient reported to say that all trouble was relieved. 



Case 8. — Charles T., aet 30. Occupation, Student. Am- 
plitude of accommodation, 1.50 D. Punctum Proximum, 200 
m. m. Muscles — 8° insufficiency of the internal recti. 
Internal recti overcame 25° of prism, base out. 
Symptoms — 

Indistinct distant vision and inability to read, severe 
headache after any near work. 
Refraction — 

O. D. — 4.00 D. sph. 

O. S.— 4.00 D. sph. 
Treatment — 

Patient was wearing — 4.00 for both eyes which were 
prescribed for distance and reading, was told to continue wear- 
ing — 4.00 for distance and given — 1.00 for reading com- 
bined with 3^° prism, base in for reading. 
Result — 

Good. Some trouble after prolonged near work. 



Case 9. — Chester B., aet 23. Occupation, student. Am- 
plitude of accommodation, 10 D. Punctum Proximum, 100 
m. m. Muscles 20°, insufficiency of internal recti. Internal 
recti overcame 8° prism, base out. 
Symptoms — 

Very severe headache after near work. 
Refraction — 

O. D.— . 25 3 — . 50 cyl., axis 120°. 

O. S. — . 50 cyl. axis 15°. 

Treatment — 



REFRACTION OF THE EYE. 125 

Full correction given and complete tenotomy of external 
rectus of left eye. 
Result — 

Over correction of 1°, patient could study well and long, 
and reported entire relief from trouble. 



Case 10. — Raymond P., 13. Occupation, Student. Am- 
plitude of accommodation, 13 D. Punctum Proximum, 77 
m. m. Muscles normal. Internal recti overcame 35° prism, 
base out. 
Symptoms — 

Had experienced headache after near work and could not 
shoot well. 
Refraction — 

O. D.— 3.00 D sph. 

O. S. Emmetropic. 
Treatment — 

O. D.— 3.00 D. sph. 

O. S. — Plain glass. 

To be worn for distance only. 
Reesult — 

Patient returned to say that no more trouble was experi- 
enced. 



Case 11.— Edgar C, aet S?4. Occupation, Student. 
Amplitude of accommodation, 5 D. Punctum Proximum, 71 
ra. in. Muscles normal. Internal recti overcame 30° prism, 
base out. 
Symptoms — 

Pain when trying to study. Indistinct vision except for 
near objects. 
Refract iom — 

O. D.— 8.00 D. sph. 

O. S.— 8.00 D. sph. 
Treatment — 

Given — 4.00 D. in both eyes, which was gradually in- 
creased to the lull correction. Given — 4.00 D. in both eyes 
for reading. 
Result — 

Good. 



126 



A HAND BOOK OF THE 



Case 12. — Mrs. S., aet 45. Occupation, Housewife 
Amplitude of accommodation not taken. Punctum Proximum 
not taken. Muscles normal. 
Symptoms — 

Patient could with difficulty do any house-work. 
Refraction — 

O. D. — 15.00 D. 

O. S. — Extreme Amblyopia. Myopia not measurable 
by subjective method. 
Treatment — 

Given — 6.00 in both eyes and increased till — 11.00 
was reached. A further increase not accepted. 
Result — 

Patient could do ordinary work. 



Case 13. — Marion L., aet 23. Occupation, Elocutionist. 
Amplitude of accommodation, 5 D. Punctum Proximum, 65 
m. m. Muscles normal. Internal recti overcame 30° of a 
prism, base out. 
Symptoms — 

Patient complained of headache when exposed to a bright 
light. 

Refraction — 

O. D. — 10.00 D. 

O. S. — 10.00 D. 
Treatment — 

Patient could use — 4.00 in both eyes for near work with 
poor illumination, but could not wear stronger lenses without 
continued and severe headache. Was given a lorgnette with 
— 10.00 D. in both eyes for distance. 



Case 14. — Nora B., aet 28. Occupation, Housewife. 
Amplitude of accommodation, 6 D. Punctum Proximnra, 160 
m. m. Muscles normal. Internal recti 40° of prism, base 
out. 

Symptoms — 

Constant headache becoming unbearable after near work, 
Pain worse in back of head. 



REFRACTION OF THE EYE. 



127 



Refraction — 

O. D. + 1.50 sph. 3 — 2.25 cyl. axis 180°. 

O. S. + .50 cyl. axis 180°. 
Treatment — 

Full correction given in each eye. 
Results — 

Headache ceased immediately after usirig correction. 



Case 15. — Nelson G., aet 28. Occupation, Student. 
Amplitude of accommodation, 7 D. Punctum Proximum, 130 
m. m. Muscles normal. Internal recti overcame 30° prism, 
base out. 
Symptoms — 

Some pain after continued near work. 
Refraction — 

— 1.00 3 — 1.00 cyl. axis 180°. 

— L.00 3 — .75 cyl. axis 180°. 
Treatment — 

Full correction given. 
Result — 

Good. 

Case 16. — Moses B., aet 13. Occupation, School Boy. 
Amplitude of accommodation, 12 D. Punctum Proximum, 150 
m. m. Muscles normal. Internal recti overcame 45° prism, 
base out. 
Symptoms — 

Headache, unable to study lessons, was considered dulL 
Refraction — 

O. D. + 4.00 3 + 3.00 D. cyl. axis 90°. 

O. S. + 3.50 3 + 4.00 D. cyl. axis 90°. 
Treatment — 

Patient was first given + 2.00 both, which was gradually 
increased until final correction. 

O. D. + 2.00 3 + 3.00 D. cyl. axis 90°. 

O. S. + 2.00 3 + 4.00 D. cyl. axis 90°. 
Result — 

Child suffered no more and was soon bright in classes. 



Case 17. — James K., aet 20. Occupation, Farm hand. 
Amplitude of accommodation, 10 D. Punctum Proximum, 150 



128 



A HAND BOOK OF THE 



m. m. Muscles normal. Internal recti overcame 40° prism, 
base in. 
Symptoms — 

Had no headache but always knew he could not see as 
well as others. 
Refraction — 

O. D. + 4.00 cyl. axis 90°. 

O. S. -f 5.00 cyl. axis 90°. 
Treatment — 

Was given full correction. 
Results — 

Did not return. 



INDEX. 



Abduction, 94. 
Accommodation — 

amplitude of, 32, 33, 59. 

centres of, 20, 21. 

effect of, 29. 

formula for obtaining, 29, 33, 
59. 

in hypei-opia, 33, 34. 

in myopia, 34, 59. 

mechanism of, 19, 30, 31. 

paresis, 118, 119. 

paralysis of, 118, 119. 

range of, 32, 33. 

region of, 33. 

spasm of, 32. 

table of range, 73. 

test for, 31, 32. 
Acuity — 

visual, 26, 27, 28, 29, 30, 31, 32. 
Adduction, 94. 
Ametropia, 25, 26, 30. 
Angle — 

alpha, 39. 

of convergence, 35. 

gamma, 39. 

of prisms, 8. 

of incidence, 9. 

of refraction, 9, 12. 

the metre, 35. 
Anisometropia, 116, 117, 118. 
Astigmatism — 

after operations, 65. 

against the rule, 65. 

axis of best vision in, 66, 67. 

compound hyperopic, 64. 

compound myopic, 64. 

correction of, 68, 69, 70. 

definition of, 21, 64. 

direction for correcting, 68, 69. 



estimation of, 68, 69, 70. 

by ophthalmoscope, 87, 88, 
89, 90, 91, 92. 

by skiascope, 80, 81. 

by lenses, 68, 69. 
irregular, 65, 71, 91. 
irregular axes, 65. 
lenticular, 71. 
mixed, 69. 

principal meridians of, 05. 

regular, 65. 

simple hyperopic, 64. 

simple myopic, 64. 

vision in, 65, 66. 

with the rule, 65. 
Atropine— 

use of, 115. 

in spasm, 48. 
Axes— 

of astigmatism, 05. 

irregular, 65. 

of lens, 9. 

secondary of lens, 9, 10. 
visual, 36, 38. 
optic, 36, 38. 
Aqueous, 22, 23. 

Cases, 122, 123. 124, 125, 126, 

127, 128. 
Centrad — 

Dennett's, 38. 
Choroid, 31. 
Ciliary Muscle — 

action of, 19. 

Cocaine— 

discs of, 115. 
how used, 115. 

in combination with homatro- 
pine, 115. 



ii 



INDEX. 



Convergence, 20, 21. 
amplitude, 35. 
angle, 35, 36. 
centrads, 20. 

metre-angle in, 35, 36, 37. 

prisms in, 37. 

range of, 35. 

test of. 
Cornea, 22, 23, 65. 
Crystalline Lens, 19. 

normal, 19. 

during accommodation, 19, 20. 
Decentering op Lenses— 

manner of, 112. 

table for, 113. 

use of, 114, 115. 
Disc- 
in astigmatism, 90, 91. 

in hyperopia, 88, 89. 

in myopia, 56, 88, 89. 
Emmetropic Eye, 18, 19, 26, 29, 
31, 33. 

determination of in refrac- 
tion. 

principal axis of, 24. 

secondary axes of, 9, 10, 24. 
Eserine — 

use of, 120. 
Eyes— 

associate movements of, 20. 
size of, 110. 

glasses, measurement of, 110, 
111. 

Focus — 

conjugate foci, 23, 24. 

of a concave lens, 15. 

of a convex lens, 15, 16, 17. 

principal, 9, 10, 41. 
Fovea Centralis, 87. 
Frames— 

angle of crest, 108, 109. 

angle of temple, 109, 110. 

height of bridge, 106. 

metals employed in making, 

position of crest, 106. 



pupillary distance, 105, 106. 
styles of bridge, 109. 
temples, 109. 

Fundus, 85, 86, 87, 91, 92. 
Heterophorias— 

see Chapter X. 
Hypermetropia — 
absolute, 28, 45. 
correction of, 42, 43, 44, 45. 
determination of, 47, 17, 48, 
50, 51. 

direction for prescribing, 
50, 17, 44, 47, 49. 

lenses for, 47, 17. 
definition of, 41. 
faculative, 44. 
latent, 43. 
manifest, 43. 
measurement of, 

with lenses, 47, 17. 
relative, 44. 
symptoms of, 45. 
total, 43. 

with insufficiency of the in- 

terni, 44. 
with insufficiency of the ex- 
terni, 44. 
Images — 
size of, 10. 

formation of, 14, 10, 24, 9. 
of a convex lens, 17. 
Insufficiency, 96. 

of external rectus, 97, 98, 99, 

100, 103, 104. 
of internal rectus, 97, 98, 99, 

100, 103, 104. 
of superior rectus, 97 to 104. 
of inferior rectus, 97 to 104. 
of inferior oblique, 97 to 104. 
of superior oblique, 97 to 104. 
Lenses — 

bi-concave, 16. 
bi-convex, 16. 
concave, 16. 
convex, 16. 

crystalline, 19, 22, 23, 31. 
crown glass, 7. 



INDEX. 



iii 



focal distance of. 

effect of placing at some dis- 
tance from the eye, 48, 
49. 

focus of concave, 
focus of convex, 17, 15, 10, 
12, 13. 

focal distance of, 12, 11, 13. 
formation of images by, 14. 
plano-concave, 16. 
plano-convex, 16. 
principal axis, 12, 24. 
measuring of, 11, 12. 
secondary axis of, 13. 
skeleton, 110. 
sphero-cylinder, 69. 
Stokes', 69. 

strength of, 11, 12, 69. 
Light — 

rays of, 7, 8, 9, 11, 13. 

refraction of, 7, 8, 9, 10, 11, 22. 
Macula Lutea, 20, 87. 
Malingers, 13. 
Meniscus, 11. 

Meter-Angle, 35, 39, 37, 61. 

sine of, 38. 

value of, 38. 
Micropsia, 119. 
Mirror— 

concave, 81. 
Mydriatics— 

choice of, 46, 115. 

disc, 115. 

in refraction, 43, 47, 104, 115. 
in spasm, 46. 

use demanded in, 46, 51, 61, 
104. 

ametropia with heterophoria, 
104. 
Myopia — 

accommodation in, 53. 
arrest of, 58, 57. 
causes of, 56, 57. 
choroidal crescent in, 88. 
convergence in, 53, 59. 
determination of, 17. 



definition of, 52, 54. 
measurement of, 17. 

by lenses, 59, 60, 61, 62, 63. 

by ophthalmoscope, 87. 

by skiascope, 78. 79. 
prescribing for, 17, 57, 58, 59, 

60, 61, 62, 63. 
progressive, 55. 
prophylaxis of, 57, 58. 
pseudo-myophia, 58. 
removal of crystalline lens for, ! 

58. 

stenopaic slit used in, 53. 
symptoms, 28, 54, 55, 55. 
treatment of, 57, 58, 59, 60, 

61, 62, 63. 

Muscles— 

anatomy of extrinsic, 93, 94, 95. 
action of, 93, 94, 95. 
insufficiencies of, 21, 101. 
ciliary, 31. 

diagnosis of insufficiency of, 

internal rectus, 97, 98. 

external rectus, 97, 98. 

superior rectus, 100, 103, 104. 

inferior rectus, 100, 103, 104. 

superior oblique, 99, 100, 104. 

inferior oblique, 99, 100, 104. 
extrinsic, 13. 

treatment of insufficiencies, 
102. 

Ophthalmoscope — 

astigmatism measured by, 89, 
90, 91. 

determination of refraction 
by, 88, 89, 90, 91, 92. 

direct method of examination 
with, 84. 

hyperopia, how measured by, 
88. 

indirect method of examina- 
tion with, 84, 85. 
light to use with, 84. 
mirror in the, 84. 
myopia, how measured by, 89. 
picture in emmetropiawith,85. 
picture in hyperopia with, 87. 
picture in myopia with, 87. 



iv 



INDEX. 



PlLOCARHINB HYDROCHLORATE, 
120. 

Presbyopia — 

cause of, 72. 
correction of, 73. 
determination of amount, 75. 
definition of, 72. 
in hyperopes, 73. 
in myopes, 73. 

method of determining, 28, 74. 

rule for correcting, 75. 

table of accommodation rela- 
tive to, 73. 
Prism Dioptre, 38. 
Prisms— 

definition of, 12, 13. 

Dennett's method of number- 
ing, 38. 

effect of, 13, 15. 

new method of numbering, 37. 

old method of numbering, 37. 

refracting angle, 8, 9, 12. 

PUNCTUM PROXIMUM — 

of emmetrope, 31. 
of hyperope, 38. 
of myope, 28. 
of presbyope, 72. 

PUNCTUM REMOTUM— 

of convergence, 36. 



of emmetrope, 31. 
of hyperope, 41. 
of myope, 53. 

Skiascopy— 

concave mirror in, 81. 

plain mirror in, 81. 

theory of, 77, 78. 

test in astigmatism by, 80, 81. 

test in hyperopia by, 80. 

test in myopia by, 78, 79. 

use of, 77. 

Second Sight, 53. 

Spherical Aberration, 17. 

Strabismus, 96, 22. 

Tenotomy, 104. 

Vitreous, 22, 23. 

Visual Acuity — 

angle of, 26. 
change of, 116. 
in hyperopia, 45. 
in myopia, 116. 
in spasm, 116. 
measure of, 26. 

pin-hole disc test in poor, 28, 29. 
report made of, 28. 
Snellen's card test for testing,. 
27.