ERRORS Ol
ACCOMMODATION
AND REFRACTION
CLARKE
r\
i:
BttKEllY
LIBRARY
UHivERSfTY or
CALIFOftNIA
nu>M
1
THE LIBRARY
OF
THE UNIVERSITY
OF CALIFORNIA
THE ERRORS OF ACCOMMODATION AND
REFRACTION OF THE EYE
THE ERRORS OF
ACCOMMODATION AND
REFRACTION OF THE EYE
AND THEIR TREATMENT
A HANDBOOK FOR STUDENTS
BY
ERNEST CLARKE, M.D., F.R C.S.
OPHTHALMIC SURGEON TO THE KING GEORGE HOSPITAL, QUEBN ALEXANDRA
HOSPITAL FOR OFFICERS, ETC.
CONSULTING SURGEON TO THE CENTRAL LONDON OPHTHALMIC HOSPITAL
CONSULTING OPHTHALMIC SURGEON TO THE MILLER GENERAL HOSPITAL
FOURTH EDITION
NEW YORK
WILLIAM WOOD & COMPANY
MDCCCCXVIII
OPTOMETRY
Printed in Great Britain
Sb--# V**"
orro
PREFACE TO THE FOURTH EDITION
The first edition of this work was published fifteen years
ago based on lectures delivered at the Central London
Ophthalmic Hospital and the Medical Graduates' College .
I have not altered its character, which is essentially
practical, all matter unnecessary for the busy practi-
tioner or overburdened student being omitted.
The whole work has been thoroughly revised and
brought up to date, many chapters having been re-
written.
The subject of Eyestrain still occupies the prominent
place that is its due.
ERNEST CLARKE.
Chandos Street,
Cavendish Square, W.
February, 191 8.
CONTENTS
CHAPTER PAGE
I. REFRACTION PRISMS LENSES - - - I
II. OPTICAL PROPERTIES OF THE NORMAL EYE - "19
III. ACCOMMODATION - - - - - 29
IV. CONVERGENCE - - - - "39
V. THE OPHTHALMOSCOPE - - - "57
VI. HYPEROPIA - - - - - - 85
VII. MYOPIA - - - - - - 99
VIII. ASTIGMATISM - - - - - "115
IX. PRESBYOPIA - - - - - -142
X. ANISOMETROPIA _ - - - - 152
XI. APHAKIA _----- 159
XII. EYESTRAIN - - - - - -1 62
XIII. HETEROPHORIA - - - " "I?!
XrV. STRABISMUS ------ 184
XV. CYCLOPLEGIA, CYCLOPLEGICS, AND CILIARY SPASM - I98
XVI. METHODS OF EXAMINATION NOTE-TAKING - - 202
XVII. SPECTACLES ------ 208
XVIII. ILLUSTRATIVE CASES - - - - -2I3
XIX. VISION TESTS FOR THE SERVICES _ - - 221
BIBLIOGRAPHY - - - - - 23O
INDEX ------ 232
VU
ERRORS OF ACCOMMODATION
AND
REFRACTION OF THE EYE
CHAPTER I
REFRACTION— PRISMS— LENSES
Light is propagated in straight lines which diverge
from any luminous point in all directions, and these lines
of direction are called " luminous rays." The propaga-
tion is produced by ether waves which are across the
path of light. The velocity of light is, in round numbers,
186,000 miles per second, and is appreciably retarded
in passing through a denser medium. Rays of light
entering the eye coming from any luminous point at a
greater distance than 6 metres may be assumed, for all
practical purposes, to be parallel.
Light is absorbed, refracted, or reflected.
Refraction of Light. — A ray of light passing from a
rarer into a denser transparent medium, if it be perpen-
dicular to the surface, and the boundaries of the medium
be parallel, will pass out of the denser medium in the
same straight line (Fig. i, l), the only effect upon it
being a retardation. If the ray enter the denser medium
other than perpendicularly, or if the boundaries of the
medium be not parallel, the ray is bent or refracted.
A simple illustration will explain this.
Explanation of Refraction. — i\s ether waves are at
right angles to the path of light, if a beam of light enter
2 TH£ RtFRACTION OF TH£ EYE
a denser medium obliquely, one end of the wave will
enter the denser meditim before the other, and conse-
quently be retarded earlier. Let A, B, c, d (Fig. i), be a
denser medium, with parallel boundaries, and n, o, p, q,
the beam of light.
The wave front will reach Q before it reaches R, and
it will at once be retarded ; and as it thus travels more
slowly from Q to s than from p to R (which is outside the
denser medium), the beam must be swung round so that
it is bent or refracted on entering the denser medium.
Fig. I
Across the denser medium the whole wave front is
equally affected, so that the beam passes across in a
straight line ; and if the sides of the medium be parallel
the converse happens, and it is again bent on passing
out, the incident and the emergent rays being parallel.
Let this be applied to the case of, for example, a prism
where the sides of the denser medium are not parallel.
Suppose A, B, c (Fig. 2), to be a triangular strip of velvet
pasted on a smooth board, and suppose d to be two
small wheels connected by an axle in such a way that
each wheel can turn independently of the other. Roll
REFRACTION 3
the wheels up to the velvet triangle; the lower or right
wheel will pass on to the velvet at e before the left wheel
reaches it, and as the velvet will retard its progress, it
will turn now more slowly than the left wheel, so that the
pair of wheels will be slewed round towards the base of
the triangle. When the left wheel enters on the velvet at
/, its progress will be the same as that of the other wheel,
and the pair of wheels will cross the velvet now in a
straight line ; when it reaches w, the left or upper wheel
will leave the velvet earlier, and will consequently travel
more rapidly, and will again swing the pair round, so
*^ '/ T----:::7
Fig. 2.
that in the transit across the triangle the pair of wheels
have been bent towards the base.
It is in this manner that light behaves; in passing
through a prism, it is bent or refracted towards the
base. *W|
A ray of light passing obliquely from a less dense
into a denser transparent medium is refracted or bent
towards the perpendicular, and when passing from a
dense to a less dense medium is refracted away from the
perpendicular.
Index of Refraction. — The index of refraction of a
transparent substance is the number that denotes the
4 THE REFRACTION OF THE EYE
refractive power of such substance compared with air,
which is taken as the unit i. In Fig. 3, let A c be the
incident ray meeting the horizontal surface of water at
c, and forming with p p', the perpendicular, an angle
A c P ; and let c b be the refracted ray in water bent
towards the perpendicular, and forming the angle B c p'.
v^
/O
~~""'^*^^
c
W
^
k
Fig. 3.
The sine l m is to the sine n o as 4 to 3, expressed
as I, or 1*33, and this is the index of refraction of
water.
The following are a few of the indices of refraction
useful to the ophthalmologist :
Air . .
I'O
Water
1*33
Cornea
1*33
Aqueous humour
1-3379
Vitreous humour
1-3379
Crystalline lens
1-4
Crown glass
1-5
PRISMS 5
Prisms. — An ophthalmic prism is a wedge-shaped piece
of glass having two of its sides, or plane surfaces, inter-
secting each other at the apex, and separated at the base,
which is the thickest part of the prism.
We have already seen that a ray of light entering one
of the sides of a prism is refracted or bent towards the
base, and the amount of this refraction depends upon —
I. The strength of the prism.
|2. The refractive index of the prism substance.
3. The position at which the light enters the prism.
The Strength of the Prism — The Numbering of
Prisms. — ^The power of a prism to deflect or refract light
depends on the size of the angle at the apex formed by
Fig.
the two plane surfaces. This is called the refracting
angle, and is written with the sign of a degree after the
numeral — thus : 4° — which is scratched on the surface of
the glass. This is the old, and even up to the present
very general, method of numbering prisms.
Maddox has suggested the word " prismetry " to denote the
numbering by the deviating angle.
IThe Deviating Angle of a Prism. — In Fig. 4, if a ray, p, enter
the prism, instead of passing out at p', it is refracted towards
O THE REFRACTION OF THE EYE
the base b c, and away from the angle b a c, and is again bent
towards the base on passing out, and emerges in the direction /
(see page 3). The angle p' o f, made by the backward prolonga-
tion of / and the forward prolongation of p, is the angle of devia-
tion, and it is equal to about half the angle of refraction.
To indicate that the angle of deviation is implied, a small d
is added; thus, prism 4° is approximately equal to prism 2° d.
Angle of Refraction. Angl* of Deviation.
1° 32'
5° 2° 42'
10° 5° 26'
As the metrical system is now universally adopted in ophthal-
mology. Prentice has suggested the numbering of prisms on the
metrical plan, and the prism dioptre is the unit, designated by
the sign A after the numeral.
A prism of the strength of i P.D. (i A) is a prism that, at a
distance of i metre, apparently displaces an object i centi-
metre. In Fig. 5, E, being the observer, sees o at o' apparently
displaced* i centimetre, the distance between o and the prism
Fig.
b?ing I metre, and the prism i A ; that is, the apparent displace-
ment of an object looked at through a prism is i per cent, of the
distance of the prism from the object, multiplied by the prism
dioptre.
A prism i A apparently displaces an object 3 metres off, 3 centi-
metres, and a prism 3 A displaces an object 2 metres off, 6 centi-
metres, and so on.
Dennett has suggested the centrad as the unit for numbering
prisms. The centrad is the hundredth part of a radian, a radian
being the angle subtended at the centre of a circle by an arc,
which is equal in length to the radius. A prism i centrad,
designated i v. deviates a ray of light one-hundredth part of the
arc of the radian.
It is interesting to note that the centrad has a relative value
to the metre angle, in that half the number of centimetres between
the pupils indicates the number of centrads in the metre angle.
This method is not much used, although probably the most
scientific.
* Note that the apparent displacement of an object viewed
through a prism is always towards the apex.
PRISMS 7
All three methods of numbering prisms are practically iden-
tical for weak prisms, and, as it is only weak prisms that an oph-
thalmologist can use, it matters little what method he adopts.
Table showing the Equivalence of Centrads, Prism
Dioptres, and Refracting Angle, of the Six Weakest
Prisms. (Index of Refraction, 1*54.)
Centrad. Prism Dioptre. Refracting Angle.
1 I I*
2 2'OOOI 2*12°
3 3*0013 3*i8°
4 4*0028 4*23°
5 5-0045 5-28°
6 6*0063 6*32°
The minimum of deviation occurs when the incident
ray crosses the prism parallel to its base; but in thin
prisms — and it is thin prisms only that the ophthalmolo-
gist uses — this has no practical importance. Hence we
can neglect the position of the incident ray.
The Uses of Prisms.—
1. To remove diplopia.
2. To ease the muscles, and so prevent muscle
strain and subsequent diplopia (see page 176) .
3. To exercise weak muscles (see page 182).
4. To test the strength of the external ocular
muscles.
5. To detect malingerers (see page 183).
In trial cases the prisms are usually cut circular, so
that they can be used in a trial frame ; the exact position
of the base of the prism is usually marked by a line on
the glass at right angles to the base.
Rotating Prisms. — If two prisms of equal strength be
placed in apposition in such a manner that the base of
the one is in contact with the apex of the other, they
neutralize each other, and if we rotate them in opposite
directions we obtain the effect of an increasingly strong
prism.
Risley's Rotary Prism (Fig. 6) is made on this principle.
If we place it in one side of a trial frame, both eyes being
used, start from zero and gradually turn the button, we
8
THE REFRACTION OF THE EYE
can ascertain the strongest prism the eyes can stand
without having diplopia, or, if we are deaUng with a
case of diplopia, we can ascertain the weakest prism that
will procure " fusion " vision.
Fig. 6.
The numbers on the frame indicate the refracting angle
in degrees. The instrument can give a total prismatic
power of 30°.
i fisms form no images and have no foci.
Fig. 7.
Lenses. — If two prisms are placed with their bases in
contact, we have roughly a bi-convex lens (Fig. 7, a),
and rays of light passing through it are bent towards
the base of the prisms — i.e., the centre of the lens; in
other words, they converge. If the prisms have their
LENSES
apices in contact, we have a bi-concave lens (Fig. 7, b),
and the rays]are]bent towards the bases — i.e., outwards —
and diverge.
Spherical Lenses. — Besides the bi-convex (Fig. 7, a)
and bi-concave lenses (b), there are plano-convex (c),
Fig.
plano-concave (d), converging concavo-convex or con-
verging meniscus (e), and diverging concavo-convex or
diverging meniscus (f). Rays of light passing obliquely
through any of these forms of lenses are refracted or
bent towards the thickest part of the lens.
Fig. 9.
The principal axis is a line drawn through the optical
centre at right angles to the lens (Fig. 8, A o), and rays
passing through this are not refracted; all other lines
passing through the optical centre not at right angles to
the lens are called " secondary axes " (Fig. 9, a '«).
Rays passing along the secondary axes are refracted.
10
THE REFRACTION OF THE EYE
but as the emergent and the incident rays are in the
same direction, and the refraction in low-power lenses
is very slight, the refraction can be ignored, and the rays
assumed to pass along in a straight line.
Convex Lenses. — Parallel rays passing through a con-
vex lens unite on the opposite side of the lens at a point
called the " principal focus " (Fig. 8, p f).
At the principal focus an inverted real image of the
object is formed. Let a b (Fig. 9) be an object at some
considerable distance from the lens. Any ray passing
from the point a through the optical centre of the lens c
will be unrefracted {vide supra), and the image of a will
be somewhere on this line on the other side of the lens
Fig. 10.
— let it be at ^; all other rays passing from A will be
refracted on passing through the lens, and will focus at a.
In the same manner an image of B is formed at b, and all
other points between A and B will form an image between
a and b, so that we get an inverted image a b oi a b
formed at the principal focus of the lens.
The distance between the principal focus and the optical
centre is called the " principal focal distance " ; it is posi-
tive, and convex lenses are known by the plus sign : + .
Rays passing from the principal focus (p f) through the
lens emerge as parallel rays on the opposite side (Fig. 8).
Divergent rays from a point l (Fig. 10) beyond the
principal focus f meet at a point / beyond the principal
LENSES II
focus f' on the other side of the lens. If the point L is
twice the focal distance of the lens, then / will be at the
same distance on the other side. These two points are
called " conjugate foci," and are interchangeable; that is,
the object may be at l or /, and the image is respectively
at / or L.
If a luminous point be between the convex lens and
the principal focus, the rays will still be divergent when
they leave the lens on the opposite side, and consequently
no real image is formed; but a magnified virtual image
is formed beyond the principal focus on the same side, at
a point called the " virtual focus," and this virtual image
Fig. II.
is seen by an observer on the opposite side of the lens,
the light from the points a and h appearing to thr^
observer to come from a' and h' (Fig. ii).
Concave Lenses. — Parallel rays passing through a con-
cave lens diverge, and consequently never come to a
focus ; but these divergent rays, if prolonged backwards,
will meet at a point f (Fig. 12).
This point is the (virtual) principal focus of a concave
lens.
If an object be placed beyond the principal focus of a
concave lens, an observer on the opposite side of the lens
will see a virtual, erect, smaller image on the same side
as the object; thus, rays from a and h will appear to
12 THE REFRACTION OF THE EYE
come from a' and h' , and the object « 6 is seen as a' h'
(Fig. 13). As concave lenses have a negative focal
distance, they are denoted by the minus sign:]-.
Cylindrical Lenses.— In addition to spherical lenses
cyhndrical lenses are required — these are lenses cut out
Fig. 12.
of a cylinder; convex cylinders are cut from a solid
cylinder (Fig. 14, a), concave cylinders from a hollow
cylinder (Fig. 14, h), which may be regarded as the mould
of convex cylinders. Cylinders have the property of not
I
Fig. 13.
refracting any rays that pass along their axis, but rays
passing at right angles to the axis undergo the maximum
refraction corresponding to the strength of the lens.
According to the angle at which the rays impinge upon
the lens, they undergo more or less refraction, as this
LENSES
13
angle is further away from, or nearer to, the axis of the
cyUnder. A cylinder has no one focal point, but a line of
foci parallel to its axis.
Cylindrical lenses are employed to correct regular
astigmatism.
The axis of a cylindrical test lens is marked by a small
line on the glass, or by making the sides of the lens,
parallel to the axis, opaque.
Fig. 14.
Numeration of Lenses. — The lens whose focal distance
is I metre is taken as a unit, and its refractive power
is called one Dioptry or Dioptre (" d "). A lens of
twice the power of this — viz., 2 d — has a focal distance
of ^-; i.e., 50 cms.; a lens of half the power — viz.,
•5 D — has a focal length of 2 metres, and so on. The
focal distance of a lens no = '
n
Under the old system, a lens whose focal distance was
I inch was taken as the unit, and a lens whose focal
length was 10 inches was called jV* 3^ inches ^, and
so on. The great disadvantage of this method of
numeration was the inability to make it international,
because the inch is not an international measure.
To convert the old numeration into the new, divide the
denominator into 40; thus, lens \ is V-==S d, and vice
14
THE REFRACTION OF THE EYE
versa, Jo convert dioptres into inches, divide the dioptre
into 40, and the result is the focal length in inches; thus,
4 D = -*3^= 10 inches focal length, expressed as to-
The following table shows at a glance the approximate equiva-
lent of the old and new numeration :
)ioptres.
Inches.
Dioptres.
Inches.
•12
320
4
10
0-25
160
4-50
9
0.37
107
5
8
0-50
80
5-50
7
0-62
64
6
6|
0-75
53
7
^1,
0-87
46
8
5"
I
40
9
4i]
1-25
32
ID
4
1-50
26J
II
3h
1-75
22i
12
3i
2
26
13
3
2'25
i7i
14
2f
2.50
16
15
2-§-
2-75
14
16
2J
3
13
17
2i
3-50
II
18
2i
20
2
Testing Lenses. — It is important to be able to test a
lens and find out its optical value. Instead of going
through the process of finding its principal focus, and
measuring the distance of this from the lens centre, we
place in front of it lenses of the opposite value; thus,
if we wish to find the strength of a convex glass, we
neutralize it with concave glasses.
A finer test is to employ the parallactic movement. If
we look at a distant object through a convex glass and
move the glass, the object appears to move in the oppo-
site direction; if we use a concave glass, the object
appears to move in the same direction. So long as
there is any movement we must place up concave or
convex glasses, according as the displacement of the
object is " against " or " with."
In testing cylinders we have to ascertain not only the
value, but also the direction of the axis.
LENSES 15
When cylinders are moved in front of the eye in the
direction of the axis, objects looked at through them are
not displaced; but the smallest rotation of the cylinder
causes displacement, which reaches its maximum when
the movement of the cylinder is in the direction at
right angles to its axis. In this position neutralize with
cylindrical lenses of the opposite value, bearing in mind
that displacement takes place " against " the movement
so long as a convex lens predominates, and " with " the
movement so long as a concave one predominates. The
axes of the two lenses must coincide.
When testing a sphero-cylindrical glass, the spherical
lens should be first neutralized.
A great saving of time is effected by testing glasses with
the Geneva lens measure and the Maddox cylinder-axis
finder.
The Combination of Lenses — Convex Spherical Lenses.
— The ordinary way for such lenses to be ground is to
work half the power needed on each surface ; thus, when
+ 4* is required, each surface of the lens is made equal
to +2, as if two plano-convex glasses of -1-2 had their
plane surfaces cemented together.
Another method of working a convex lens is to grind
the surface away from the eye as a convex lens of higher
power than is required, and the other surface concave of
such a strength as to reduce the convex surface to the
desired amount. Thus, when -1-4 is ordered, one surface
can be made +7 and the other surface -3, or one
surface can be -1-6 and the other -2. Such lenses are
called " periscopic *'; they enlarge the field of distant
vision, as the eye in all its movements is at the same
distance from the surface of the glass, but their chief
advantage is in enabling the glasses to be placed nearer
the eye without being touched by the lashes.
* In the following pages, the numeration of lenses will always
be in dioptres, and " d " after the numeral will be generally
omitted.
l6 THE REFRACTION OF THE EYE
Concave Spherical Lenses. — These are usually made
with half the strength required, on each surface. A
certain amount of periscopic effect may be obtained by
grinding the surface nearer the eye more concave, and
reducing the other: thus, —8 may be, and usually is,
made — 4 on each surface, or the surface nearer the eye
may be made —6 and the other — 2 ; or the lens may be
ground as a diverging meniscus (Fig. 7, f) ; thus, if - 3 is
required, the surface next the eye is ground as -5, and
the other surface as +2.
Cylindrical Lenses. — When a cylinder only is pre-
scribed, it is ground on one surface, the other surface
remaining plane. When combined with a spherical lens,
it is usual to grind the sphere on one surface, and the
cylinder on the other. When a convex sphere and
cylinder are required, the periscopic effect can be pro-
duced by grinding a concave cylinder on the surface
nearer the eye, and increasing the spherical strength by
the amount of the cylinder. Thus, supposing +2 cyl.
axis vert, c:; +3 sph. is needed, it may be ordered thus:
— 2 cyl. axis horizontal c^ +5 sph.
In ordering glasses for mixed astigmatism (see page
137), the periscopic effect is produced by combining a
convex cyhnder with a concave sphere, and mounting the
latter next the eye.
As it is very important to divide the strength of the
lens between the two surfaces when dealing with high
powers, a cylinder, if also required in such a case, must
be worked on one of the spherical surfaces. These
lenses are called Toric lenses, and any combination of
spherical lens (•25 to 18), with cyHndrical lens (from •25
to 3) can be supplied. Thus, if — 14 sph. oj - 2 cyl. axis
horizontal is required, one surface is made - 7, and the
other - 7 sph. c::> - 2 cyl. These toric lenses are very
useful in high myopia, and also in aphakia.
Bi'Focal Lenses (see Presbyopia, page 151). — Where
BI-FOCALS 17
different lenses are required for distance and reading,
bi-focal lenses are generally prescribed.
The earliest forms of these glasses were straight split
bi-focals, also called " Franklin " glasses, the lenses being
two separate lenses divided horizontally and in the
middle. Against the many disadvantages of this form
of bi-focal was the one advantage that the lower lens
could be slanted and made more or less parallel with the
book or paper that was being read.
An improvement on these " split " bi-focals consisted
having the reading addition cemented on by Canada
dlsam, either as in Fig. 15 or as a small round wafer.
Fig. 15.
The disadvantage of both these forms was chiefly
manifested by the dividing line between the lenses being
visible and constantly causing annoyance to the wearer.
Another disadvantage of the cemented bi-focals is the
tendency of the balsam to dry or crystallize.
The next improvement did away with the visible line,
and these bi-focals are called " invisible bi-focals." They
are of two kinds, one the so-called " Kryptok," where
a concavity is ground in the lower part of the distance
glass, and the reading glass, of a higher refractive index,
is fused into it; the other, the final perfected form of
invisible bi-focals is the " Luxe," which consists of
one glass only. The glass is made from a solid piece
of crown glass with the two lenses ground invisibly
on its surface. The chief advantage of these " Luxe "
l8 THE REFRACTION OF THE EYE
bi-focals is that the centring of both portions is more
under control, and there is no chromatic aberratioii,
which is so often present in the fused form.
Spherical Aberration. — In most lenses the rays passing
through the peripheral part of the lens do not focus at
the same spot as those which pass through the central
portion. In convex lenses, when the peripheral rays
focus in front of the central rays, the aberration is spoken
of as positive ; and when the peripheral rays focus
behind the central, as negative. The crystalline lens
suffers from spherical aberration, but it is more or
less hidden by the contraction of the pupil. During
mydriasis this aberration may interfere considerably
with vision, but may be corrected by placing in the
trial frame an opaque disc with a central circular open-
ing. If the refraction is being estimated, this opening
should not be smaller than 4 mm. in diameter.
CHAPTER II
OPTICAL PROPERTIES OF THE NORMAL EYE
The eye is constructed in the form of a photographic
camera. As in the camera, there is a closed darkened
box open in front, where there is an arrangement of
lenses to focus an object on the back, at which spot
there is the apparatus for receiving the perfectly-formed
image : the plate in the camera, the retina in the eye.
As in the camera, there are two conditions which must
exist in the eye : firstly, the media must be transparent ;
and, secondly, the focusing must be so arranged that a
perfect image of the external object is formed on the
retina — i.e., the principal focus of the eye must coincide
with the retina.
All deviations from this latter condition are called
errors of refraction and accommodation.
The Refraction of the Normal Eye at Rest— i.e., in
the Absence of any Effort of the Accommodation —
Dioptric Apparatus of the Eye. — The simplest form of a
dioptric apparatus is when two media of different refrac-
tive power are separated by a spherical surface.
Such a system is represented by Fig. i6, where
X y z is di spherical surface separating a less refractive
medium on the left from a more refractive medium on
the right. The line o A passing perpendicularly to the
surface of the sphere and through its centre at N is called
the " optic axis."
All rays passing .^normally to the surface, such as R N
and s N, like the optic axis, pass through n, and undergo
19
20 THE REFRACTION OF THE EYE
no refraction; N, the centre of the sphere, is called the
" nodal point."
The point y where the optic axis cuts the sphere is
called the " principal point."
Rays c, d, parallel to the optic axis in the less dense
medium, unite somewhere on the optic axis at the point
F, called the " posterior principal focus."
On the optic axis, in the less dense medium, there is
another point (f'), called the " anterior principal focus,"
whence divergent rays passing into the denser medium
are refracted, and become parallel to the optic axis as at zf.
Fig. i6.
These four points — the principal point, nodal point,
and anterior and posterior principal focus — are called
" cardinal points " of the system.
In the eye the system is much more complicated. A
ray of light passing into the eye meets the following sur-
faces and media in the order named: Anterior surface
of the cornea, substance of the cornea, posterior surface
of the cornea, aqueous, anterior surface of the lens, sub-
stance of the lens, posterior surface of the lens, and
vitreous. Thus there are four surfaces and, if we include
the air, four media. As the anterior and posterior sur-
faces of the cornea are parallel, we may neglect the sub-
stance of the cornea, and consider the two surfaces as
one. Again, as the indices of refraction of the aqueous
THE CARDINAL POINTS 21
and vitreous are identical (see page 4), we may assume
them to be one medium. In this manner the eye is
reduced to three surfaces and three media.
These three surfaces — the cornea, and the anterior and
posterior surfaces of the lens — are symmetrically centred
round the optic axis of the whole system, which may
now be reduced to a compound system, consisting of the
cornea and a bi-convex lens ; we find the principal points
p' p" (Fig. 17) and the nodal points n' n" of the cornea
and lens ; and, finally, take the mean of these two points,
Fig. 17.
and get p the principal focus and N the nodal point.
Such an eye is known as the " reduced eye," and was
suggested by Listing.
The positions of the cardinal points of the reduced eye
are —
Principal point, in the aqueous, 2 '3448 mm. behind the
anterior surface of the cornea.
Nodal point, in the lens, '4764 mm. from its posterior
surface, and about 15 mm. from the retina.
Posterior principal focus, 22 •81 9 mm. behind the an-
terior surface of the cornea — i.e., on the retina of the
normal eye. (This is the length of the standard eye.)
Anterior principal focus, 12 '8 mm. in front of the
anterior surface* of the cornea.
22 THE REFRACTION OF THE EYE
The principal plane r p s (Fig. i8) is where the one
surface of this reduced system passes through the prin-
cipal point p which is considered the centre of refraction
of the eye.
The optic axis (o a) is an imaginary line passing
through the centre of the cornea and the nodal point,
and meeting the retina a little above and to the nasal
side of the fovea.
The nodal point corresponds to the optical centre,
and, as we have already seen, all rays passing through
it are unrefracted.
We can now ascertain how an image is formed on the
retina.
Let X Y (Fig. i8) be an object in front of the eye; each
Fig. I 8.
point of this sends out a pencil of divergent rays, and all
those which pass into the eye, by the dioptric system,
are made to converge into a point on the retina.
Each pencil of rays from the point x has a principal
ray x a, which is normal to the surface, and, passing
straight through the nodal point n without refraction,
impinges on the retina at x'. The other rays from x are
increasingly divergent, and are represented by x 6, x c;
they undergo refraction, and converge together at some
point on the principal ray, which in the normal eye will
be at x'. In like manner we can trace the rays from the
other extreme point Y, which forms an image at y', and
so for all the other points.
In tracing the formation of an image on the retina.
SIZE OF RETINAL IMAGE 23
we can ignore all the rays from a point of the object,
except the principal ray, which we trace through the
nodal point; and by tracing all the luminous points
from an object through the nodal point, we obtain in
the normal eye an inverted image of the object on the
retina.
The nearer the object is to the eye, the larger will be
its image, and vice versa. The size of the retinal image
is therefore directly proportional to the distance of the
object from the eye.
It is sometimes important for the oculist to determine
the size of the retinal image of an object in order to dis-
cover the size of a diseased area; this can be estimated
if the size of the object and its distance from the eye be
known.
The triangles A N B and a ^ h (Fig. 19) are similar,
hence ah : hB : : « N : A n — that is, the size of the area
on the retina is to the size of the object as the distance
from the nodal point to the retina is to the distance
of the nodal point from the object. Let the latter be
10 metres and the size of the object i metre; we know
that the distance a; N is 15 mm. — consequently :
ah : 1,000 : : 15 : 10,000;
, 15,000
.-. ah = ^^ =1*5 mm.
10,000
24 THE REFRACTIOnIoF THE EYE
The perfect type of eye is that in which the retina
coincides with the posterior principal focus, and is called
Fig. 20.
Showing parallel rays focused on the retina in emmetropia (e),
behind the retina in hyperopia (h), and in front of the retina
in myopia (m) .
the " emmetropic " eye (e, Fig. 20), and any deviation
from this is called ametropia.
VISUAL ANGLE
^5
The following table gives an idea of the relative
frequency of the different forms of ametropia :
(a) Emmetropia (see
Presbyopia be-
2500 individuals
whose sight
after correc-
tion was nor-
mal and who
had no disease
of the eyes.
:) Same refrac-
tion in both
eyes (O57)
low)
9
(b) Hypermetropia .
^3
(c) Myopia
22
{d) Astigmatism —
Hypermetropic
43«
Myopic .
113
Mixed .
12
U^)
Refraction different in the two eyes
(Anisometropia) . . . .
^843
2500
5000 eyes (as above) —
Emmetropia , ....... 56
Hypermetropia ....... 425
Myopia ........ 216
Astigmatism . . . . . . . .4303
5000
Of the 2500 individuals, 961 were presbyopic, and only 9 of
these were emmetropic.
If the posterior principal focus is beyond the retina,
the eye is too short, and parallel rays, when they meet
the retina, have not yet come to a focus, and only con-
vergent rays come to a focus. This is called " hyper-
opia " (h, Fig. 20).
If the principal focus is in front of the retina, the eye
is too long ; parallel rays focus in front of the retina, and
only divergent rays focus on the retina. This condition
is called " myopia " (m, Fig. 20).
The Visual Angle and Visual Acuity. — Rays of hght,
proceeding from the two extremes of an object into the
eye, meet at the nodal point n (Fig. 21) before crossing
and forming the inverted image on the retina, and the
angle included at n is called the " visual angle." A n b
is the visual angle of the object a b (Fig. 21).
The size of the visual angle depends on the size of the
object and its distance from the eye; thus, a' b', which
26
THE REFRACTION OF THE EYE
is the same size as A b, subtends a larger angle, and the
image is larger; and, again, a" b" subtending the same
visual angle as A b would appear to be the same size,
whereas it is much smaller. Fortunately, we do not
gain our estimation of the size of objects by the visual
angle alone; experience and comparisons with other
objects of known size are brought into play, and enable
us to correct any erroneous judgment.
The smallest visual angle in which the standard eye
can recognize an object is an angle of one minute, so
that two points of light, such as two stars, separated by
an angular interval of less than one minute would
appear on the retina as only one point.
Fig. 21.
Test Types. — It is most important to have a standard
measure for acuteness of vision, and Snellen has arranged
test types on such a plan that each letter is made up of
several parts, each of such a size that it subtends an
angle of one minute vertically and horizontally, the
whole letter subtending an angle of five minutes verticallj*
and horizontally when read at the standard distance.
Thus, in Fig. 22, the F is made out of twenty-five
squares, each subtending an angle of one minute (the
whole letter subtending an angle of five minutes) when
read by the normal eye at 12 metres; and the l, which
is constructed on the same plan, subtends the same
angle when read by the normal eye at 6 metres.
The numbers of the different-sized letters in Snellen's
types represent the distance in metres at which the
VISUAL ACUITY 27
standard eye can read them; in other words, at that
distance they subtend an angle of five minutes. For
instance, the largest type, d = 60 (see type at end of
book), can be read by the normal eye at 60 metres, and
it subtends the same angle as the type d = 24 read at
24 metres, and d =^ 6 read at 6 metres. The acuteness
of vision is represented by a fraction which has for its
numerator the distance in metres at which the type is
read, and for its denominator the distance at which it
ought to be read. The line d = 6 means that this type
can be read by the normal eye at 6 metres, and if the
patient under examination can read it at 6 metres, the
fraction is | — that is, normal vision. If the patient
cannot see a smaller type than d = 12 at 6 metres, his
Fig, 22.
vision == tV ; if D = 60 is the only letter that can be read
at 6 metres, his vision = A — i-c-, one- tenth of the
normal. If d = 60 cannot be read at 6 metres, the
patient must be made to approach the type; if he can
just read this letter at 2 metres, his vision is i~^', he has
only one-thirtieth of normal vision. Although J is the
standard of normal acuteness of vision, many eyes can
see better — viz., |, or even J; i.e., such eyes can read at
6 metres type that the standard eye cannot read at a
greater distance than 5 and 4 metres respectively.
If the visual acuity is so lowered that the patient
cannot see any letter at any distance, it can be measured
by finding whether he can count fingers, and if so, at
what distance, and failing this, by finding whether he
2S THE REFRACTION OP THE EYE
can distinguish between black and white. If vision is
even worse than this, we take him to the Hght and pass
the hand in front of the eye — i.e., between the eye and
the Hght ; if movement is recognized, we find out whether
he can distinguish the direction of the movement.
Finally, if he fails at all these tests, he should be
taken into the dark room, and a strong beam of light
should be directed on to the eye; if this is not perceived,
vision = o; if it is perceived, we ascertain whether he
has good projection, by reflecting the light on to the eye
from different positions, and ascertaining whether he
can tell whence the light is coming.
Type for Near Vision. — As the " Schrift-scalen " of
Professor Jaeger represent no standard, this type is being
superseded by Snellen's, which is on the same principle
as his distant type, the figure over the type signifying the
greatest distance at which the normal eye can read it,
and, of course, subtending an angle of five minutes at
that distance. The sizes range from d = '5 to d = 4
(see type at end of book), j '2 (Jaeger) is the equivalent'
of D 'S (Snellen).
D = 0,5.
JU to my boat, it was a very good one, and that he saw, and told me he would buy it of
me for the ship's om and asked me what I would hare for it.
D = 0,6.
In this distress the mate of our vessel la3rs hold of the boat, and with the help
of the rest of the men, they got her slung over the ship's side.
D = 0,8.
A little after noon I found the sea very calm, and the tide ebbed so
far out, that I could come within a quarter of a mile of the ship ;
D = l.
In search of a place proper for this, I found a little plain
on the side of a rising hill, whose front towards this little
plain was steep as a house side.
D = l,25.
Then I took the pieces of cable which I had cut
in the ship, and laid them in rows one upon
another, within the circle between these two rows
of stakes.
D = l,5.
When I had done this, I began to work
my way into the rock, and bringing all
the earth and stones, that I dug down,
out through my tent.
D=2,25.
For in this way you may
always damp our ardour.
D = 3.
I sai^T no one there.
For the ensuiner
D=60f200)
D-36 (120)
D=24 (80)
D=I8(60)
D-12 (40)
DF O E
D=9 (30)
G L Z T O
D«6 (20)
L T R F P
D-5 (16)
A P O R F D
CHAPTER III
ACCOMMODATION
When, with one eye closed, the other eye focuses a
needle a metre from the eye, another needle placed half
a metre from the eye will appear blurred.
If A (Fig. 23) be the first needle, a clear image is
formed by the exact focusing of it on the retina at a',
Fig. 23.
Fig. 24.
while the image of B will be focused beyond the retina
at b', the rays from B impinging on the retina in the
form of a collection of diffusion circles.
On the other hand, if the needle b be focused on the
retina — i.e., if its image be clearly seen — the needle A
30 THE REFRACTION OF THE EYE
will appear hazy or out of focus, because its image is
focused in front of the retina, and, after crossing, the
rays impinge on the retina as diffusion circles.
Diffusion Circles. — Two points of light, if near one
another and out of focus, appear as two diffusion circles
overlapping each other if near enough (Fig. 24, a).
As a line in focus may be considered to be an infinite
number of points of light in focus, so a line out of focus
consists of a series of overlapping diffusion circles
(Fig. 24, b) which makes the line appear as a broad
band, as Fig. 24, c. The further the rays focus from the
retina, the larger will be the diffusion circles. In Fig. 25
both A and b are " out of focus," but b is more so than
A, and consequently the diffusion circles formed by b
Fig. 25.
occup3 a larrer area; and, again, the larger the pupil the
larger the area of diffusion circles, because as the pupil
contracts it cuts off the outside rays.
The alteration of the eye by its focusing mechanism
is called accommodation. The photographer focuses
by lengthening or shortening the distance between the
back of the camera and the lens, but he could also focus
by adding a convex or concave lens to that he is already
using.
It is in this latter way that the eye focuses; the eye
cannot lengthen, but the lens can become more convex,
which has the same result as adding a convex lens.
In the normal standard eye, parallel rays, coming from
a distance beyond 6 metres, are focused on the retina
when the eye is at rest — i.e., when the apparatus of
ACCOMMODATION 31
accommodation is not being used; but when the eye
wishes to see clearly any object nearer than 6 metres,
the lens must become more convex.
After looking at the needle A, when we look at needle
B and obtain a clear image, we are distinctly conscious
of an effort, and the nearer we approach b to the eye
the greater is the effort, till we reach a spot near the eye
when no effort will produce a clear image, because the
rays from the needle are too divergent to be focused on
the retina. The nearest point to the eye at which the
object is recognized as a perfectly clear image is called
the near point "P." After looking at the needle close
to the eye, and again looking at the distant needle, we
are conscious of a relaxation of our efforts.
How do we know that this focusing or accommodation
is caused by an increased convexity of the lens ?
The Mechanism of Accommodation. — If we take a
patient into the dark room and hold a candle in front o
the eye a little to one side, we shall see three images oi
this candle in the eye. One, the brightest, is upright,
the reflection coming from the anterior surface of the
cornea; the second, duller, is also upright, and is the
reflection from the anterior surface of the lens; and the
third is inverted, duller, and smaller, and is from the
posterior surface of the lens. The patient is told to look
into distance, and the size and position of these images
is noted, and then, carefully watching them, he is told to
gaze at a near point. No change will be seen in the first
image (proving the fallacy of the old theory that the
cornea becomes more convex during accommodation),
and little change in the third; but the middle image —
viz., that from the anterior surface of the lens — becomes
distinctly smaller and moves forward, showing that this
surface has become more convex.
In accommodation, then, the lens becomes larger in
its antero-posterior diameter, and as it does not alter in
volume, it becomes narrower in its equatorial dimensions.
32
THE REFRACTION OF THE EYE
Fig. 26. — Diagrammatic Section of the Ciliary Region
OF the Eye.
C, Cornea; c S, Schlemm's canal; O s, era serrata; / p, pectinated
ligament ; e F, Fontana's space ; T, tendinous ring ; m, merid-
ional fibres; r, radiating fibres; c, circular fibres of the
ciliary muscle; Z, zone of Zinn.
The full lines indicate the lens, iris, and ciliary body at rest, and
the dotted lines the same in a state of accommodation.
(Reduced from Landolt.)
ACCOMMODATIOK 33
We will now inquire how this change is brought about.
According to Iwanoff, the ciliary muscle arises from a
tendinous ring (Fig. 26, t) close to the insertion of the
iris and Schlemm's canal {c S), at the posterior surface
of the sclerotic, close to its junction with the cornea.
The muscle then passes backwards, and may be divided
into three parts: (i) The outermost part or meridional
portion, passing into the posterior tendon (m), to be
inserted into the choroid; (2) the radiating portion (r) ;
and (3) the annular portion or circular muscle of Miiller
(c), passing directly backwards and inwards respectively,
to be inserted into an agglomeration of fibres called the
" zone of Zinn " (Z). These fibres arise partly from the
ciliary portion of the retina at the ora serrata (0 s) , and
partly from the ciliary processes and the intervals
between them, and they pass forwards and backwards,
to be inserted into the anterior and posterior capsule of
the lens.
The annular muscle of Miiller is a sphincter, and does
the principal work; hence it is always larger in hyperopia,
because of the extra accommodation work necessary, and
is badly developed in myopia.
There are two theories as to the modus operandi of the
ciliary muscle when accommodating. The old theory
started by Helmholtz, and supported by Hess, is as
follows :
When the ciliary muscle contracts, it pulls forwards
and inwards the capsule of the lens, the inward pull
being specially brought about by the contraction of the
circular muscle of Miiller. The contraction of the longi-
tudinal fibres pulls forward the choroid and the portion
of the ciliary body near it.
By this process, they contend, the tension on the lens
capsule is relaxed, and the lens, which has been in a
state of compression, is allowed to assume a more convex
form.
The new theory advanced by Tscherning maintains
3
34 THE REFRACTION OF THE EYE
that the action of the ciliary muscle is to increase the
tension on the fibres of the suspensory ligament, and to
alter the lens from a spherical to a hyperboloid form,
and this theory is founded on the work of Thomas
Young. According to this theory, the lens becomes
more conical under accommodation, and the contraction
of the pupil, that occurs at the same time, masks the
increased aberration which results from the flattening
of its periphery.
The posterior surface of the lens does become slightly
mere convex during accommodation, but it does not
change its position, the increase of thickness of the lens
being effected by the advance of the anterior surface.
Tscherning's theory of accommodation is entirely supported
clinically. Under the Helmholtz theory it is difficult to under-
stand the possibility of meridional asymmetrical accommodation,
and as difficult to believe in the possibility of obtaining 20 d of
accommodative power which is frequently seen in young subjects.
Lastly, the Helmholtz theory is totally against the idea of rest.
Amplitude of Accommodation. — At rest, the eye is
adapted for the most distant point it can see distinctly — ■
viz., its punctum remotum (R) ; while the greatest possible
contraction of the ciliary muscle adapts the eye to the
nearest point it can see distinctly — viz., its punctum
proximum (P), which represents the greatest possible
contraction. The force required to change the eye from
R to P is called the " amplitude of accommodation," and
is represented by the difference between the refraction of
the eye at rest and the refraction when doing its utmost
work.
Donders represented the equation thus:
III
A~P~R
or ' a = p-r.
Where " a " equals the numbers of dioptres represented
by the accommodation, " p " equals the numt^er of diop-
ACCOMMODATION 35
tres represented by the eye when in a state of maximum
refraction—/.^., when adapted for its nearest distinct
point — and " r " equals the number of dioptres repre-
sented by the eye at rest — i.e., when adopted for its
furthest distinct point. In other words, " r " represents
the static refraction of the eye.
In emmetropia, as R is at " infinity, " r " can be
ignored,
.-.a = p.
Therefore the ampHtude of accommodation is represented
by the nearest distinct point ; if this is 9 cms. off, " a " =
-f- = II — that is, the power of accommodation is equal
to a lens of eleven dioptres.
In myopia •' r " has a positive value. Take, for ex-
ample, a person whose furthest distinct point with the
eye at rest is 33 cms. (that is, a myope of 3), and sup-
pose that his nearest distinct point is 7 cms. ; then
a = p - r
100 100
= -7 5^
= 14-3
= II.
In other words, 14 would represent his amplitude of
accommodation if he were emmetropic; but being
myopic to the extent of 3, we must subtract that, which
leaves us 11 to represent his amplitude.
In hyperopia, as we shall see later, " r " is negative;
therefore the equation is
a = p - (-^r)
= p +r.
Thus, an eye hyperopic to the extent of 5, having its
near point at 25 cms. from the eye, has an amphtude of
accommodation equal to a lens of 9. To see 25 cms.
36 THE REFRACTION OF THE EYE
off, the eye requires an accommodation of 4 (-V/), but
it has already expended 5 for distance, so that
a = p - {- r)
= 4 - (- 5)
= 4+5 = 9-
We thus see that to determine the ampHtude or range
of accommodation we must find R and P.
R is represented by the refraction of the eye at rest.
P we find as follows :
Take a tape graduated on one side in centimetres, and
on the other in corresponding dioptres ; the zero-end of
the tape is attached to the handle of a frame, into which
may be introduced either a perforated diaphragm or a
paper with fine print upon it, or threads or hairs ; or the
ordinary near vision test card and separate measure may
be used. The test object is brought towards the eye
under examination (the other one being covered) until it
begins to appear indistinct ; we then read off on the tape
the distance of P from the eye, and the corresponding
dioptres (p) representing the maximum refractive power
of the eye.
If from any cause, such as presbyopia or high
hyperopia, the patient's near point is so far that the
above tests cannot be employed, we place in front of
the eye such a convex glass as will bring the punctum
proximum (P) closer, and enable him to read d = .5 or
see the words in the frame, such glass to be, of course,
deducted afterwards. Thus, supposing a person with
+ 2 can bring the test object up to 25 cms. and no nearer,
we read off on the other side of the tape 4, and we
subtract the + 2 from this, which gives us p = 2 — that
is, P is 50 cms. off. If he is an emmetropic presbyope,
this represents his amplitude of accommodation. If he
is hyperopic to the extent of 6, then
a = 2 + 6
= 8.
ACCOMMODATION 37
Or suppose the patient, being hyperopic and presbyopic,
requires +5 to read at 33 cms., if his hyperopia = 6, then
a = p + r
= (3 - 5) + 6
= 4-
We can also find the ampHtude of accommodaf ion by
ascertaining the strongest concave glass the patient can
" overcome." In emmet ropia such glass represents the
amplitude of accommodation. In hyperopia the amount
of hyperopia must be added, and in myopia the amount
of myopia must be deducted.
As an example, we find a patient who is hyperopic to
the extent of 2 can still read J with - 4, but he cannot
do so with - 5; thus his amplitude of accommodation
is 4 + 2 = 6.
It necessarily follows that to determine the amplitude of
accommodation of an eye, its refraction must be accurately
ascertained and the patient must wear the full correction of
the error when the examination is made.
The Region of Accommodation is quite different from
the range, and gives very little idea of the work done.
1 —————— -^
WD
a^ WD
00 R'lO'^'" P'-S'"
i _ .
lOD 2.0D
a = too
Fig. 27.
Thus, the region of accommodation in an emmetropic
eye, as Fig. 27 (i), is from infinity (R) to 10 cms. (P) in
38 THE REFRACTION OF THE EYE
front of the eye, while in Fig. 27 (2), a myopic eye, it is
only from 10 cms. (R) to 5 cms. (P) in front of the eye,
and yet in each case the same amount of accommodation
work is done, which is equal to a lens of 10.
Accommodation is spoken of as absolute, binocular,
and relative.
Absolute accommodation is the full amount of accom-
modation of one eye, the other being excluded.
Binocular accommodation is the full amount of accom-
modation which both eyes, converging, can exert
together.
Relative accommodation is the limit within which
accommodation may be increased or decreased, the con-
vergence remaining the same (see Convergence, page 54).
CHAPTER IV
CONVERGENCE
Anatomical and Physiological Considerations. — The orbit con-
tains the eyeball, the optic nerve, muscles, lachrymal gland
vessels and nerves, and a quantity of fat. These structures are
all firmly connected by a system of fasciae. Surrounding the
eyeball, these fasciae are condensed in a fibrous capsule — the
fascia bulbi or Tenon's capsule. This capsule consists of an
external capsule and an internal capsule. It is perforated by the
muscles just before their insertion into the globe, and its reflection
unites with the cone of fascia surrounding the muscle ; prolonga-
tions and thickenings of the orbital fasciae of these muscles are
inserted into the margins of the orbit, and constitute the check
ligaments.*
The muscles which move the eye are six in number, and,
with the exception of the inferior oblique, which arises from
the anterior and inner part of the floor of the orbit, they all
arise from the apex of the orbit. These muscles may be con-
sidered as three pairs, each pair rotating the eye round a par-
ticular axis. The four recti — viz., superior, inferior, internal,
and external — -pass forwards, pierce Tenon's capsule, from which
they receive a sheath, become tendinous, and are inserted into
the sclerotic not far from the margin of the cornea, the most
anterior insertion being that of the internal rectus, which is about
6 mm. from the margin of the cornea. The superior oblique
passes forwards to the upper and inner angle of the orbit, where
it becomes temporarily tendinous, and passes through a pulley,
after which it becomes muscular again, and changes its direction,
passing backwards and outwards through Tenon's capsule to be
inserted (tendinously) into the sclerotic, at the back and upper
part of the eye. The inferior oblique passes outwards and back-
wards, underneath the inferior rectus, and then between the
external rectus and the eye, to be inserted into the outer, pos-
terior, and lower part of the eyeball, not very far from the
entrance of the optic nerve.
The axis of rotation of the internal and external recti is ver-
tical, and that of the superior and inferior recti horizontal, with
the inner extremity more forward than the outer (Fig. 28) . That
* See Maddox, " Ocular Muscles," 1907, page 26,
39
40
THE REFRACTION OF THE EYE
of the oblique muscles lies also in the horizontal plane, with its
anterior extremity tilted outwards.
The movements of the eyeball are produced by the association
of various muscles, as shown below:
I. Elevation. — The movement of the eye straight up is pro-
duced by the superior rectus and inferior oblique, probably
obi 6UP,
Fig. 28. — Diagram of the Attachments of the Muscles
OF the Left Eye and of their Axes of Rotation as
seen from Above. (Michael Foster.)
The attachments of the muscles are shown by the beginning of
the thick lines, and the direction of the pull is shown by the
arrows, v x represents the visual axis, and h h a line at
right angles to it.
The axis of rotation of the internal and external recti, being
perpendicular to the plane of the paper, is not represented ;
that of the other muscles is indicated by the broken lines.
steadied by the internal and external recti, the superior rectus
assisting in the elevation of the lid.
2. Depression. — Looking straight downwards is produced by
the inferior rectus and the superior oblique, steadied by the
lateral recti, the inferior rectus assisting in the depression of the
lower lid.
3. Abduction. — The eye is turned straight;^ out by the external
CONVERGENCE
41
rectus, assisted at the extremity of its action by the superior
and inferior recti.
4. Adduction. — The eye is turned straight in by the internal
rectus, assisted at the extremity of its action by the superior and
inferior recti.
When both eyes look to the right, we have contraction of the
Fig. 29. — Diagram of the Connections of the Nuclei of the
Lateral Recti Muscles. (After Ross.)
C C, Cortex of right and left cerebral hemispheres; i, 2, fibres
of the pyramidal tract uniting C, the cortex of the right
hemisphere, and r' and er', the nuclei of the left internal
and external rectus; i', 2', fibres of the pyramidal tract
connecting the cortex of the left hemisphere with r and er,
the nuclei of the right internal and external rectus muscles ; c,
fibres of the corpus callosum uniting identical regions of
the two hemispheres; c', commissural fibres connecting the
spinal nucleus of the internal rectus of one eye with that of
the external rectus of the opposite eye ; c", suggested com-
missural fibres connecting the nuclei of the two internal recti.
right external and left internal recti, and when they look to the
left, contraction of the left external and right internal recti.
Movement of the eyes up and in is produced by i and 4 — viz.,
superior rectus, inferior oblique, and internal rectus, movement
down and out by 2 and 3, and so on.
42 THE REFRACTION OF THE EYE
The external rectus is supplied by the sixth nerve, the superior
oblique by the fourth, and the others by the third.
Convergence of the eyes is produced by the associated move-
ments of both the internal recti. The nuclei {r r', Fig. 29) of
that part of the third nerve which supplies these muscles may be
connected by fibres {c"), illustrating the principle that there is
bilateral association of the nerve nuclei of muscles bilaterally
associated in their action (Broadbent). This explains the con-
vergence of a covered eye. A. Graefe says that one of the factors
causing the covered eye to converge is a " Convergenzgefiihl,"
or, as Hansen Grut expresses it, a " Nahebewusstsein " — a con-
sciousness of nearness. Landolt denies this, and asserts that the
excluded eye fixes correctly through the connection between
accommodation and convergence alone.
It is important to remember that when a stimulus passes pri-
marily to the nucleus of the internal rectus, it is associated with
the same muscle of the opposite side, and convergence takes
place; whereas the conjugate movements of the eyes to the right
or left are produced by stimuli passing primarily to the nucleus
of the external rectus, which nucleus is connected with the nucleus
of the internal rectus of the opposite side (Fig. 29). We may
have both these stimuli occurring at the same time — viz., primary
stimulus to the internal recti to converge, and to the external
rectus of one side associated with the internal rectus of the other
side — to produce lateral movements of the eyes.
The oculo-motor centre (Fig. 30, o.m.c.) is situated beneath the
floor of the aqueduct of Sylvius. It includes (i) the accommoda-
tion centre (a), lying most anteriorly near the middle line, and
(2) the pupil constrictor centre (p) . The nucleus of the internal
rectus (i.R.) lies further back. Filaments pass along the third or
oculo-motor nerve from these centres to the ciliary muscle, the
sphincter of the iris and the internal rectus, and are so associated
that contraction of the ciliary muscles for accommodation, of the
pupils, and of the internal recti for convergence, are all three
associated in their actions. One impulse — viz., a psychical
impression, a wish to look at a near object — passes from the
motor centre in the cortex of the brain to these nuclei, and the
result of this one impulse is the united action of these different
muscles; the action is not always simultaneous, for convergence
often lags behind accommodation (see page 56).
Many people can voluntarily squint inwards, but they will be
found to accommodate for a near point at the same time; some
few can, however, do so without accommodating, and in such cases
the psychical impression probably passes straight to the nucleus
of the rectus internus by vs' (Fig. 30).
Binocular Vision. — Man has binocular vision — that is,
the image from an object falls upon the retina of each
eye simultaneously, and in normal binocular vision on
exactly the same region of the retina; for if the images
BINOCULAR VISION
43
did not overlap, two images would be seen, and so-called
" double vision " would be the result. The absence of
double vision does not necessarily imply the presence of
normal binocular vision with fusion of the two images,
for one eye may be blind or its image suppressed by the
brain (monocular vision). Many people use one eye
only, for years, without discovering the fault. The best
and quickest test for determining whether binocular
Fig. 30. — Scheme showing the Oculo-Motor Centre and
Some of its Connections. (Adapted from Erb.)
P5, Psychical impression (the wish to accommodate being the
stimulus) ; ps', psychical impression for voluntary converg-
ing strabismus; a, accommodation centre with motor nerve
to ciliary muscle, and p, centre for the sphincter of the iris
with motor nerve, the two forming the oculo -motor centre,
o.M.c; I.R., internal rectus centre, with motor nerve to
internal rectus muscle; o.n., optic nerve from retina to o.c,
optic centre, and connected with p, the papillary centre;
X is the seat of the lesion causing reflex pupillary immobility.
vision is present or not, is Snellen's apparatus, described
on page 153.*
Whereas, then, in discussing accommodation we con-
sidered the eye simply as an optical apparatus, now we
* Any of th3 tests for latent deviation, mentioned later, may
also be employed.
44 THE REFRACTION OF THE EYE
must consider the two eyes together as forming one whole,
and on their proper associated movements must depend
perfect binocular vision.
If binocular vision be impossible, through some great
defect of the optical apparatus or the muscles, no attempt
will be made to produce it, and no strain will follow. On
the other hand, apparently normal binocular vision may
exist; but to produce this, a demand in excess of the
power is put upon a muscle or a set of muscles, and the
result is strain, either producing or tending to produce
the symptoms of muscle strain.
The Relation of the Two Eyes to Each Other in
Normal Distant Vision. — Michael Foster says that the
primary position of the eyes is " that which is assumed
when, with the head erect and vertical, we look straight
forwards to the distant horizon; the visual axes of the
two eyes are then parallel to each other and to the
median plane " — that is, in ideal binocular distant
vision, the eyes being at rest and all the muscles in
equilibrium with respect to each other, the visual axes
are parallel.
Test for Latent Deviation of the Eyes for Distance. — If a
person with normal vision be directed to look at an object
in the distance, and one eye be covered for twenty or
thirty seconds, if there be any latent deviation it becomes
as a rule manifest, and on uncovering the eye there will
be diplopia for a brief space of time, the covered eye
moving (in order to fuse the two images) — in, if there be
latent divergence, and out, if convergence. A more
accurate method of conducting this test is to destroy the
possibility of binocular vision — i.e., fusion — by means of
a prism, with its base up, placed before one eye, or, better
still, by the apparatus suggested by'^Maddox, called the
" glass rod test "; by which means we can not only at
once detect concealed deviation, but can also measure
the amount.
The Maddox Test. — A glass rod (Fig. 31, a) is arranged
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MADDOX ROD TEST 47
in a metal disc, which fits into the trial frame.* If this
rod be placed before one eye, the other eye remaining
uncovered, and a small flame be looked at from a distance
of more than 4 metres, the eye in front of which the rod
is placed sees the flame merely as a streak of light, and,
the images of the tv/o eyes being so dissimilar, there is
no desire on the part of the brain to fuse them; conse-
quently the two eyes assume their position of rest. If
the rod be placed horizontally in front of the right eye,
there is a vertical streak of light, and if this streak
coincide with the image of the candle seen by the left
eye, the visual axes are parallel (orthophoria) ; but if it
Fig. 31.
do not, then when the streak is on the same side as the
rod (in this case the right side) there is latent convergence
(homonymous diplopia), when on the other side there is
latent divergence (crossed diplopia). If a scale be used
as suggested by Maddox (see plate), the number on the
scale through which the streak of light passes records
the amount of diplopia ; or prisms may be put up in front
of the other eye, or the rotary prism used (see page 7).
The weakest prism that causes the two images to coin-
cide records the amount of diplopia. The Maddox
* Fig. 31, b, represents a simple form of this apparatus which
can be made by uniting four or five glass rods with seaUng-wax.
This must be held before the eye, as it does not fit into a trial
frame.
4^ THE REFRACTION OE THE EYE
distance scale is marked for 5 metres, and roughly every
3j° represents a metre angle. To be quite exact, every
3° 40' or 32 cms. is a metre angle. If the scale be used
at 4 metres, then every 25*5 cms. represents a metre
angle.
If we wish to measure vertical deviations, we turn the
rod vertically, and thus obtain a horizontal streak of
light. If this streak pass through the middle of the
flame there is no vertical deviation, but if it be above or
below there is hyperphoria of that eye which sees the
lower image — i.e., if the streak of light be lower there is
a tendency to upward deviation (latent hyperphoria) of
the eye in front of which the rod is placed. To measure
the vertical deviation we must use a scale, similar to
that in the Plate, placed vertically.
Before this test is applied any refractive defect must
be corrected. By making a large number of examina-
tions by this method, we can easily prove the correctness
of the statement, ihs^., for all practical purposes, the visual
axes of the two eyes in normal binocular vision are parallel.
So much for what is called the " static equilibrium " of
the ocular muscles. Now we proceed to examine the
dynamic condition — that is, the relation of the muscles
in binocular near vision; in other words, during con-
vergence.
Convergence " is the direction that the eyes must give
to their lines of fixation in order that they may be
simultaneously directed toward the point of fixation."
When both eyes are fixing an object 6 metres (or more)
distant, they are parallel, and C (which represents con-
vergence) = o; when the eyes simultaneously fix an
object I metre off in the median line, both internal recti
contract and the eyes converge ; convergence is then said
to be I metre angle, C = i m.a. This metre angle is the
unit of convergence. If the eyes converge to a point
50 cms. off, then C = -Vxr = 2 m.a.; if 20 cms. off,
C = ^■^^- = 5 m.a.; and if the object be 3 metres off.
AMPLITUDE OF CONVERGENCE 49
C = i = *33 m.a. In Fig. 32, E e is the base line con-
necting the two eyes, and E r' and e r' are two lines at
right angles to this base, and therefore parallel. If the
two eyes look at a point R, the angle r' e r is the " metre
Fig. 32. (After Nagel and Landolt.)
angle," or, better still, as r' e R is equal to E R p, the
latter may be called the " metre angle." To Nagel
belongs the credit of devising this method of measuring
the amount of convergence. The metre angle (or " Meter-
4
50 THE REFRACTION OF THE EYE
winkel," as he calls it) of convergence corresponds to
the dioptre of accommodation. Thus, an emmetfope
who is fixing binocularly a point i metre off is using
I dioptre of accommodation, and convergence is i m.a. ;
and if the point be 25 cms. off, he is using 4 dioptres
of accommodation, and his amount of convergence is
-W- = 4 metre angles, and so on.
Amplitude of Convergence. — ^We again use Bonders'
formula, and, expressing the equation in metre angles,
c a == c p — c r,
where "c a" represents the amplitude, "c p" the
maximum, and " c r " the minimum, of convergence.
When R is at finite distance (Fig. 32), we have
ca^cp- cr; that is, the amplitude of convergence
is the amount of convergence required to direct the
visual axes of the two eyes simultaneously to the point
P, starting from the binocular distant point R. When
the visual axes are parallel, " r " can be ignored, and
the equation stands —
c a = c p.
When the visual axes diverge, E r", e r", the axes will, if
prolonged backwards, meet at a point - R, which is
negative; the equation will then be —
ca = cp- (-cr) = cp+cr.
We distinguish the equation from that used in accommodation
by prefixing or affixing a "c," thus :
ca=:cp-cr,
a": = pc-rc.
The Punctum Remotum of Convergence. — Just as the
punctum remotum of accommodation is the expression of
the refraction of the eye when completely at rest, so the
punctum remotum of convergence is the expression of the
THE NEAR POINT 51
position of the eyes when at rest — that is, when the impulse
to fusion brought about by binocular vision is removed — so
that to find R we must find the latent position of the eyes
for distance. This we do by the Maddox test, and the
number of metre angles read off on the scale gives us
" c r." When there is no latent deviation " c r " = o,
when there is latent divergence " c r " is negative, and
when latent convergence it is positive.
To find " c p," the maximum of convergence, we
direct the person to fix binocularly a small test object
held, say, J metre from the eyes, equidistant between
them and on the horizontal plane of the eyes. This may
be a fine hair or wire stretched vertically in a frame, or
it may be a luminous slit, as in Landolt's ophthalmo-
dynamometer (Fig. 33); when the object is approached
to such a distance that the test line appears double, we
measure off the distance in centimetres, and divide this
into 100, which gives us the number of metre angles
that " p " is equal to. Suppose " c P " to be 10 cms.,
" c p " = "W = 10 m.a., and if c R be at " infinity,"
a = 10;
but if there be latent divergence, say of i m.a., r =
— I m.a., and
a = 10 - (- i) = 10 + I = II m.a.
In this test we must be careful to distinguish between
mere haziness of the test object, which is the result of its
being within the patient's accommodation near point,
and doubling of it, because the near point of convergence
is often nearer than that of accommodation. We should,
therefore, always first ascertain the accommodation near
point in each eye.
It is generally considered that the normal amplitude of
convergence is 10*5 m.a., although it may be 15 or even
17 m.a.
52 THE REFRACTION OF THE EYE
The Relative Range of Accommodation and Convergence.—
If the latent position of the eyes be tested, not only during the
fixation of distant objects and of objects at a reading distance,
but also for intermediate distances of fixation, it will be found
that, as a rule, there is quite a gradual lagging of the non-fixing
Fig. 33. — Landolt's Ophthalmo-Dynamometer.
This apparatus rests on a candle, which, when lighted, causes the
slit in the cylinder to appear as a luminous line.
eye behind the fixing one : a gradual increase of latent divergence.
This divergence is greater in myopia and less in hyperopia than
in emmetropia. Fig. 34 represents the average curve of relative
latent deviation in emmetropia. According to this figure, we
see that with parallelism, or a condition almost approaching to
CONVERGENCE
53
parallelism for distance, there is ^ metre angle of divergence on
accommodating for ^ metre, and a whole metre angle for ^ metre
accommodation — that is, that whereas, with both eyes fixing, on
accommodating for J metre, 4 d of accommodation is used, and
both eyes converge to a point using 4 m.a. of convergence,
when the possibility of fusion is removed both eyes only con-
verge to a point ^ metre off, using 3 m.a. of convergence.
This is no proof of the existence of " insufficiency " of con-
vergence ; all it shows is that the intimate relation between accom-
modation and convergence is not absolute.
All the more, then, should we expect to get latent divergence
for near points when there is initial latent divergence for distance.
When there is initial latent divergence for distance, the " lagging "
of the convergence behind the accommodation for near points is
-OC9, ID 22)
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more marked than when the position of the eyes is parallelism,
and this produces a " convergence insufficiency." We can
ascertain the presence of latent deviation in near vision by the
Maddox test. A scale (see plate, page 45) is held \ metre from
the eyes, and a prism of 12°, base up, is held before the right eye.
The scale consists of a horizontal line with fine print below it,
in the centre of which is an arrow pointing upwards. The line
is divided in degrees which are marked by figures, black on the
right of the arrow, red on the left. Every 3^° from the arrow
is marked by a small cross representing i m.a. The prism
causes two lines and two arrows to be seen, and the patient is
instructed to fix the upper arrow, or, better, the fine print just
below it. When there is no latent deviation the two arrows are
in a vertical line. When the lower arrow points to the left (red
side) of the upper arrow there is latent divergence, and when it
54 THE REFRACTION OF THE EYE
points to the right (black side) there is latent convergence for
^ metre, the amount of deviation being read otf on the scale.
Graefe's " dot and line " test is inferior to the foregoing, as it
does not record the amount of the defect.
Maddox maintains as a result of his experiments that in near
binocular vision there is always relative divergence — that is, con-
vergence always lags behind accommodation. This convergence
is composed of three factors: (i) " initial convergence " (this, of
course, exists only when there is latent convergence) due to
the relaxation of the external recti which are maintaining paral-
lelism {p p, Fig. 35), and the eyes assuming their position of rest
i i ; (2) accommodative convergence — i.e., the amount of con-
vergence which is called forth by the accommodative effort
which brings the axes to a a ; and, lastly, (3) the " fusion supple-
ment," which is the result of the desire for single vision, and
brings the axes to 0. This " fusion supplement " is demon-
FiG. 35. (Maddox.)
strated by holding a pen midway before the eyes of a patient at
the distance of the convergence near point, and telling him to fix
the tip of the pen; if now one eye is covered, this covered eye
will markedly turn out, and, on uncovering, the patient will for
a moment have diplopia, the eye making an incursion to recover
binocular vision. The amount of the excursion on covering, or
incursion on uncovering, represents the fusion supplement which
the demand for binocular vision calls forth. This experiment
can be made on most people, and is no proof of " insufficiency "
of convergence.
Although accommodation and convergence are intimately
connected, this connection is not absolute. We can prove this
experimentally by altering our accommodation without changing
our convergence, as in looking at an object with both eyes before
which we place weak convex and concave glasses, and also by
altering our convergence without changing our accommodation
CONVERGENCE
55
by placing before the eyes weak prisms, base in or out. The
amount of dissociation between the accommodative and con-
vergence efforts is limited, and varies with and in the individual ;
it can be increased by practice, and it differs for varying degrees
of accommodation and convergence. Fig. 36 shows the relative
amount of accommodation that can be used with different degrees
of convergence in an emmetrope aged 15.
The horizontal figures record the degrees of convergence in
metre angles, and the vertical figures record the degrees of
accommodation in dioptres. The diagonal d d represents the
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convergence, starting from zero — i.e., " infinity " — and stopping
at 5 cms. (20 metre angles). The vertical divisions between the
upper curved line p p and the diagonal, represent the amount of
maximum or positive part of accommodation, ascertained by the
strongest concave glass that can be borne without prejudice to
binocular and distinct vision, for any given point of convergence,
and those between the diagonal and the lower curved line y v
represent the amount of minimum or negative part of accommo-
dation, ascertained by the strongest convex glass. Thus, take
convergence for 6 m.a.: above we have 2*5 dioptres of positive
accommodation, and below 3 of negative accommodation — that
56 THE REFRACTION OF THE EYE
is, the relative amplitude of accommodation for 6 m.a. of con-
vergence is 5*5 in this individual. It will be seen that when the
convergence has reached lo m.a. the whole of the range of accom-
modation is negative.
Accommodation remaining fixed, we can estimate the amount
of relative convergence by means of prisms ; the strongest prism,
base out, that can be borne with fusion represents the positive,
and base in, the negative part, of the amplitude of convergence,
and, as Landolt has pointed out, we find that Fig. 36 can be made
use of to represent this. The diagonal d d represents the accom-
modation starting with eyes adapted for infinite distance; the
positive portion of the relative range of convergence is on the
right of the diagonal, and is represented by the horizontal
divisions between d d and r r, and the negative portion is on the
left. Thus for accommodation at 25 cms. — i.e., 4 dioptres — we
see that we have 3 m.a. on the right and 3 '5 m.a. on the left — that
is, while maintaining the same amount of accommodation,
an adducting prism producing a deviation of 3 m.a., and an
abducting prism requiring a diminution of 3-5 m.a., can be over-
come by the eyes. Thus for 4 dioptres of accommodative power
in this individual, an amplitude of convergence of 6-5 m.a. exists.
It is fortunate for the ametrope that this dissociation between
accommodation and convergence is possible. A hyperope of
3 D who fixes an object binocularly 7,^ cms. off must use an addi-
tional 3 of accommodation — that is, he must use 6 altogether —
but he will only require to converge to 3 m.a. If the association
between accommodation and convergence were absolute, he
would either have to converge to 6 m.a., and consequently squint,
and thus lose binocular vision, or he could keep binocular vision
on the condition that he did not accommodate for this near
point; in other words, he has the choice between distinct vision
and binocular vision — he cannot have both. Many hyperopes
dissociate these two efforts, and can by practice and " nerve
education " accommodate in excess of their convergence (see
page 90). The difference in the power to dissociate these two
efforts is one of the explanations of the well-known fact, that of
two individuals having the same refractive defect, one will squint
and the other not.
The same necessity for dissociation between convergence and
accommodation occurs in myopia. A myope of 3 d can see an
object 33 cms. off without any accommodation, but must con-
verge to the extent of 3 m.a. Thus he uses his convergence in
excess of his accommodation.
Donders stated that accommodation can only be maintained
for a distance when, in reference to the negative, the positive
part of the relative range of accommodation is tolerably great,
and that the relative range of accommodation in ametropic eyes
is quite different from that of emmetropic eyes, but that it tends
to approach the latter when the correction of the error has been
worn for some time.
CHAPTER V
THE OPHTHALMOSCOPE
To understand the action of the ophthalmoscope, the
following facts connected with the Optics of Reflection
should be remembered :
I. When light falls on a plane mirror (Fig. 37, A b),
the angle of incidence is equal to the angle of reflection.
The incident ray f d makes with the perpendicular
p D an angle f d p, and the reflected ray D E also makes
Fig. 37.
an angle E d p, and these two angles are equal to one
another. Both incident and reflected rays are in the
same plane, which is perpendicular to the mirror.
2. "When parallel rays of light (Fig. 38, A b and c d)
fall on a concave mirror, they are reflected to a focus (f)
in front of the mirror, and this principal focus is midway
51
58 THE REFRACTION OF THE EYE
between the mirror and the centre of curvature of the
mirror (o) and on the principal axis.
3. Rays of light coming from a point near the mirror,
but beyond its centre, as at L (Fig. 38), come to a focus (/)
between the centre and the principal focus, and the two
points are conjugate foci.
The Ophthalmoscope.
If by some contrivance we can manage to send rays of
light from a spot in front of our eye into another eye, we
shall get some of those rays returning to our eye after
Fig. 38.
being reflected from the retina of the observed eye, if
the media be clear, and the pupil of the observed eye,
instead of appearing black, will appear red.
This can be done in the simplest manner by a piece
of glass plate. If Ohd is the observed eye, and Ohr
the observer's eye (Fig. 39), in front of which is inclined
a glass plate G l, the rays of light passing from l are
reflected partly at g l into Ohd, return along the same
path, passing through the plate, and enter the observer's
eye. As only a few rays find their way to the observer's
eye, the light is very feeble. This was the principle
of Helmholtz's first ophthalmoscope, and he improved
THE OPHTHALMOSCOPE 59
it by placing together several glass plates, and thus
increasing the luminosity. If for these glass plates a
mirror with a central hole is substituted, more rays still
will pass into the eye; and these rays returning, more
will pass through the hole in the mirror into the ob-
server's eye, and a brighter image of the fundus will be
seen. A still greater improvement results if we use a
concave mirror, as the light is more concentrated.
Such is the simple ophthalmoscope — viz., a mirror
with a central sight-hole supported on a handle.
The ophthalmoscope has been further improved by
adding an arrangement of lenses of different strength,
which can be turned into position in front of the sight-
hole, so that if the eyes of the observer or observed are
ametropic, a clear image of the fundus can be obtained
by the correcting lens.
The Qualities of a Good Refraction Ophthalmcscope. —
The mirror should be concave, with a focus of from 14 to
17 cms. It should be oblique, and capable of being
turned round so that it can be used for either eye. This
obliquity of the mirror enables the observer to approach
very near the observed eye without cutting off any of the
light, and also permits the correcting glass, when used,
to be in a position parallel to the vertical plane of the eye.
When the oblique mirror is not required, as in the " in-
direct " method and in the " shadow test," a " straight "
6o
THE REFRACTION OF THE EYE
mirror should be substituted for it. This can be done by
changing the mirrors, or, better still, by an arrangement
like the nosepiece of a microscope, to which both mirrors
are attached, either of them being turned into position
as required. A further improvement can be made by the
" straight " mirror being plane on one side and concave
on the other, and fixed with a spring hinge, so that either
side of the mirror can be used as desired. The mirror
should be perforated; imperforate mirrors (with a
central hole in the silvering) are not so good, as the glass
reflects some of the light that should enter the observer's
Fig. 40. — Morton's Ophthalmoscope.
The rotating wheel is made to serve as a pupiliometer, the discs
being numbered from i to 8 mm.
eye. The aperture in the mirror should not be too small,
otherwise too little light will reach the eye of the ob-
server; its diameter should be about 3 mm. anteriorly
(the glass side), and somewhat wider behind, and the
sides of the tube should be well blackened. The cor-
recting lenses of the ophthalmoscope should not be too
small; they should have a diameter of not less than
5 mm. There should not be too many of them, and
never more than two superimposed. The best plan is to
have the glasses ordinarily used arranged round the rim
of one disc, and those less used arranged, either on
another disc, or on a movable quadrant. The number
THE OPHTHALMOSCOPE
6l
of ophthalmoscopes on the market is large, but the best,
and certainly the most popular, is Morton's (Fig. 40).
Fig. 41. — Electric Ophthalmoscope.
The Electric Ophthalmoscope (Fig. 41).— This instru-
ment has completely revolutionized direct ophthalmo-
62 THE REFRACTION OF THE EYE
scopy. The management of the Ught, when using the
old-fashioned instrument, has always been a trouble
to the beginner or the practitioner who only occasionally
uses the instrument; this trouble is removed in the
electric ophthalmoscope because the Ught is concealed
in a tube near the mirror and is fed by a battery in the
handle. But even for the oculist who is an adept at
using the old instrument the advantages of the electric
ophthalmoscope are very marked. As the light has
not to be considered, the instrument can be brought so
near the eye that it can almost touch the cornea, and
consequently a dark room is not necessary, as, by turn-
ing the patient with his back to the window, we can
easily examine the whole of the fundus, and, moreover,
the patient can be examined in any position.
A further improvement is obtained by using a Marple
Mirror. This mirror, instead of having a central
Fig. 42. — The Marple Mirror.
opening, has a U-shaped one (see Fig. 42), with the
result that there is little or no reflex from the centre of
the cornea, and it is often quite possible to examine the
macula through a pin-point pupil.
With this instrument and a combined concave and
plane mirror (Fig. 50, page 83), and a focusing lens, no
further apparatus is required, and the old-fashioned
refraction ophthalmoscope can be dispensed with.
The convex lens or focus glass used in the " indirect "
method should have a focus of about 8 cms. — i.e., be
about 13 D — and should have a diameter of about 6 cms.
The lens usually supplied with ophthalmoscopes is much
too small. The glass should be kept clean and free from
scratches.
THE OPHTHALMOSCOPE 63
The Different Methods of Examining the Eye with
the Ophthalmoscope.
1. The indirect method.
2. The direct method.
3. The " shadow test," or retinoscopy.
The patient should be in a darkened room.
The light used should be on an adjustable bracket if
possible; any kind of light will do if it has a broad,
steady, white flame, but the electric light in a ground-
glass globe is the best, as it gives off less heat.
Before commencing the ophthalmoscopic examination,
the eye should be thoroughly examined by the oblique
or focal illumination. For this purpose put the light
on a level with the patient's eye, on the same side as the
eye to be examined, and about 12-15 inches from it,
and with the focus-glass throw a luminous spot on the
cornea. By moving the lens about, the whole surface
of the cornea, the anterior chamber, iris, and anterior
surface of the crystalline lens, can be examined. This
examination is further aided by viewing the illuminated
spot through a strong magnifying-glass, and one of the
best is Voigtlaender's. This preliminary examination
gives valuable information as to the translucency of the
media, etc.
I. The Indirect Method. — Place the light close to
the patient's head and a little behind, so that no light
can reach the eye to be examined directly.
Use the " straight " concave mirror, holding it about
15 inches from the eye, thus lighting up the fundus,
and making the pupil appear red (if the media are
transparent), and detecting opacities of the cornea, lens,
and vitreous (the latter are best seen with a plane
mirror and faint light) .
Still using the same mirror, put up the focus-glass,
holding it by the left index-finger and thumb, and
64
THE REFRACTION OF THE EYE
steadying it by resting the remaining fingers of the
left hand on the patient's brow. By this means an
inverted image of the fundus is seen. This is called
the " indirect method."
The observer recognizes that the picture is inverted
by slightly moving his head or the focus-glass, and
finding that the image moves in the opposite direction.
The manner in which this inverted image is formed
is shown by the following figures.
Fig. 43. (After Fick.)
The focus-glass held in front of the eye makes the
eye myopic, and, according to the refraction of the
eye, this inverted image will be nearer or further from
the lens.
When the observed eye is emmetropic, the rays
coming from the eye (Fig. 43, e) are parallel, and focus
at the principal focus of the focus-glass; and, more-
over, as the rays emerging from the eye are parallel, it
does not matter where the focus-glass is placed ; nearer
THE OPHTHALMOSCOPE 65
or further from the eye, the image must necessarily
always be the same size.
In hyperopia (Fig. 43, h) the rays emerging from the
eye are divergent, and, passing through the focus-
glass, they form a larger image than in emmetropia,
and this image is further from the lens in front of its
principal focus; on withdrawing the lens from the
eye, the image is formed on the other side of the lens,
nearer and smaller.
In high hyperopia the image is so far in front of the
focus-glass that the observer will have either to move
back, or to accommodate, in order to get a distinct
view of the inverted image.
If with the mirror alone, still held at some distance
from the eye, we can recognize fundus details nol in-
verted — that is, in their true position — we are dealing
with high hyperopia.
In myopia (Fig. 43, m) the rays emerging from the
eye are convergent, and form an inverted aerial image
in front of the eye, and the focus-glass shows this
image smaller than in emmetropia, and nearer to the
lens — in fact, within its principal focus; on withdraw-
ing the lens the inverted image becomes larger.
In high myopia no focus-glass is required to see
the fundus, as the rays proceeding from the eye are so
convergent that they come to a focus at the punctum
remotum and form an inverted image.
In astigmatism the disc may appear oval, and the
shape will alter as the focus-glass is withdrawn, accord-
ing to the refraction of the different meridians.
The advantages of the indirect method are —
1. The examiner is further from the patient than in
the direct method (a distinct advantage in dealing with
certain patients).
2. A general " bird's-eye " view of the fundus is
obtained.
3. No correcting glasses are needed in the ophthal-'
66 THE REFRACTION OE THE EYE
moscope; thus, a simple concave mirror with a central
hole is sufficient.
4. It is sometimes easier to see the fundus when the
pupil is small.
In looking at the right disc, the patient should be
directed to look past the observer's right ear, for the
disc is on the nasal side of the posterior pole of the
eye, and on looking at the left disc he should look
past the left ear. It is important to remember that
the patient must look with the eye not being examined ;
therefore, in examining the left eye by this method,
take care not to obscure the right eye with the hand
that is holding the focus-glass.
2. The Direct Method.— As already stated above, this
method is much simplified by the use of the electric
ophthalmoscope. If the old-fashioned instrument is
used the light must be brought quite close to the patient's
head and slightly behind, and on the same side as the
eye to be examined. The observer sits (or stands in a
stooping position) close to the patient, and on the same
side as the eye to be examined, using his right eye for the
patient's right eye, and his left for the patient's left.
Use the refraction ophthalmoscope (without the
focus-glass) and the oblique concave mirror. Hold-
ing the ophthalmoscope a few inches from the eye,
reflect the light on to the eye and observe the red
pupillary reflex through the central hole of the mirror,
and then, without allowing the light to leave the eye,
approach the eye as near as possible; in fact, the ob-
server's forehead ought to touch the patient's forehead.
The fault that most beginners make is not getting
near enough to the eye. The observer must not ac-
commodate, but look, as if trying to see through the
patient's head, into distance. If the observer or
patient have an error of refraction, the wheel of the
ophthalmoscope must be turned until the suitable
glass is found. To see the macula, the patient should
THE OPHTHALMOSCOPE b'/
be told to look horizontally, in front; if the disc is to
be examined, he should look slightly to the nasal side.
Fig. 44. — Examination of the Erect Image when the Eye
EXAMINED IS HyPEROPIC, EMMETROPIC, OR MyOPIC. (HaAB,
after Fick.)
In each figure three rays are shown emanating from a luminous
point on the eye-ground. In hyperopia they diverge after
leaving the eye, in emmetropia they are parallel, in myopia
they converge : /, the posterior focus ; H, principal plane of
the dioptric system of the examined eye ; Be., observer. The
ophthalmoscope is not shown.
Only a small portion of the fundus can be seen at
one time, but this portion is considerably magnified
68 THE REFRACTION OF THE EYE
(about 15 diameters), and consequently the minutest
details are visible.
By this method the refraction of an eye can be esti-
mated, which as an objective method has, of course, a
distinct advantage.
The first duty of the observer — and most beginners
find this very difficult — is to relax his accommodation.
The person whose eye is being examined must also
relax his accommodation, which can be done by direct-
ing him to look at some object 5 or 6 metres off with
the other eye, or, better, by paralyzing the ciliary
muscle with a cycloplegic. If both the observer's and
obr
Fig. 45.
the observed eye are emmetropic, all the details of the
fundus will be clearly seen (Fig. 44, B). We can easily
understand this, when we remember that rays passing
from the mirror to the back of the eye that is being
examined, are reflected as parallel rays if the eye be
not accommodating and be emmetropic, and that
parallel rays must be focused on the fundus of the
observing eye if it also be emmetropic, and its accom-
modation be relaxed (Fig. 45). If, on the other hand,
the observer's eye under these circumstances accom-
modate, the image, instead of being sharp, is blurred.
It is not only necessary to observe these rules in order
to get a clear picture of the fundus, but it is of para-
THE OPHTHALMOSCOPE 69
mount importance if we wish to estimate correctly
, the refraction of the eye we are examining. For this
reason it is important that the observer should esti-
mate his own refraction, and, if there be any error,
correct it.
If the observer be myopic, the fundus will be indis-
tinct, just as is the case wuth all distant objects, for
the rays coming from the observed ieye are parallel —
that is, as if coming from a distant object. In order,
therefore, to obtain a clear view of the fundus, the
myope must use a concave glass, and the weakest
concave glass he can see distinctly with will be the
measure of his myopia, if his accommodation be relaxed .
A hyperopic observer is in a somewhat better posi-
tion, because he can see the fundus if he accommo-
dates; but as he must relax his accommodation in
order to estimate the refraction of the eye he is exam-
ining, he must first find his own refractive defect and
correct it. Unless he had his defect properly corrected
in early youth, he has become so accustomed to use his
accommodation that it will be most difficult-^almost
impossible — for him to relax it, and the probability is
that, although the convex glass he uses corrects his
defect, he nevertheless cannot help using some accom-
modation, and will thus overcorrect himself, render-
ing himself myopic. It necessarily follows, therefore,
that it is most difficult for a hyperope to estimate the
refraction of an eye correctly by this method. He
should use some other method, such as the " shadow
test," which will be explained later.
We have supposed up to now that the observed eye
was emmetropic. We will proceed to examine the
conditions that exist when the observed eye is myopic
or hyperopic.
Examination and Measurement of a Myopic Eye
by the Direct Method. — The retina of a myopic eye is
at the conjugate focus of an object situated at finite
7& THE REFRACTION OF THE EYE
distance (see page 99); consequently rays proceeding
from the retina of a myopic eye are focused at the far
point when the accommodation is relaxed (Fig. 44, C).
As this far point is at finite distance — in fact, near the
eye — the rays are convergent; consequently they will
not be focused on the retina of an emmetropic eye
unless they are made parallel by using a suitable con-
cave glass in the ophthalmoscope. This is done by
turning the wheel of the instrument and bringing con-
cave glasses before the opening, and the weakest con-
cave glass required is the measure of refraction (if the
accommodation of both the observer's and the ob-
served eye is relaxed).
The observer will, of course, be able, by using his
accommodation, to see the fundus with a stronger
concave glass than is required, but it will not then
be the measure of the myopia. If the observer be
a myope, and his myopia be not corrected with glasses,
to ascertain the refraction of the observed eye he
must deduct from the concave glass he requires the
amount of his own myopia. When, for instance, the
weakest concave he requires to see clearly the retina
of the myopic eye is -5, and he himself is -2, then
the observed eye is — 3. When he is hyperopic, he
must add the amount of his hyperopia — i.e., when
he has hyperopia of 2, and the weakest glass he re-
quires is -5, the amount of myopia in the observed
eye is - 7.
The Examination and Measurement of a Hyperopic
Eye by the Direct Method (Fig. 44, A). — The rays emerg-
ing from a hyperopic eye are divergent (see page 85), and
as they must be made parallel for an emmetropic ob-
server if he wish to see the fundus clearly, a convex
glass, representing the amount of hyperopia, must be
turned into position. If the patient have hyperopia
of 4, then + 4 must be used. The fundus could be
seen clearly without a glass, by accommodation; but
THE OPHTHALMOSCOPE 7 1
then, as it would be impossible to measure the amount
of accommodation used, so would it be impossible to
estimate the amount of hyperopia in the observed eye.
A hyperope who is examining a hyperopic eye with
the ophthalmoscope must deduct the amount of his
own hyperopia from the strongest lens he requires to
see the fundus with; i.e., if he be hyperopic to the
extent of 3, and the strongest convex glass he can
clearly see the fundus with, is +6, the observed eye
has a hyperopia of 3 d.
A myope, on the other hand, as he requires a weaker
correcting glass, must add in dioptres the amount of
the defect; thus, when he is myopic to the extent of 5,
and requires no glass to see the fundus clearly, the
eye that is being examined is hyperopic to the extent
of 5. Again, when his myopia is 3, and the strength
of the convex glass he can use is 2, the amount of
hyperopia present in the observed eye is 5 ; or when he
has myopia of 7, and he cannot see the fundus clearly
with any glass less concave than 3, the amount of
hyperopia present is 7 + ( - 3) — i.e., 4 d.
To summarize, we may say that if an ametrope, to
see clearly the fundus of an eye with the ophthalmoscope
and to estimate correctly its refraction, requires —
(i) A glass of the same kind as his own ametropia,
but stronger, he must deduct the number of his own
from that glass.
Example. — He has a myopia of 3, and requires - 5 in the
ophthalmoscope, then the error of the observed eye is - 2.
(2) A glass of the same kind, but from one to ten
dioptres weaker than his own ametropia, then the eye
that is being examined has an ametropia of from one
to ten dioptres of the opposite kind.
Example. — He has a myopia of 6, and requires - 5 ; the refrac-
tive defect of the observed eye is + 1 . If he require - 4, it is
+ 2, and so on. He has hyperopia +4, and requires +3; then
the refractive error of the observed eye is - i ,
72 THE REFRACTION OF THE EYE
(3) A glass neither of the same kind nor strength,
then the refraction of the observed eye is the opposite
to that of the observer's, and the amount is equal to
the addition of the number of dioptres of each.
Example. — He has myopia of 5, and requires + 3 ; the refrac-
tion of the observed eye is + 8. He has hyperopia of 3, and
requires - 2 ; the error is - 5.
It should be borne in mind that, to insure the exact
measurement of the patient's refraction by means of
the ophthalmoscope, the yellow spot must be looked
at. If the patient be not under the influence of a
mydriatic, this is not always easy, for not only does
the pupil contract when the examined eye is turned
towards the mirror, but the light reflex from the cornea
interferes very much with the view unless the electric
ophthalmoscope with Marple mirror is used; and,
further, the absence of any large structure, such as the
retinal vessels, makes it diihcult to secure the correct
focus. As a rule, all that is seen at the macula is a
slight stippling, produced by the irregular deposit of
retinal pigment, and when this pigment is specially
pronounced we get a bright ring or crescent at the
fovea. This is the foveal reflex; and although it is
slightly in front of the retina, the distance is so small
that it can be ignored, and this foveal reflex can be
focused and made use of in this manner, for ascertaining
the refraction. If it cannot be used in this way, through
being too faint, we must focus a small retinal vessel
passing from the disc to the macula.
The beginner will find that the easiest part to focus
is the temporal side of the disc, for its margin here is
generally very well defined.
The Measurement of Astigmatism. — It is very diffi-
cult, if not impossible, to diagnose low errors of
astigmatism by the ophthalmoscope, but an error of
one dioptre or more is revealed by portions of the
fundus picture being out of focus, and by our inability
THE OPHTHALMOSCOPE 73
to get a clear picture of all parts at the same time by
any of the spherical glasses in the ophthalmoscope.
Some ophthalmoscopes have cylindrical glasses fixed
in them, but this is not at all necessary, as the astigma-
tism can be approximately estimated without much
difficulty by measuring the refraction of the meridians
at right angles to each other in the following manner:
Focus, for instance, the vessels that pass in a horizontal
direction from the disc to the macula, and note the
glass in the ophthalmoscope (the weakest concave and
strongest convex) that is required to give a clear
definition; this will give the refraction of the meridian
at right angles to the horizontal one — viz., the ver-
tical. Then focus the vessels that pass vertically
upwards and downwards from the disc; this will give
the refraction of the horizontal meridian, and the
difference between the two glasses (if any) is the amount
of the astigmatism (if any).
When the chief meridians are not vertical and hori-
zontal, but oblique, we can then, say, focus the vessels
passing upwards and outwards from the disc, and
when we have focused these vessels, if astigmatism
exist, the vessels passing downwards and outwards will
not be in focus, but will be either blurred or invisible,
and we proceed to find the glass that is necessary to
bring these latter vessels into focus, and so on. If
the correcting glass be a large one, we must be care-
ful to look through the centre, for if we look through
the glass obliquely we shall get an appearance as if
produced by astigmatism, which might not be present.
In estimating the refraction by means of the ophthalmoscope,
as above explained, the observer shoald approximate his eye as
much as possible to the eye that is being examined, as the value
of the lens is altered by altering the distance; a concave glass is
weakened and a convex glass strengthened by removal from
the eye. It is for this reaso.i that old people are often seen to
wear their glasses low down o.i the nose, the strength of the
CO a vex glass being slightly increased. This, of coarse, specially
refers to lenses of high po.ver; therefore, the further away we
74 THE REFRACTION OF THE EYE
hold the ophthalmoscope the more shall we overcorrect in
myopia and undercorrect in hyperopia — i.e., the myopia of the
eye being examined will be less, and the hyperopia more, than
that represented by the ophthalmoscope glass.
3. The Estimation of the Refraction by the " Shadow
Test " ; Retinoscopy ; Skiascopy. — Seated at a short dis-
tance from the patient in a dark room, if we throw the
light on to the patient's eye by means of an ophthal-
moscopic mirror, provided the pupil is normal and the
media are clear, we observe the red reflex of the fundus ;
and if we gently rotate the mirror, the red reflex dis-
appears, and darkness takes its place. The manner in
which this darkness or shadow appears varies according
to the refraction of the eye.
We will examine the behaviour of the shadow under
three conditions :
1. When the observer is beyond the patient's far
point.
2. When he is within the patient's far point.
3. When he is exactly at the patient's far point.
I. Let us suppose the surgeon Ob (Fig. 46, A) examin-
ing the patient Pt by this method, and using the plane
mirror, and we will assume Pt to have a refractive error of
over I of myopia. Ob is seated i metre off Pt, and is
consequently beyond P/'s far point. Ob reflects the light
into Pt^s eye and observes the red reflex ; and if he rotate
the mirror, making the light pass, say, across Pfs face
from the nose to the temple, he will notice that the red
reflex disappears, and that darkness takes its place, and
in this example the darkness or shadow comes over the
eye from the temple towards the nose — that is, in the
opposite direction to the rotation of the mirror.
Let us see how this has come about. In Fig. 46, for
the sake of clearness, the mirror and the light have been
omitted, and only the rays proceeding from P/'s fundus
have been drawn.
All luminous rays proceeding from the fundus of Pt
Fig. 46. (After Fick.)
RETINOSCOPY 77
through the pupil P p (Fig. 46, A) either do not reach the
eye Ob, or they impinge on Ob's retina between P' and p\
Thus, all rays from p, from whatever part of Pt's fundus
they come, must unite at p' of Ob if they are intercepted
by 06' s pupil.
Let a he a, luminous point on the fundus of Pt (who in
this case is assumed to have a myopia of ov^ i), then
at Pi's far point, somewhere on the line between a and
the nodal point, an aerial image a' of a will be formed.
Some of the diverging rays from a' will reach Ob, and,
passing through the refractive media, will unite at a"',
but as Ob's fundus intercepts these rays, a bright
diffusion circle will be formed on the upper part ot P' p'
{Ob's fundus), while the lower part of P' p' will be in
darkness. Now, as our retinal images are projected
inverted, Ob sees the pupil of Pt light below and dark
above. If the luminous spot a descend to b in Pt, its
image ascends to b\ and we have a bright area below in
P' p', and Ob sees in P^'s pupil a bright area passing
from below upwards.
We thus see how in myopia of over i d, with the
observer i metre from the patient, and using a plane
mirror, the " shadow " moves against the rotation of t) (
mirror.
2. The reverse obtains when Ob is within Pt's far poin^.
Let us suppose (Fig. 46, B) Pt to be hyperopic. The
image of a will be at a', but those rays that pass through
Ob's pupil are refracted, and meet at a" in front of the
retina, and, diverging again, meet Ob's retina at p' as a
diffusion circle ; in this case the bright area, being below,
is projected inversely, and Ob sees P^'s pupil bright
above and dark below, and if a moves down to b, it will
be seen that Ob projects the bright area moving down
also. Thus, with a plane mirror, if Ob be within Pt's
far point, the shadow moves with the mirror; if Ob
be seated i metre off Pt, this will occur in hyperopia,
emmetropia, and myopia of less than i d.
78 THE REFRACTION OF THE EYE
3. £,astly, let us consider what happens when Ob is
exactly at P^'s far point, which, of course, occurs if Ob
be seated 1 metre off Pt, who has a myopia of i d
(Fig. 46, C). The illuminated point a has its image
a' exactly on the pupil of Ob, and as the ray p a' is re-
fracted to p' , and the ray P a' to 0', the entire area P' p'
is illuminated, and the entire pupil of Pt appears illumi-
nated to Ob. Movement of « to 6 produces no effect;
the area P' p' is still illuminated; but when the luminous
point on P^'s retina passes below b or above a — that is,
outside the area a b — it is focused on 06's iris, and no
rays reach 06's retina ; consequently Ob sees the pupil Pj^
becoming suddenly dark, and there is no moving shadow.
This point, when the observer's eye is exactly at the
patient's far point, is called the " point of reversal," and
the whole principle of retinoscopy is to find this point.
In myopia Ob can move nearer to or further from the
patient, and measure off the distance of the point of
reversal, and so obtain the refraction of that particular
meridian ; but in hyperopia this cannot be done, so that
the best method is to work always at one fixed point —
say I metre — and make the patient artificially myopic,
if hyperopia or emmetropia exist, by placing before
P^'s eye convex glasses ; if he be myopic, make him less
myopic by using concave glasses.
Let us now examine a patient. The patient's eyes
should (if possible) be under the influence of a cycloplegic,
which not only gives us a dilated pupil and makes the
retinoscopy easier, but insures the relaxation of the
ciliary muscle, which of course is essential. The patient
should be seated in a dark room, with the light above or
on one side of his head and slightly behind, so that no
rays can reach the eye except from the mirror. We pro-
vide ourselves with a set of test lenses and a trial frame,
and seated, say, i metre off the patient, we reflect the
light by means of the plane mirror into the patient's eye,
"directing him to look at the sight-hole of the mirror.
RETINOSCOPY 79
Suppose we are. examining the right eye, and rotate the
mirror so that the Hght passes across from the patient's
nose to the temple, and suppose we notice that as the
Hght leaves the pupil a dark shadow takes its place,
passing across in the same direction — i.e., from the nose
to the temple — we know from Fig. 46, B, that we are
within the patient's far point, and that we are dealing
with a hyperope or emmetrope, or myope of less than i .
Let us place in the trial frame + 2 : we find, say, that the
shadow is still moving with the mirror; we are therefore
dealing with hyperopia. Put up +4: the shadow now
moves against the mirror, which means we are outside
the patient's far point; put up +3, and we find on
rotating the mirror that the pupil becomes suddenly
dark, and there is no shadow following with the rotation,
or passing against it. This, then, is the point of reversal.
We have found the point of reversal with a +3 lens,
seated i metre from the patient, which means that this
meridian has a myopia of i with a +3 lens in front, and,
deducting i from 3, leaves us 2 as representing the
hyperopia. If the patient had been emmetropic in this
meridian, a + 1 lens would have given us the point of
reversal at i metre.
If the patient's eye have a myopia of over i, when
we are seated i metre off we must be outside his far
point, whatever the amount of myopia, and the shadow
moves " against " the plane mirror, and that glass which
gives us the point of reversal represents the amount of
myopia of that meridian with - i added if we are seated
I metre off (or - 2 added if seated 50 cms. off, or - •5
added if 2 metres off) — -that is, if - 5 gives the point of
reversal, - 6 is the amount of myopia. Some surgeons
always aim at reversing the shadow — 'that is, they
purposely go beyond the point of reversal, and slightly
overcorrect. This is quite safe if allowance be made fof
the overcorrection. <
In the examples, we have been ascertaining the far
80 THE REFRACTION OF THE EYE
point of one meridian only — viz., the horizontal; we
must now proceed to examine the meridian at right
angles — viz., the vertical — and if the same glass give us
the point of reversal, we know that no astigmatism is
present; but if there be a difference, that difference
represents the astigmatism. When the astigmatism is
great, and especially when one meridian is emmetropic
or made emmetropic, the light is seen to pass across as a
bright band (Fig. 47), and sometimes two bright bands
are seen with a dark band in the centre, and as the
bright bands approximate each other, the central dark
band disappears, and one bright band remains. This
has been called the " scissor movement." ,
Fig. 47.
In oblique astigmatism, of course, the meridians are
not vertical and horizontal, and when the astigmatism is
marked, the appearance of the shadow is very character-
istic, and a bright band is seen passing obliquely across
the pupil, although we may be moving the mirror hori-
zontally or vertically (Fig. 47). Suppose we are dealing
with oblique mixed astigmatism, and, rotating the
mirror horizontally, we observe a bright band fol-
lowed by a shadow passing obliquely across the pupil
" with " the plane mirror, we note the axis of this bright
band, and also note that the meridian is hyperopic;
if we then rotate the mirror at right angles to this bright
band, we find that the shadow passes against the move-
RETINOSCOPY 8l
ment of the mirror, showing that this meridian is myopic.
Suppose the point of reversal of the horizontal oblique
meridian is obtained by -1-3, and that of the vertical
by - I, by this we know that the refraction of the hori-
zontal meridian is +2, and that of the vertical meridian
is - 2, and there is therefore a total astigmatism of 4.
The greater the ametropia, the nearer is the far point
to the eye, and it is of great practical importance to
remember that, the greater the ametropia, the less
distinct is the shadow and the slower it moves; and as
we approach the point of reversal by using correcting
glasses, we obtain an increasingly defined shadow which
Fig. 48.
moves more and more rapidly. We can thus, at once,
make a rough estimate of the degree and kind of
ametropia.
In Fig. 48, if R be the far point of the myopic eye, on
rotating the mirror, the shadow moves from R to r' ; but
if the myopia be less, and the far point at M, the shadow
will have to describe the larger arc M m' in the same
time — that is, it will move more quickly.
In the same way, if the far point of a hyperope be at
R (Fig. 49), the shadow will move more slowly than
when the hyperopia is less and the far point at H.
Some surgeons use the plane mirror at a distance of
4 metres always, and as only •25 d has to be deducted
from the retinoscopy, this small amount can be ignored,
6
82 THE REFRACTION OF THE EYE
and the point of reversal of a meridian represents the
measurement of that meridian.
When the surgeon is nearer than i metre, he must, of
course, deduct more than i. For instance, suppose at
33 cms. the point of reversal of a meridian is obtained
with +5, as his far point is J metre off, we have made
the patient artificially myopic to the extent of 3, and
we must deduct this from 5; therefore 5-3 (that is, 2)
represents the hyperopia of this meridian.
Many surgeons still use the concave mirror. The
mirror should have a focus of 25 cms., and the observer
should be seated a little over a metre from the patient.
The movement of the shadow is the reverse of that which
takes place with the plane mirror — that is, the shadow
Fig. 49.
moves " with " the mirror in myopia of over i, and
" against " the mirror in myopia of less than i, and in
hyperopia and emmetropia. This is easily understood
if we remember that with a concave mirror the rays of
light converge to the focal point of the mirror, and then
cross and diverge ; consequently the image thrown on a
screen by a concave mirror is inverted.
[If we reflect a lighted candle on to a dark screen by a concave
mirror held further from the screen than its focal distance, and
if we then focus the divergent rays with a convex lens, we shall
get an erect image, because the rays have been twice inverted,
whereas with a plane mirror used in the same manner we obtain
an inverted image, because there has been only one inversion.]
If the observer be not emmetropic, he should wear his
correcting glass. This, of course, especially applies if
RETINOSCOPY 83
he be myopic. If he be hyperopic, he may correct his
defect by accommodation if he choose. The point to be
remembered is, that to practise retinoscopy accurately
the observer requires a normal acuity of vision. He
Fig. 50.
may accommodate as much as he likes, as it does not
affect the result.
As the point of reversal is more definite when the
shadow moves with, it is not a bad plan to use a plane
''^■^#;"'^'. ^ ^^ 4r -^ ^
Fig. 51. — Marple's Skiascopes.
mirror when estimating hyperopia, and a concave mirror
when estimating myopia. These two mirrors can be
hinged together, and thus each mirror is the handle and
cover of the other (Fig. 50).
Marpie's skiascopes (Fig. 51), made by Meyrowitz of New
York, are very useful, and obviate the necessity for keeping a
84 THE REFRACTION OF THE EYE
separate test case in the dark room; they are designed to be held
by the patient before the eye during retinoscopic examination.
Each contains a series of six lenses, ranging from i to 6 dioptres,
plus and minus respectively. In addition to these lenses there
is on one side a movable slide containing a 6 d lens, which can be
quickly slipped up over the other lenses one after the other,
making further combinations from 7 d to 12 d. To determine
smaller errors within i d, a slide containing three lenses -25,
•50, and '75 d, respectively, is placed on the other side, and
can easily be brought before the other lenses. On the skiascope
containing the plus lenses the movable slide carries minus fraction
lenses, and vice versa.
CHAPTER VI
HYPEROPIA
Hyperopia or Hypermetropia. — The hyperopic eye is
the undeveloped eye in which, with accommodation
at rest, parallel rays come to a focus beyond the retina
(Fig. 20, h), and only convergent rays focus on the
retina; but as in Nature all rays are either parallel
or divergent, it follows that the hyperopic eye at rest
sees everything indistinctly.
Rays coming from a point on the retina diverge,
and, on passing through the dioptric system, emerge
from the normal eye as parallel rays. In hyperopia,
although they are not so divergent as they were before
refraction, they still diverge if the eye be at rest, and
therefore never come to a focus in front of the eye;
Jut when prolonged backwards, they will meet at
a point behind the eye — the punctum remotum. This
punctum remotum of the hyperope is therefore not
the actual focus of the distant rays, but the virtual
focus, and is represented by the negative sign -R
(Fig. 52).
It will be seen from Fig. 52 that the more divergent
the rays are in front of the eye, the nearer will their
" backward prolongation " focus ; hence the nearer
- R is to the eye, the higher will be the hyperopia.
This is the same as in myopia — viz., the higher
the myopia, the nearer is r to the eye; but the differ-
ence is that in myopia R is in front of the eye, and
in hyperopia it is an imaginary point behind the eye.
«5
86 THE REFRACTION OF THE EYE ^ . '
Thus, the degree of hyperopia is in inverse ratio to
the distance of the punctum remotum. In myopia
this point can be measured directly, but in hyperopia
it can only be done indirectly by employing convex
glasses.
Suppose the punctum remotum of a hyperope is
33 cms. behind the retina. We have seen that a convex
lens whose focal point is 33 cms. is 3 d — that is, such a
lens has the power of converging parallel rays to a
point 33 cms. on the other side of the lens, and con-
versely, rays diverging from a point 33 cms. in front of
Fig. 52.
Showing the punctum remotum of a hyperopic eye.
such a lens become parallel on passing through. If
this lens be put in front of the eye of this hyperope, it
will so act that, assisted by the dioptric system of the
eye, it will cause parallel rays to focus on the retina.
Hence the measurement of hyperopia is that convex
lens which enables the hyperopic eye, at rest, to see
distinctly objects at a distance, and the focal length of
such a lens represents the distance of the virtual far
point from the eye. In the above example it was found
that +3 was this lens, and we accordingly say that
this eye has a hyperopia of 3.
HYPEROPIA
87
A hyperope differs from a myope in that he can
correct his defect up to a certain point; he can by
accommodation produce the same effect on parallel
rays as if a convex glass were placed in front of the
eye. This apparent advantage brings with it many dis-
advantages — viz., all the troubles incident to eyestrain.
The hyperopic eye is never at rest; it has to accom-
modate for distant as well as for near objects. The
emmetrope's ciliary muscle is at rest when he is looking
at any object 20 feet off, or beyond, but the hyperope's
Fig. 53.
Showing parallel rays focused on the retina of a hyperopic eye
by means of a convex lens.
eye is never at rest if he attempt to see distinctly;
and, moreover, when he wishes to look at a near object,
he starts with a deficit, which deficit is the amount of
accommodation he required for distant vision. ' Thus,
a hyperope of four dioptres, with five dioptres of
accommodation, can focus distant objects clearly,
but then he has only one dioptre left for near vision;
this will only bring his near point to i metre from the
eyes. Again, take a hyperope of two dioptres, with
five dioptres of accommodation: he has only three
dioptres available for accommodation for near objects;
88
THE REFRACTION OF THE EYE
this brings his near point to 33 cms., but he is using the
whole of his accommodative power for this, and it is
impossible for him to do this for long without fatigue,
and so we get all the symptoms of eyestrain.
We have seen that in hyperopia a = p-(-r) = p+r
(page 35), therefore p = a-r ; in other words, the
fO /S 20 2S 50 3S 40 46 SO 66 GO 66 70 76 SO
//
m
3
\
H
\
7
\p
fi
\
5
\,
4-
N
J
\
?
k.
•^/
Sj
V
a
-/
--■
—
•--
---
--
V
^
-_.
.__
--■
...
.--.
?
'v
,T
v^
^
<t
^
S
6
r
" —
^v^
P
f
r
^
7
\4
8
Fig. 54.*
Showing the range of accommodation of an uncorrected hyperope
of 4 D at different ages.
The numerals above represent years, those on the left, dioptres.
The line p p represents the curve of the punctum proximum, and
the line r r that of the punctum remotum.
available amount of accommodation is represented by
the total accommodation less the amount required
to correct the hyperopia, so that although, as we have
* As Bonders' diagrams are still universally used I have re-
tained them, but I would refer the reader to Chapter IX.
(Presbyopia).
HYPEROPIA 89
seen, age for age the hyperopic eye has the same total
amount of accommodative power as the normal eye,
it has less to use for near vision, if uncorrected. In
Fig. 54 the amount of available accommodative power
in the uncorrected hyperope of 4 d is represented
by the number of dioptres between p and the zero
line; thus, at the age of 30 we find only two and a
half, representing 2»5 d of accommodative power,
because, although he has 6*5 d total power like the
emmetrope, 4 of this is used up to correct his defect.
At the age of 40 we see that p crosses the zero line;
in other words, all available power is lost. He has
4 D left, but this is used up in correcting his defect.
Beyond this age he loses still more accommodative
power, and this means that he cannot even correct
his defect ; in other words, he cannot obtain clear images
of anything far or near.
This condition will obtain, of course, at an earlier
age if the hyperopia be higher. Thus, a hyperope
of 10 D at 25 has only 8 d accommodative power,
and consequently sees everything indistinctly. Such
persons often approach their eyes very near their work
in order to obtain larger retinal images, and an erroneous
diagnosis of myopia is liable to be made.
The Varieties of Hyperopia. — The hyperopia which
is at once recognized, the patient confessing to im-
proved vision with a convex glass, is called manifest,
and this manifest hyperopia (Hm) is expressed in
amount by the strongest convex glass the patient
accepts. For instance, a patient sees |, but with +1
in front of the eye f : +i«5 makes the letters hazy,
then +1 D = Hm.
Again, when the defect is hidden by the patient
using his accommodation and obtaining perfect distant
vision, hyperopia is present if he accepts a convex
glass and the total manifest hyperopia is represented
by the strongest convex glass with which he can see
90
THE REFRACTION OF THE EYE
Total
Hyperopia
(Ht)
Latent H.
[HI)
Manifest H.
[Hm)
equally well as with the naked eye. (It should be
noted that neither the emmetrope nor the myope will,
under any circumstances, accept even the weakest
convex glass for distance.)
The latent hyperopia is the additional hyperopia,
which shows itself when the accommodation has been
relaxed by a cycloplegic. If the patient quoted above,
when under atropine, see | only when +3 is used, in
his case +2 represents the latent hyperopia (HI), the
total hyperopia (Ht) being the sum of Hm and HI.
Hm+HUHt.
Thus:
I Only revealed under a cycloplegic.
'Absolute (Hma), which no amount
of accommodation can correct,
represented by the weakest con-
vex glass.
Facultative (Hmf), when distant
objects can be clearly seen with
or without convex glasses, repre-
sented by the difference between
the strongest and weakest convex
glass.
The want of harmony between the accommodation
and the convergence is a constant cause of eyestrain
in uncorrected hyperopia. We have seen (page 48)
that in normal vision, the two eyes converging for a
point I metre off, form a metre angle, and use i d of
accommodation. Eyes converging for a point 50 cms.
off have to converge 2 metre angles and accommodate
2 D, and so on. Now, a hyperope of 2, when looking at
a point ^^ cms. off, is using W — i-^-y 3 d added to the
correction of his hyperopia — viz., 3+2 = 5 d; but he
is only converging 3 metre angles instead of 5, con-
sequently he is using 2 d of accommodation in excess
of convergence.
Nature has endowed many hyperopes with the power
of increasing their accommodation, to a certain extent,
without varying their convergence; this faculty is the
HYPEROPIA 91
result of " nerve education." We shall see when deal-
ing with myopia that the same thing occurs, only in
this case it is the convergence that is used in excess of
the accommodation. There is a minimum amount of
effort when convergence and accommodation work
harmoniously together, as it were supporting each
other ; but when one is used in excess of the other, it has
to work unaided and alone, and strain is liable to ensue.
Many hyperopes never become capable of using
their accommodation in excess of convergence, and
therefore they are less likely to suffer from strain
(although the unconscious effort to do so may induce
it) ; but a worse evil befalls them — they lose binocular
vision, and squint. A hyperope, under these circum-
stances, finds himself in the following dilemma: if he
wishes to see binocularly, he must use less accon^mo-
dative power than he requires to see distinctly; or,
if he wishes to see distinctly, he must sacrifice binocular
vision, which ends in squint. He must choose between
distinct vision and binocular vision. Distinct vision is
more craved for, and more useful, than binocular vision,
especially if the latter be not quite perfect, owing to
one eye being more defective than the other; conse-
quently, he sacrifices binocular vision, and squints,
and the eyestrain ceases* (see Strabismus, page 184).
Conditions causing Hyperopia — i. Axial Hyperopia.
— This is the commonest form, and is the condition of
most eyes at birth. It is due to the shortening of the
antero-posterior diameter of the eye, and may be due
to a flattening of the globe or to a general diminution
in size. Roughly, every 3 d of hyperopia represents a
diminution of i mm. of the axial line.
This condition is due to an arrest of growth of the
eye, and is often associated with arrest of growth of
the neighbouring bony parts; thus, the face of a
hyperope often shows want of relief.
* Bonders called this form of hyperopia relative.
92 THE REFRACTION OF THE EYE
The tendency is for hyperopic eyes, at birth, to grow
towards the normal and even to become myopic; but
after 50, owing to the increase in size and the flatten-
ing of the lens with age, there is a tendency for all eyes
to become hyperopic as life advances; this is called
acquired hyperopia (see page I45)-
2. Curvature Hyperopia : due to a lack of convexity
of the refractive surfaces; it is generally associated
with astigmatism (see page 119).
3. Index Hyperopia : due to diminution in the index
of refraction of the media.
4. Hyperopia may be due to absence of the lens
{aphakia), or its total or partial dislocation.
5. Tumours or exudations causing advance of the
retina in the eye will cause hyperopia.
Symptoms. — It is the facultative hyperopia which
the patient can correct, and thus more or less conceal at
will, which is one of the most common causes of eye-
strain, and the reason is very apparent.
In absolute Hm, vision is never acute, and the patient
makes no attempt to strain his accommodation, because
he finds the result of little or no use.
In relative Hm, it has been seen that only monocular
vision can be acute, and that eyestrain generally ceases
when the squint appears.
Patients with facultative Hm are most frequently
quite unaware that they are suffering from any defect
of the eyes; they can see well at a distance, their near
vision is as good as they want, they have no idea that
the headaches that come on after near work are caused
by eyestrain, and they will in all probability be treated
for all manner of diseases before the real cause is dis-
covered. It is true that when this eyestrain lasts
long, signs of inflammation will often show themselves
in the eye and its appendages, such as conjunctivitis
and blepharitis, and lead the patient to the oculist;
but even he may miss the true cause unless he make it
HYPEROPIA 93
a rule to examine, under atropine, the eyes of all young
people suffering from chronic inflammation of the lids
or conjunctiva, they being especially the subjects of
this condition.
The facultative hyperopia of the young becomes
absolute after middle life.
Although it is quite possible to suffer from faculta-
tive Hm, and pass through youth without any symp-
toms of eyestrain, sooner or later they will appear.
Good health, plenty of outdoor exercise, and not too
much application to books, will ward off eyestrain for
a long time; but in these days of examinations the
day must surely come when the young student must
" cram," when he must read four or five hours a day
by artificial light, when he must do more work and
take less play — in other words, when he must use his
muscles of accommodation for a much longer time.
As a result, after a few hours' reading, his head aches,
his eyes pain, and the type appears to run together.
Many such cases may occur from simple overwork in
emmetropes, but there is little doubt that many a
young man has broken down reading for his " Tripos "
simply because he is hyperopic and has overstrained
his eyes. Patients often suffer from eyestrain for the
first time through taking up German or Hebrew; the
fine strokes that have to be recognized in order to dis-
tinguish the different letters (especially is this so in
Hebrew) put an extra strain on the accommodation,
and if the eye start with a deficit, as it does in hyper-
opia, eyestrain is sure to ensue.
If symptoms of eyestrain occur among the upper
classes who suffer from this form of hyperopia, how
much more must they occur in those who spend their
lives at close work, in badly lighted and badly venti-
lated rooms, with little or no outdoor exercise and often
insufficient food. Yet the large body ol seamstresses
and compositors, who find their way to the out-patient
94 THE REFRACTION OF THE EYE
room of an ophthalmic hospital, are only a small
fraction of the number who really want relief, but
do not recognize it because they see well without glasses.
Those who do come for advice generally tell the same
tale; they are at work, say, from eight in the morning
till eight or ten at night, and towards evening they
complain that their vision becomes less acute, and their
eyes and head ache. It is but natural; their ciliary
muscles have been working at high pressure all day, and
in their way have done as much work as the leg muscles
would in a thirty-mile walk. Surely in an emmetropic
eye we should expect fatigue under such circumstances ;
how much more, then, in a hyperopic eye !
With advancing years the latent hyperopia becomes
gradually and finally, about the age of 40, entirely
manifest, and with the diminution of accommodation
range the hyperope necessarily becomes prematurely
" presbyopic." As might be expected, symptoms of eye-
strain are much more common in presbyopes who are
hyperopic than in those who are emmetropic or myopic.
One of the results of eyestrain in young hyperopes,
or in those who have to make great efforts to see small
objects, as watchmakers, is spasm of the ciliary muscle,
whereby vision is accommodated for near objects, and
the patient rendered artificially myopic (see page 109).
This spasm is usually accompanied by a contracted pupil
from associated spasm of the sphincter of the iris, both
conditions being caused by direct or indirect irritation
of the third nerve (see page 201).
Hyperopic headache is often accompanied by twitch-
ings of the eyelids.
Besides the many symptoms of eyestrain already
described, the special indications of hyperopia are —
1. Spasm of ciliary muscle, often producing apparent
myopia.
2. Sudden failure of the ciliary muscle from fatigue,
causing obscurations of vision.
HYPEROPIA 95
3. Convergent strabismus.
4. Apparent divergent strabismus.
The angle gamma is larger — i.e., the angle formed by
the visual and optic axes is increased, which causes
apparent divergence of the axis of the cornea (see
page 187).
There are also certain physical signs noticeable in
hyperopes.
The eye is flatter than normal and often markedly
smaller; if the eyeball be turned strongly in or out, the
equatorial region presents a much sharper curve than in
the normal or myopic eye, showing its shortened axis.
The cornea is often smaller than usual. The face is
sometimes flat-looking.
The ciliary muscle of a hyperope is always larger than
normal ; this is especially marked in the annular muscle
of Miiller, and is due to excessive use.
The Diagnosis of Hyperopia by Examination — i. Vision
is improved by, or is as good with, convex glasses.
2. Retinoscopy gives a shadow moving " with " with
a plane mirror, and a reverse shadow with a concave
mirror, and the more defined the shadow and the quicker
its movement, the lower the hyperopia.
3. The indirect ophthalmoscopic examination shows
an image of the disc larger than normal, and diminishing
on withdrawing the lens from the eye.
4. In the direct ophthalmoscope examination, if the
observer's accommodation be relaxed, a convex glass is
required in the ophthalmoscope to obtain a clear image
of the fundus. If the hyperopia be high, the mirror
alone, a short distance from the eye, shows an erect
image of the fundus moving with the observer.
The Influence of Age on Hyperopia. — See Presbyopia,
page 142.
Treatment. — Up to 6 years of age atropine should
be applied to the eyes for at least three days twice a
day before the examination.
96 THE REFRACTION OF THE EYE
During this period of life the eye has not fully de-
veloped, most children have a certain amount of
hyperopia, which should only be corrected if high in
amount, or if strabismus be present.
As the examination before the type at this age is of
no practical value, the surgeon has to depend on the
result of his retinoscopy.
The glasses ordered should be weaker than the atropine
estimate. Suppose the retinoscopy, under atropine,
gives +4 at a metre, this would mean +3 before the
type if the patient can read, and the glass ordered
should not be stronger than +2. It will be found that
when the error is greater, more than +1 must be
deducted from the atropine correction.
The correction of hyperopia, when adopted with those
who have developed convergent strabismus, has often
the happiest results, for the squint may be cured with-
out resorting to an operation, and the result is in direct
ratio to the youth of the patient. Moreover, these
are the hyperopes who can most readily be made practi-
cally " emmetropic," and therefore get the greatest
gain from the glasses ; for their very defect, the squint,
shows that they have been unable to dissociate their
accommodation and convergence. It is this habit of
using their accommodation in excess, which has led to
hypertrophy of the ciliary muscle, which prevents
many hyperopes from taking full correction. They
cannot overcome the habit, and naturally the older
the patient, the greater the probability is there of
this being possible. In such cases a much weaker
convex glass must be given at first, and its strength
increased later.
When the amplitude of accommodation is great,
the patient will probably prefer a weaker glass than
we wish to give, and vice versa. When the hyperopia
is small in amount, and equal in the two eyes, and
there is no strabismus, it is unwise to order any glass.
HYPEROPIA 97
From ages 6 to 15 the atropine should be appUed
to the eyes twice daily for two days before exami-
nation.
By this time the eye should be emmetropic, and as
the child is entering the active period of school career,
the hyperopia should be corrected, unless very small
in amount, and even then when associated with astig-
matism or anisometropia, or both.
At this period we must recognize the " personal
equation." Some patients will stand a fuller correction
of their defect than others.
After ascertaining the amount of error under atropine,
we must, whenever possible, arrange for a final examina-
tion before the test types, when the effects of the cyclo-
plegic have passed off. We should prescribe the fullest
correction of the error the patient will accept, consistent
with good vision. For instance, under atropine a child
of 12 reads years J with +3. When the effects of the
atropine have passed off, deducting + 1 for the atropine,
we find that + 2 gives J hazily, and that it is only when
we reach +i'25 that § is clearly read. This is the
correction we order, with a cylinder correcting the
astigmatism, if present (see page 138) .
We must remember that such a patient did not come
to us for improvement in vision; he probably read f
quite well, and if we insist on his wearing a correction
which makes his distant vision worse, he will seize every
opportunity of not wearing the glasses.
When the hyperopia is slight (say only i»75 or 2 under
atropine), equal in the two eyes, and unassociated with
astigmatism or strabismus, glasses are not necessary.
From ages 15 /o 25 or 30 atropine is the best cyclo-
plegic, but in a large number of cases we have to be
content with homatropine, because atropine necessitates
too long a period of rest from work. Glasses should not
be prescribed until. the patient has recovered from the
effects of the cycloplegic, and then the fullest correction
7
98 THE REFRACTION OF THE EYE
of the error that the patient will accept, consistent with
clear vision, should be ordered to be worn always.*
In high hyperopia, especially in cases where the patient
will only accept a very partial correction of the error, it
is advisable to give the full atropine correction, as special
near- work glasses, for use by artificial light.
From ages 30 to 45 homatropine should be used as the
cycloplegic, and in the case of patients with high hyper-
opia, or when the smallest fear of glaucoma is present,
one drop of eserine (2 grains to the ounce) should be
put into the eyes when the examination is finished.
The treatment is the same as for younger patients.
A larger proportion of the correction will be accepted,
and the older the patient, the more necessary is it to
give the full cycloplegic correction for near work.
Over age 45 no cycloplegic is necessary, as all the
hyperopia has by this time become manifest.
The strongest convex glass the patient will accept
must be prescribed, and as the presbyopic period has
arrived, a still stronger glass will be required for reading,
and the two are best prescribed in one glass in the form
of invisible bi-focals (see Presbyopia, page 151)4
* When the patient cannot return for a final visit, glasses
correcting the whole of the manifest, and about one-third of the
latent hyperopia, may be safely prescribed. (Manifest hyperopia
is expressed in amount by the strongest convex glass the patient
accepts when not under atropine. Latent hyperopia is the addi-
tional hyperopia which is revealed under atropine.)
t It is important to remember that hyperopia tends to de-
crease towards emmetropia. A child may be hyperopic to the
extent of 2 D, and as the growth and development of the different
parts of the body proceed, the flatness of the eye may disappear,
and by puberty the eye may have become emmetropic. For
this reason it is necessary to re-examine the eyes of children at
least once a year, in order to make sure they are not wearing too
strong a convex glass, which would induce an artificial myopia,
which in turn might lead to real myopia. This tendency for the
eye to grow normal is often interfered with by the presence of
eyestrain; hence one of the benefits derived from glasses in
young children is the increased chance of the eye improving and
growing to the normal shape, and the possibility of discontinuing
the use of spectacles.
CHAPTER VII
MYOPIA
Myopia {fiveLv, to close, o)\p, the eye, from the habit of
myopes to partially shut the eyes in order to lessen the
circles of diffusion), or short-sight, is a condition of the
eye in which the retina is situated behind the principal
focus (Fig. 20, m), and only divergent rays from a near
point (Fig. 55), or parallel rays made divergent by a con-
cave glass (Fig. 56), can come to a focus on the retina.
Fig.
The retina of a myopic eye is the conjugate focus of
an object situated at a short distance in front of the eye,
or, in other words, the punctum lemotum of a myope is
always at a definite distance (less than 6 metres), the
distance being measured by the amount of myopia.
Thus, a myope of i has his far point i metre from the
eye, a myope of 2 has his far point J metre, or 50 cms.,
and a myope of 5, 20 cms., from the eye.
99
100
THE REFRACTION OF THE EYE
A myopic eye sees distinctly distant objects (when
accommodation is relaxed) with that concave glass whose
focal length is equal to the distance of the far point from
the eye, and the converse is true: the measurement of
myopia is that concave glass with which the myopic
eye sees distinctly objects at a distance, and its focal
length is equal to the distance of the myope's far point
from the eye. If the accommodation be relaxed,
the strongest concave glass is the measure of the
myopia.
We can ascertain the punctum remotum of a myope
directly by measuring the furthest distance at which he
Fig. 56.
can sec objects distinctly ; thus, if such a spot be 2 metres
from the eye, this is R, and its expression in dioptres is
r= I or '5, hence the myopia = - •5.
Myopia may be produced in the following ways :
1. By elongation of the axis of the eye — axial myopia.
This may be due to —
(a) General elongation of the eye — typical myopia.
(6) Localized protrusion of the sclerotic, particularly
at the posterior pole — staphyloma.
2. By increase of the refractive power of the eye —
refractive myopia.
This may be due to —
(a) Increase in curvature of the cornea, as in myopic
astigmatism.
MYOPIA 10 1
(b) Increase in the curvature of the lens —
(a) In spasm of the accommodation.
(/3) In luxation of the lens.
(c) Increased density of the lens, as at the beginning
of senile cataract.
3. A combination of i and 2, as in conical cornea,
when elongation of the axis and increase in the curvature
of the cornea coexist.
Thus, we see that typical myopia is due to an elonga-
tion of the antero-posterior diameter of the eye, and
every dioptre of myopia represents a lengthening of this
axis by about J mm.
The punctum remotum and punctum proximum of a
myope are ascertained according to the methods already
given.
The punctum proximum is nearer the eye than in
emmetropia, and the higher the myopia the nearer
it is.
As r has a positive value in myopia, the amplitude
of accommodation is the difference between p and r;
thus a=p- r.
The Influence of Age on a Myope.— See Presbyopia,
page 144.
The diagram (Fig. 57) represents the amplitude of
accommodation of a myope of 3 d at the different ages.
The Hue ^ begins at the figure 17, showing that at the age
of 10 the near point is 6 cms. from the eye; therefore
a = -§- - 3 = 17 - 3 = 14 D. At 30 years of age ^ = 10 d,
and p is 10 cms., while R is still 33 cms. on the positive
side, a = io-3 = 7 d.
At the age of 55 r begins to curve downwards, and
reaches the zero line at 80, so that at this age the myope
of 3 has lost all his myopia; p and r unite (showing
that all accommodation is lost) at the age of 75.
The Relation between Accommodation and Con-
vergence. — A myope of 5 d can see a point 20 cms.
from the eye without using his accommodation,but he
102
THE KEFRACTION OF THE EYE
must converge to 5 m.a. in order to see binocularly. As
a compensation for the visual defect, most myopes have
the power of using their convergence in excess of their
accommodation, just as a hyperope has often the power
of using his accommodation in excess of his convergence ;
but, it has been shown (page 91), they both have to pay
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Showing the range of accommodation of an uncorrected myope
of 3 D at different ages. (Donders.)
a penalty for this, the liabiUty to strain always being
greater when either effort is used in excess of the other.
The " fusion supplement " must be greater than the
emmetropia, and the greater the "fusion supplements^
the greater the fatigue to the internal recti ; the fatigue
leads to " insufficiency " of the muscles, and matters
* See note, p. 88,
MYOPIA 103
are made worse. But it is not only the excess of con-
vergence, but the excessive convergence that tends to
produce strain and fatigue of the internal recti. The
uncorrected myope sees nothing distinctly beyond his
far point, and when he wishes to see clearly he must
bring everything within that point; for instance, an
emmetrope wishes to know the time by the clock: he
can see the clock across a room, but the myope must
go up to the clock and bring it within his far point;
and, moreover, the incentive to use this remedy is
great because the remedy is perfect. A high hyperope
has the same difficulty with distant objects, but he
has not the same remedy. Naturally, the greater the
myopia, the nearer is the far point, and the greater is
the convergence strain.
A myope requires more convergence of the visual lines
because vision takes place closer to the eyes, and, as
Bonders has shown, precisely in myopia is this for two
reasons more difficult — first on account of impeded move-
ments, due to the altered shape of the eyeball, which
becomes elUpsoidal in form, and which has to move in a
cavity of similar shape ; and, secondly, on account of the
altered direction of the visual lines, the angle 7 (angle
formed by the visual and optic axes) being smaller than
in emmetropia or hyperopia (see page 187). If a myope
cannot dissociate his accommodation and convergence,
he has the same difficulties as a hyperope : he can either
see distinctly, but sacrifice binocular vision to remove
the diplopia, or he can use his accommodation when he
does not require it, and see indistinctly.
From the observations of Bonders, Nagel, and
Landolt, we find that the relative amplitude of accom-
modation and convergence (see page 52) vary consider-
ably, not only according to the refractive error, but also
in different individuals with the same error. There is
a tendency for the accommodation to adapt itself to
the altered state of refraction* hence most myopes can
104 THE REFRACTION OF THE EYE
converge in excess of their accommodation; and when
the myopia increases, the excess of convergence over
accommodation also increases.
The Causes of Myopia. — Although myopia is heredi-.
tary, it is, with few exceptions, not congenital. Almost
all eyes are hyperopic at birth.
The savage is rarely myopic: it is civilization that is
responsible for it. The necessity for constantly adapt-
ing the eye for near objects means undue convergence.
Myopia generally first shows itself from the age of
8 to 10, when school work begins in earnest — that
is, when convergence is first used in excess — and there
is no doubt that excessive convergence is mostly re-
sponsible for the development of myopia. The over-
used internal recti constantly pulling at the sclerotic
(assisted by the pressure of the other muscles) tend to
lengthen the antero-posterior diameter of the eye, be-
coming then the most potent factor in the causation of
myopia, and as this lengthening of the antero-posterior
axis necessitates still greater convergence, a vicious
circle is produced, and the myopia tends to increase.
The hereditary character of myopia is explained by
the existence in such eyes of an " anatomical predisposi-
tion " to myopia. The sclera is unusually thin, and con-
sequently less able to resist the pull of the internal recti,
and the relative position of the recti and the position of
the optic nerve, both of which may be hereditary, may
be factors in the production of this defect.
Anything which causes young subjects to approach
their work too near the eyes may be the starting-point of
myopia. Bad illumination, or the light coming from the
wrong direction (for instance, in front), or defective vision
produced by corneal nebulae, or lamellar cataract, etc.,
all necessitate over-convergence in order to obtain clearer
images, and myopia may be produced.
It is interesting to note that when the work is ap-
proached very near the eye, but convergence is not used;
MYOPIA 105
as in the case of watchmakers, who habitually use a
strong convex glass in one eye, there is no special
tendency to myopia.
Symptoms and Diagnosis of Myopia. — (i) Distant
objects are seen indistinctly, because parallel rays focus
in front of the retina and cross and form diffusion circles
on the retina, and the higher the myopia the larger the
diffusion circles; these are reduced, commonly, by the
myope " screwing up " his eyes, and in later life by the
contraction of the pupils.
(2) Near objects are seen distinctly, and the near
point is much nearer than in normal eyes.
(3) Acuteness of vision is often lowered, and in high
myopia this is invariably the case, because the stretching
of the eye leads to atrophic changes in the retina and
choroid.
(4) The presence of convergence insufficiency and
latent divergence (diagnosed by the Maddox test,
page 44) for distant and near objects, often becoming
manifest later, and ending in divergent strabismus.
(5) An apparent convergent strabismus due to the
angle 7 being negative (page 187).
(6) Many of the symptoms of eyestrain, but not so
frequent or so marked as in hyperopia (see Heterophoria,
page 171).
(7) Spasm of the ciliary muscle, which apparently in-
creases the amount of myopia, so that young subjects
will choose a stronger concave glass than they require.
(8) A prominence of the eyeball is sometimes noted in
high myopes, but is not always present. A dilated pupil
and dreamy stare are sometimes present.
(9) Muscse volitantes are often complained of. These
are probably due to the indistinct vision allowing the
vitreous to be seen against a hazy background.
(10) In high myopia vitreous opacities are sometimes
numerous and most annoying.
(11) Myopes often stoop very much and become
I06 THE REFRACTION OF THE EYE
" round-shouldered " from their habit of poring over
their work, and this stooping at near work tends to
produce congestion of the eyes and appendages.
It should be noted that, in low degrees of myopia, often
the only symptom present is indistinct distant vision,
and this, very often, is not recognized by the patient as
a defect. Such people learn to recognize indistinct out-
lines by the aid of other senses in a way that emmetropes
can hardly understand, and when, in later life, they can
put off the wearing of glasses for near work for many
years, or till extreme old age, what wonder that they
and their relations imagine them to be possessors of
remarkably good sight !
The Diagnosis of Myopia by Examination.
Objective Examination — The Ophthalmoscope ; (i) The
Indirect Method, — By this method the disc appears
smaller than in emmetropia. On withdrawing the focus-
glass from the patient's eye the disc becomes larger.
Without the focus-glass, in high myopia the fundus
is seen very large and inverted, if the observer be not
nearer the aerial image than his own near point, and if
the observer's head be moved, the image of the disc
appears to move in an opposite direction.
(2) The Direct Method. — By this method — viz., with
the ophthalmoscope close to the eye — the fundus is
indistinct, and the concave glasses have to be rotated in
front of the opening; the weakest concave glass that
gives a distinct image is the measure of the myopia, if
the observer's accommodation is relaxed.
Retinoscopy. — With a plane mirror the shadow moves
" against," and with a concave mirror " with," if the
observer be beyond the patient's far point. With the
observer seated i metre from the patient, the measure of
myopia is that concave lens which gives the point of
reversal, with -id added (see page 79).
THE FUNDUS OF THE RIGHT EYE OF A MYOPE.
The amount of myopia is 9D, and correction gives normal vision.
The retinal vessels are very straight, and they are seen to curl over
the tilted nasal margin of the disc.
The " myopic cre,scent," instead of being limited to the outer or
temporal side of the disc, is seen to surround it.
[The fellow eye is much the same, only the changes at the macula
are more marked, and vision is only j'j].
MYOPIA 107
Subjective Examination. — Having ascertained the
amount of error by the above methods, we seat the
patient before the test types, and proceed to correct with
concave lenses in the trial frame.
As the accommodation is never so relaxed as when the
eye is under the influence of a cycloplegic, the patient
may require a slightly stronger lens than the objective
examination indicated. Thus, if retinoscopy at a metre
from the patient gives us the point of reversal with - 5,
we call the amount of defect - 6, and before the types
the patient may prefer - 6'5.
Note the weakest lens that gives the most distinct
vision with each eye separately, and then try the glasses
binocularly when binocular vision exists, as sometimes
the patient will accept and prefer a slightly weaker glass.
As the circles of diffusion are removed by correction,
the myope often finds that concave glasses reduce the
size of objects. In Fig. 21, page 26, m represents the
position of the myopic retina, and the image of an
object on it is seen to cover a larger area than the same
object does either in emmetropia or hyperopia, H.
Changes in the Fundus in Myopia. — In most myopes a white
crescent is observable on the temporal side of the disc; this is
the myopic crescent. It may not be limited to this part, and may
even surround the disc (see frontispiece). In high myopia it
often extends on the outer side towards the macula. Its
margins are often pigmented. This crescent, which is a localized
atrophy of the choroid, is brought about by the stretching of
the tunics in the formation of the posterior bulging or staphy-
loma posticum of the eyeball. The " dragged disc " is due to the
resistance of the optic nerve (often shorter than normal) on the
one side, and the posterior staphyloma on the other. Whether
the atrophy is secondary to choroiditis or merely due to insuffi-
cient nutrition, it is difficult to say. Von Graefe asserted that
the staphyloma posticum is due to a sclerotico-choroiditis. If
this be so, then this particular spot bulges, because it is unsup-
ported by the recti which compress the sides of the globe. This
also explains why undue convergence, by increasing the intra-
ocular pressure by pressing the recti on the globe, is such an
important factor in causing and increasing myopia. The dragged
or tilted disc is very characteristic, and becomes deeply cupped
in some cases. This cupping is quite distinct from glaucomatous
I08 THE REFRACTION OF THE EYE
cupping, in that it is most marked on the nasal side, the vessels
rising up and dipping down over the tilted edge in a very charac-
teristic fashion, and it does not occupy the whole area of the disc
(see frontispiece).
If the myopia progress, the changes may become general,
and after a time white patches of choroidal atrophy, with masses
of black pigment forming their boundary, are scattered all over
the fundus. These changes extend to the vitreous, causing
liquefaction of that body and the subsequent shrinking, and the
consequent loss of support to the retina may end in detachment
of that membrane. Unfortunately, very often some of the most
serious changes occur at the macula, as this is the region of the
bulging, and haemorrhages and consequent atrophy lead to a
result as disastrous as the detachment.
Some cases of high myopia have been termed " malignant,"
and it is very probable that many of them ought not to be classed
under myopia at all, but that the myopia is only a symptom of
a disease attacking the eye.
The milder cases of progressive or malignant myopia (wrongly
so called) are often the result of wrong treatment, as we shall see.
Treatment—
Donders said: "The effect of wearing glasses is, in
fact, that the relative range of accommodation is
displaced, becoming gradually the same as the position
proper to emmetropic eyes, and therefore the binocular
furthest point approaches the eye while the absolute
furthest point r by no means does so. The myopia
thus neutralized is less progressive, because both too
strong convergence and a stooping position are avoided."
Since his day some ophthalmologists have advocated
the practice of allowing those suffering from low myopia
to do near work without glasses, and when the myopia
was high, have given weaker glasses for near work.
The consequence has been that as the convergence was
still being used in excess, the myopia tended to progress.
If Bonders' teaching had been followed, this error
would never have been made, an error which has kept
progressive myopia and malignant myopia dreaded for
so many years.
Following on Bonders' teaching, and making use of
our increased knowledge, which indicates the im-
portance of correcting low errors of astigmatism, we
MYOPIA 109
have made great advances in recent years. The full
correction of the error (with the smallest minus cylinder) ,
except in cases of very high myopia, leads not only to
the arrest of the progress of the myopia, but in some
cases to its distinct diminution.
In a paper read in 1904 at the British Medical Association
meeting at Oxford, I cited 532 myopes who had been treated
by full correction.
The myopia ranged from '75 to 20, and the average period of
observation was four and a half years. The following table
shows the result :
'469 remained stationary, and of these, in 162, the visual
acuity improved.
532<| 63 progressed. f Increase limited to i d, 47.
Average age, 15. <^ ,, ,,2 D, 13.
1^ M. from -I to -II. [ ,, ,, 4 D, 3.
If we exclude the 27 whose increase was limited to i d, we
have 16 left — i.e., only 3 per cent, progressing.
In early life the treatment of myopia is mostly
preventive. It is rare to come across a child under
the age of 6 with actual myopia. Apparent myopia
may show itself — (a) from spasm of the ciliary muscle,
distant vision being subnormal and improved by con-
cave glasses, and near work being approached very
close to the eyes; (b) in a young patient with high
hyperopia, in which case distant vision is poor, and near
work is held very near to the eyes in order to acquire
large retinal images; (c) in children who have acquired
the habit of holding their work near the eyes, either
through faulty illumination or on account of reduced
visual acuity, produced by some disease of the eyes, such
as corneal nebulae.
In all these cases the true error is revealed by a cyclo-
plegic, consequently no attempt should ever be made to
treat a young myope without previously paralyzing the
accommodation, and atropine should, if possible, always
be used.
The preventive treatment is more especially indicated
in all children whose parents are myopic, for they have
I3t0 tHfi REFRACTION OF THE EYE
probably inherited an " anatomical predisposition *' to
myopia.
Bearing in mind that excessive convergence is the
most potent cause of myopia, the most rigid attention
should be paid to ophthalmic hygiene. The schoolroom
should be lofty and large, and have high windows on one
wall. The seats and desks should be arranged in rows
so that the students sit with the windows on their left.
When practicable, each scholar should have an adjust-
able seat and desk, but when this cannot be arranged^
as most children of the same age are of the same height
while sitting, and in the same class, the height of the
desk from the seat should increase gradually with the
classes, the highest class having the highest desks.*
The school work that needs close application of the
eyes should be continued only for a short period at a
time, the period alternating with other work which does
not require the use of the eyes, such as mental arithmetic^
demonstrations, or play.
Schoolmasters should teach more — that is, they should
explain and impart knowledge by demonstrations and
simple lectures, and reduce as much as possible the time
spent in " home preparation," which is usually work
done by bad light, and when the student is physically
and mentally tired.
Even in the nursery the greatest care should be taker!
with the children's sight. They should have large toysy
and among these there should always be a large box
of plain wooden bricks; picture-books should be dis-
couraged, and close work that entails undue converg-^
ence, such as sewing, threading beads, etc., should be
forbidden. The nursery governess can teach them theif
letters and small words, and even simple arithmetic, by
means of the wooden bricks.
* The desk should have a slight slope, and its height should be
so regulated that the scholar can sit with head upright and the
eyes about 33 cms. from the work.
MYOMA Itl
No child with a tendency to myopia or with a myopic
family history should be allowed to learn to write or to
draw until at least 7 years old.
The child's bed should not be allowed to face the
window; preferably it should be back to the Ught.
Having ascertained the concave glass that corrects the
myopia of each eye under atropine, we may, in quite
young patients, order such glasses for constant use; in
those over puberty it is wise to delay prescribing until
the effects of the atropine have passed off — not only
because an increase of '5 may very distinctly improve
vision, but because it is important to try the glasses
binocularly when the eye is in the normal state. It is
too often forgotten that the eyes are not single optical
instruments, and we often find that a weaker pair of
concave glasses give as good vision as a stronger glass
used monocularly.
The only certain method of arresting the progress of
myopia is to establish a normal state, in which the
ciliary muscle is strengthened by being forced to work,
the excess of convergence over accommodation stopped,
and excessive convergence made impossible, and this can
only be achieved by insisting upon the constant use of
the glasses, and refusing to give weaker ones for near work.
The patient can see his near work so much better without
glasses that we may have some trouble at first in enforcing
this treatment.
Of course, the precaution must be taken of re-
moving the glasses when rough games are being
played.
In adults the treatment of myopia should be carried
out in the same manner, substituting homatropine for
atropine in those who cannot afford the time from work
that the latter entails.
If the myopia be somewhat high, say 6 d or over, and
has never been fully corrected, we may have to give
glasses for near work 1*5 or 2 d weaker, but the patient
112 THE REFRACTION OF THE EYE
should be strongly advised only to wear these on special
occasions when fine work is being done, or by artificial
light.
The older the patient and the higher the myopia the
more difficult will it be for him to use the distance
glasses for near work, because his accommodation has
been so long idle that the ciliary muscle is considerably
atrophied.
If the patient be a student or engaged in literary or
other work which entails close application for many
hours a day, and if he be free to regulate his work, he
should be advised to work for shorter periods and take
longer intervals of rest, and be especially careful to have
his work always in a good light.
In patients of 30 years of age and up to 40, homatro-
pine should be used when practicable. Over 40 no
cycloplegic is required. If the patient has never had
the full correction, he will at this age be unable to read
with his distance glasses, and weaker ones must be given,
preferably in the form of bi-focals. All the more will this
be the case when he arrives at 40 or 45, the emmetrope's
presbyopic period ; no rigid rule should be observed, but
each case should be treated according to its requirements.
After carefully testing the patient, we should find his
working near point, and keep more accommodation in
reserve than would be required in the emmetrope because
the ciliary muscle is weaker (see Presbyopia, page 150).
Some adults with a small amount of myopia obsti-
nately refuse to wear the constant correction: ladies
will wear lorgnettes at the theatre, etc. ; men will wear
a monocle. If no astigmatism be present, this may
be allowed, so long as no increase in the myopia takes
place, but only on the condition that the patient is
re-examined at frequent intervals.
ffigh Myopia. — ^The treatment of high myopia is some-
what different. When the young adult has never worn
the full correction, it will be useless to prescribe it, even
MYOPIA 113
for distance, at first. We should reduce the glass as
little as possible, and test binocularly. For instance, the
myopia may be 20 in both eyes, but - 18 before each
eye is the strongest glass the patient will tolerate. These
we order for constant use, and we may find that later
the full correction will be accepted. In older patients,
not only have we to be satisfied with a reduction in the
distance glasses, but we must often take off as much as
4 D for near work.
Adults who have, say, 10 d of myopia, will very often
refuse to wear this correction because of the discomfort
entailed; in such cases it will be found that the best
method is to give, say, 8 d for constant use, and to
give them 2 d either as lorgnettes or as a " spy " glass
to put up in front of the 8 d when they want particularly
to see at a distance.
The advantage of this treatment is that the " constant "
glass, being weaker, may do quite well for reading.
When recent fundus changes are present in young
patients, the eyes should be kept under atropine for a
long period, the correcting glasses should be well tinted,
or made with Crookes's " B " glass (see below), and a
country, open-air life should be strongly recommended,
with complete cessation of all close work while the
changes are active; older patients should be warned
against stooping or straining, and should be strongly
advised to do little, if any, near work. In all these cases
so much depends on the general health that it is wise for
the surgeon to place them under the care of a physician,
who, among other things, can advise as to aperients, the
means of reducing high blood-pressure when present,
etc., and by this care we may avert retinal haemorrhage
which is so liable to occur.
When any fear exists as to the possibility of detach-
ment of the retina ensuing, the patient should be especi-
ally warned against riding on horseback, jumping, or
doing any act which may jar the body.
114 THE REFRACTION OF THE EYE
The Treatment of High Myopia by Discission and
Removal of the Lens. — When the patient is not older
than 25, the myopia very high, vision very poor, and not
improved by glasses beyond yt> and when the fundus
changes are not very marked, and especially when the
myopia is progressive, the lens may be removed by
discission. The improvement is sometimes very great,
the patient being able to see better without a glass than
he did previously with a strong concave lens. (For
details of this treatment the reader should consult a
textbook on ophthalmology.)
Crookes's Glass (see page 211). — It is advisable to have
the correction of myopes made with Crookes's " A " glass.
In high myopia this special glass is imperative, and
where the myopia is progressive Crookes's " B " should
be used.
CHAPTER VIII
ASTIGMATISM
In discussing errors of refraction, it has been shown
that both hyperopia and myopia are mainly due to an
alteration in the shape of the eye as a whole, the antero-
posterior axis being too short or too long — axial
ametropia; but the fact was also mentioned that altera-
tions in the curvature of the cornea or lens would
produce these errors, that if the curvature were too fiat.
Fig.
parallel rays would focus beyond the retina, and if too
great, in front of the retina. It is these errors of
curvature that will now be considered.
In the normal standard eye, if an opaque disc with a
sUt aperture (Fig. 58) be placed in front of it, at whatever
angle this slit is rotated, distant objects will be seen
through it distinctly — that is, parallel rays will focus on
the retina. In simple hyperopia and myopia the same
"5
Ii6
THE REFRACTION OF THE EYE
result is obtained after correcting the ametropia with a
spherical lens, because the surfaces of the dioptric ap-
paratus are perfect spheres, and consequently all the
meridians have the same curvature. But suppose we
examine an eye in which the vertical meridian is normal
— i.e., parallel rays, passing through this meridian, focus
on the retina — but in which the horizontal meridian has a
flatter curvature, then parallel rays passing through this
flatter curvature will focus beyond the retina, and as they
impinge on the retina will form diffusion circles. In
such a case all the meridians, between the horizontal and
o - O I
Fig. 59.
vertical, will have a different focus point, such points
gradually approaching the retina as the meridian becomes
more vertical.
Such an eye is astigmatic, and astigmatism may be
defined as an ametropia of curvature, a condition in
which rays of light, passing through the dioptric appar-
atus, do not all focus at one point.
In regular astigmatism, of which the above is an
example, and which is now under discussion, the
meridians of greatest and least curvature are always at
right angles to each other, and are called the principal
* For the sake of simplicity, the two principal meridians only
are shown.
ASTIGMATISM II7
meridians; the meridians in between these have a
greater or less curvature, according as they are nearer
to the former or to the latter. The meridian exactly
between the two (corresponding to an angle of 45° if the
meridians of greatest and least curvature are vertical
and horizontal) has its focus point exactly between that
of the greatest and that of the least curvature.
The bowl of a spoon is an exaggerated example of an
astigmatic surface, the curve from side to side being
much greater than that from the handle to the tip of the
spoon.
Fig. 59 shows in a highly diagrammatic manner the
shape of the images formed by a regular astigmatic sur-
face. Let the vertical meridian have a curvature a b,
so that parallel rays passing through it focus at /. Let
the horizontal meridian c d have a flatter curvature, so
that rays passing through it focus beyond / at /'.
If a beam of light pass through such a surface, and the
resulting cone of light be intercepted at the different
positions i to 5, the image will be altered in shape
according to its position.
At / the vertical rays have come to a focus, and
therefore form a point of light ; but the horizontal rays
have not come to a focus, and will be spread out, as at
(2), into a horizontal line, called the anterior linear focus.
The reverse obtains at f\ as shown at 4, because the
horizontal rays have come to a focus, and the vertical
have crossed and form diffusion circles and spread out
the points of light into a vertical Ine, called the " pos-
terior linear focus." At i none of the rays have
focused, but the vertical are nearer the focus than the
horizontal, so the figure here will be an oblate ellipse.
At 3 the vertical rays, having crossed, are diverg ng as
much as the horizontal are converging, and here the
figure is a circle. At 5 the vertical rays are more out of
focus than the horizontal, so that the figure is a prolate
ellipse.
Il8 THE REFRACTION OF THE EYE
When the retina is situated at any of these positions
(i to 5), the image on the retina will be something like the
diagrammatic sketch. The interval between / and /' —
i.e., between the focal points of the principal meridians —
is called the " focal interval of Sturm," and represents
the amount of astigmatism.
The vision of an astigmatic person, when the astigma-
tism is sufficiently high to cause a defect of vision, is
different from that of the defective vision of the hyperope
or myope. Objects may not appear blurred generally,
but only in parts ; lines are lengthened or broadened,
and circles appear elliptical. He may be able to read
some letter in §, but even in line ys he may not read all
correctly; he supplies the visual deficiency by guessing.
Fig. 6o.
In every eye affected with regular astigmatism there
is one direction in which straight lines appear most
dist nct,|and another at right angles to it in which the
line ismost indistinct; hence, if two lines at right angles
to each other are held before an astigmatic eye, they
cannot both be distinct: if one is in focus, the other is
b urred.
In Fig. 59, where the vertical merid an is more sharply
curved than the horizontal, at the anterior Unear focus (/)
a horizontal line will appear in focus, but a vertical line
blurred; and at the post-linear focus (/') a vertical line
will appear in focus and a horizontal line blurred; they
cannot both be in focus at the same time.
ASTIGMATISM II 9
Thus, when two lines at right angles to each other (a
and B, Fig. 60, a) are looked at by an eye affected with
simple astigmatism, if the vertical meridian be defective,
A will appear defined and B blurred, because A is spread
out vertically and this does not affect the definition,
while the vertical " spreading out " of B makes the line
appear blurred (Fig. 60, h). If the horizontal meridian
be defective, the reverse happens (Fig. 60, c).
Varieties of Regular Astigmatism (Fig. 6i) :
v„_:^* CK »• r — Refraction of the Position of the
Variety of Astigmatism. principal Meridians. Principal Focus.
1. Hyperopic Astigmatism —
(a) Simple. /Emmetropic. On the retina
^ ' ^ \ Hyperopic. Behind the retina.
{b) Compound. Both hyperopic. Both behind the re-
tina, one being nearer
than the other.
2. Myopic Astigmatism —
(n\ QimT-ii^ /Emmetropic. On the retina.
W simple. ^Myopic. In front of the retina.
(6) Compound. Both myopic. Both in front of the
retina, one nearer
than the other.
3. Mixed AsTiGMA- f Hyperopic. Behind the retina.
TiSM. \ Myopic. In front of the retina.
Generally, the vertical meridian, or one near it, is most
convex, and this is called " direct astigmatism." Thus,
in direct astigmatism the horizontal meridian (or one
near it) is hyperopic in simple and mixed astigmatism
and most hyperopic in compound hyperopic astigmatism,
and the vertical meridian (or the one near it) is myopic
in simple and mixed astigmatism and most myopic in
compound myopic astigmatism. In Fig. 61 all the
examples show direct astigmatism. If the conditions
are reversed, it is called " inverse astigmatism."* When
the meridians are exactly oblique — i.e., at an ang e of
45° or 135° — it is called " oblique astigmatism."
Symmetric Astigmatism is when the axis of the principal
* The old nomenclature of these two forms was " astigma-
tism according to (or with) the rule " and " against the rule."
120
THE REFRACTION OF THE EYE
meridian in each eye is identical; for instance, the
meridian of greatest curvature is vertical in both eyes, or
is 15° from the vertical passing down and in, in both
eyes; and Asymmetric Astigmatism is the reverse.
ninf>lp
Hup A ■^h(j
Compoii
Simple Mi^op A&ti<j Compound
Fig. 61.
V, Rays passing through the vertical meridian; H, rays passing
through the horizontal meridian.
Homonymous Astigmatism is when the axes of the
principal meridians in each eye are more or less parallel ;
for instance, the axis of the correcting cylinder passes
down and in 15° from the vertical in the right eye and
lo*** 15°, or 20° down and out in the left eye.
ASTIGMATISM 121
The Seat of Astigmatism. — In regular astigmatism
the seat is chiefly in the cornea, due (i) to congenital
malformation of the cornea, often traced to heredity ; or
(2) to acquired alteration in the curves of the cornea,
produced by operations, such as iridectomy and opera-
tions for cataract, or inflammation of the cornea; or
(3) to pressure from tumours in the lid.
Transient astigmatism can be produced by pressure on
the eye with the finger, or by contraction of the lids or
the extra-ocular muscles. Congenital corneal astigma-
tism is, more or less, stationary through life; acquired
astigmatism of the cornea alters, and very often is con-
siderably reduced by time.
Even in the normal eye there is a certain amount of
astigmatism, but this " physiological " astigmatism is so
small that it can, in most cases, be ignored.
The lens may also be the seat of astigmatism, which
may be " static " or " dynamic."
The static lenticular astigmatism is generally small in
amount, and, being in the same meridian, adds itself to
that of the cornea, thus increasing the total astigmatism
of the eye ; but sometimes this lenticular astigmatism is
the reverse of that of the cornea, and so corrects it.
Dynamic Lenticular Astigmatism is nearly always cor-
rective, and is the opposite of that of the cornea. It is
produced by an unequal contraction of the ciliary muscle,
and is a most potent factor in causing eyestrain.
Symptoms of Astigmatism. — When the astigmatism is
pronounced, acuteness of vision is below the normal.
Spherical glasses may improve the distant sight to a
certain extent, but the correction is never complete. On
directing the patient to look with one eye at the " Fan "
(Fig. 62), placed 5 or 6 metres off, or nearer if necessary,
we find that he can see certain lines more distinctly than
others. The vertical lines may be seen quite black and
distinct, the horizontal lines being faint, or vice versa ; or
the oblique lines on one side may be distinct, those on
122
THE REFRACTION OF THE EYE
the other side, at right angles to the former, being in-
distinct. If all the radiating lines are indistinct, we must
make one of the meridians emmetropic, by placing before
the eye the weakest concave or strongest convex spherical
glass that is required to make one set of lines distinct and
black.
As we have already seen, when rays coming from a
point are refracted at an astigmatic surface, a linear
image of the point is formed at the focus of each principal
meridian, and the direction of the linear image is at right
angles to the meridian at whose focus it is formed. Thus,
when a patient sees the horizontal lines distinctly and the
lOO 90 QQ
Fig. 62.
lines as they pass to the vertical become less distinct,
reaching the maximum of indistinctness in the vertical
lines, we know that the vertical meridian is emmetropic,
or nearly so. Such a patient would complain that the
letters of the test type were spread out horizontally,
and if we place before the eye a stenopaic disc with the
si t vertical, we shall find the phenomena of astigmatism
disappear ; he sees all the lines with equal clearness, and
the letters appear normal, because the vertical slit has
cut off all the horizontal rays that caused the blurring.
Astigmatic Headache. — Eyestrain is the commonest
symptom of astigmatism, and of all the forms of eye-
ASTIGMATISM 123
strain, headache is by a long way the commonest. The
strain is produced in two ways :
1 . When the astigmatism is pronounced, the eye has to
accommodate in order to obtain clearer images.
Sturm asserted that accommodation was not made for
either of the linear foci, but for a point between the two
where the image is approximately a circle ; but Javal, in
his later researches, found that it was the vertical focal
line that was sought after.
Tscherning points out that among the reasons for this
preference is the fact that in reading, the legibility of the
letters depends especially on the distinctness with which
the vertical lines are seen. We can prove this for our-
selves by holding up before one eye (the other being
closed) a concave cylinder of, say, i»5 d, with its axis
vertical; if we turn the axis round to the horizontal,
making ourselves vertically hyperopic, the letters are
spread out vertically, and the words are very much
clearer.
2. Meridional Asymmetrical Accommodation. — When
the astigmatism is small, the error can be corrected by an
unequal contraction of the ciliary muscle, producing an
astigmatism of the lens the opposite of that of the cornea.
With few exceptions, the seat of regular astigmatism is
in the cornea, due to a difference in the curvature of the
different meridians; added to this there is sometimes
found a " static lenticular astigmatism," due to a differ-
ence in the curvature of the different meridians of the
lens, and the two together make up the total astigmatism
of the eye which is revealed under an ordinary examina-
tion. But most frequently, although astigmatism of the
eye is suspected, where it is of low degree it may be im-
possible to detect it without resorting to a cycloplegic.
Bonders in 1864 first drew attention to this, and he
pointed out that the corneal astigmatism, when smill in
amount, was corrected by an astigmatism of the lens ;
thus, if there was direct astigmatism of the cornea of •25,
124 THE REFRACTION OF THE EYE
there would be inverse astigmatism of the lens of the
same amount, neutralizing and masking the defect.
Dobrowolsky in 1868 asserted that this lenticular
astigmatism was produced by an unequal contraction of
the ciliary muscle ; and Hensen and Voelckers, later,
have shown by experiments upon animals that this
unequal contraction is possible. They showed that
when a filament of the ciliary nerve was divided, the
portion of the muscle supplied by it was relaxed, and that
on stimulating the cut end a local contraction took place.
But, in addition to this physiological proof, the
clinical proofs are even more conclusive.
Let us take a typical case. A patient complains of
headache, accentuated by near work. Examination re-
veals no refractive error. The ciliary muscle is paralyzed,
and astigmatism is discovered. This is corrected by
cylinders, the glasses are ordered to be worn always,
and in a short time the headache disappears.
Again, very often when the effect of the cycloplegic
has passed off, the patient refuses the cylinder that im-
proved his vision under atropine. He tells you that it
makes his vision worse. In spite of this you prescribe it,
and — this is a very important point — you insist on the
glasses being worn always. He returns in a month or
two, assuring you that his headache has entirely dis-
appeared, that he has become accustomed to the glasses,
but that he cannot now see as well without them as he
could before using them.
What has happened ? At first, when the effect of the
atropine has passed off, the ciliary muscle returns to its
old habit of unequal contraction, and consequently the
correcting glasses, instead of helping, make matters
Worse ; but by constantly wearing them the necessity for
this unequal contraction disappears, the muscle resumes
the normal condition, and allows the glasses to do the
work. Vision is apparently worse without the glasses,
because the muscle has forgotten its bad habit; but, of
ASTIGMATISM 125
course, like all bad habits, it can be easily re- acquired.
The patient has lost nothing but his headache. What
stronger proofs could there be that this unequal con-
traction does occur ?
Further, as lenticular astigmatism must necessarily he
very small, probably rarely higher than *$, it can only
neutralize a low degree of astigmatism in the cornea, and it
is in these cases where headache is most frequent.
It is easy to understand how this unequal contraction
of the ciliary muscle causes discomfort or pain, especially
in a neurotic subject. It may be also that several causes
are present, such as constipation, worry, etc., and that
this form of eyestrain is the " last straw on the camel's
back." Sometimes it is only towards middle age, when
the accommodative power is lessened, or the nerve
energy lowered, that the strain shows itself.
This unequal contraction of the ciliary muscle must
interfere with the nutrition of the lens, and it is most
likely a very potent factor in the causation of cataract.
In high astigmatism there may be some asymmetry of
the face, but otherwise there are no physical appearances
that indicate astigmatism. In oblique astigmatism the
patient may acquire the habit of holding the head on one
side, but this is not always the case ; on the other hand,
if a patient, in reading the distant types, does hold the
head obliquely, we may be almost certain that oblique
astigmatism is present.
Diagnosis and Measurement of Astigmatism. — ^There
are numerous methods for detecting and measuring
astigmatism, and they may be ranged under two heads :
I. Objective Methods —
(a) The Shadow Test, or Rhinoscopy (see page 74). —
The patient being, if possible, under the influence of a
cycloplegic, the refraction of the different meridians is
estimated in the manner described on page 78. When
all the meridiansjhave the same refraction, there is no
astigmatism; when there is a difference, astigmatism is
126 THE REFRACTION OF THE EYE
present, and its degreee is estimated by the difference
between the meridian of least and the meridian of
greatest refraction. The axis of the correcting cyHnder
will be in the same direction as that of the meridian of
least refraction (see Shadow-test, page yS). For in-
stance, supposing we find the vertical meridian shows
a hyperopia of 1-5 and the horizontal a hyperopia of 2 '5,
then the astigmatism equals i, and the axis of the convex
cylinder is vertical. Or, again, using a concave mirror,
supposing we find the shadow moves " against " in the
direction 15° from the vertical down and in and a +2
corrects, and the meridian at right angles gives a shadow
" with " corrected by - 1, we have mixed astigmatism
amounting to 3 d and corrected by a concave cylinder
-3 placed 15° from the vertical down and in and a +1
sphere, or a convex cylinder +3 with its axis 15° from
horizontal down and out and a - 2 sphere.
Retinoscopy is a very valuable help in estimating
astigmatism; it is accurate and simple, but is not so
delicate as the ophthalmometer.
(6) The Ophthalmometer. — The instrument first suggested by
Helmholtz, and improved by Javal and Schiotz, has been made in
many forms, but perhaps the model made by Meyrowitz is the
best (Figs. 63 and 64).
The ophthalmometer measures the curvatures of the cornea,
and thus enables us to ascertain the presence of astigmatism, and,
if present, its amount, the direction of the principal meridians,
and the character — i.e., whether we are dealing with direct,
inverse, or oblique astigmatism — and all this information is
acquired in a very short space of time by the expert. Moreover,
the patient is passive, the examination being purely objective,
and, as such, of immense value in checking the subsequent
subjective test.
It is true that the ophthalmometer only gives information
concerning the astigmatism of the cornea, and not the total
astigmatism of the eye, but this is exactly the information we
want; for, as we have seen (page 121), the lenticular astigmatism
when dynamic is a corrective astigmatism, and disappears under
a cycloplegic. It naturally follows that, useful as this instru-
ment is in every case, in those cases where for some reason a
cycloplegic is prohibited, the ophthalmometer is exceptionally
valuable, and in the diagnosis and estimation of low errors of
astigmatism it is indispensable.
THE OPHTHALMOMETER 12 7
The essential parts of the instrument are two mires, whose
images are reflected on the cornea of the patient, and which are
seen by the observer through a telescope containing a double
prism between two bi-convex lenses. The patient's chin rests
on a support, and his forehead should press against the top of the
stand to insure perfect rest, and the eye not being examined is
covered by a sliding clip. The mires are carried on an arc
which can be rotated into any position, and there is a graduated
Fig. 63.
disc on the observer's side of the instrument, which, by means of
a pointer, shows the meridian of the arc (Fig. 64) .
The two mires are made of porcelain, and illuminated by elec-
tric lamps behind them; they are operated by means of a gear
movement, and are thus made to approach or separate from each
other, their position being indicated by pointers working on a
disc on the observer's side (Fig. 64) .
When electricity is not available, gas or lamps must be used at
128
THE REFRACTION OF THE EYE
the side of the patient's head and reflected on to the mires, as
ordinary daylight is an insufficient illuminant.^,^j^
Fig. 64. — ^Latest Model of Meyrowitz Ophthalmometer
(Observer's Side).
Seated on the other side of the stand, the surgeon looks through
the eyepiece and points the telescope to the eye, and, by means of
THE OPHTHALMOMETER 12^
a rack and pinion on the upright, moves it up or down until he
sees four figures on the patient's cornea, which are two reflections
of each of the mires.* Ignoring the outside figures, he now
accurately focuses the two inside ones by means of a rack and
pinion.
The first step is to ascertain the axis of the astigmatism when
present, and we start with the arc horizontal, and note whether,
in the reflection of the two mires, the deep black line which runs
through the centre is a continuous black line running through
both; if not, the arc must be revolved to the right or left (but
never more than 45°) until this result is obtained; we then read
off the axis on the dial. The next step is to so adjust the mires
that their reflections are just in contact, as in Fig. 65, a. This,
then, is the primary position of the ophthalmometer — ^viz., with
the central black lines of the two mires forming an unbroken
black line, and the two inner edges of the mires just in contact.
We now revolve the arc through a complete right angle, and
if the relative position of the reflected mires has not changed,
then there is no corneal astigmatism ; but if, with the arc vertical
Fig. 65.
or nearly so, the reflections overlap (as in Fig. 65, b), there is
direct astigmatism, each step of the reflected mire overlapping
representing one dioptre of astigmatism; or, to be more correct,
we note the exact position of the underneath pointer on the disc,
bring the superficial pointer exactly over it, then readjust the
mires so that they are again in contact. If astigmatism is present,
the two pointers are separated by an interval which represents
the amount of the astigmatism, which amount is indicated by
the divisions on the disc.
If, on revolving the arc from the horizontal to the vertical
position, the reflections separate, we are dealing with inverse
astigmatism, and we must then make the secondary position our
primary position, and proceed as before.
If used as a servant, and not allowed to become master, the
ophthalmometer is one of the most valuable adjuncts to the
ophthalmologist's consulting-room, for after some practice, and
* The telescope has an adjustable eyepiece at the surgeon's
end, and in the centre an achromatic objective, which has
between its two bi-convex lenses a bi-refringent prism.
9
130 THE REFRACTION OF THE EYE
when thoroughly mastered, in about one minute the observer
ascertains (i) whether astigmatism is present, {2) the amount,
and (3) the direction of the axis of the principal meridian; and,
moreover, this is done with such delicacy that one-eighth of a
dioptre of astigmatism is revealed. It is also the quickest method
of diagnosing irregular astigmatism (see page 139).
In low errors, probably owing to some static astigmatism of the
lens, the total astigmatism of the eye generally shows about '25
of inverse astigmatism, in addition to the ophthalmometric
measurement. Thus, when the ophthalmometer shows no
astigmatism, there is '25 inverse astigmatism; when it shows
direct astigmatism, there is '25 less; and when it shows inverse
astigmatism, there is generally about '25 more, but this varies
in different instruments; the particular idiosyncrasy of any
instrument is very soon discovered.
The following points should be very carefully observed, in
order to prevent inaccurate results:
1. Impress upon the patient the imperative necessity of
keeping the forehead pressed against the stand. If on turning
the arc into the secondary position the mires have to be re-
focused, we know the patient has moved. Once the mires are
focused in the primary position, the focusing must not be altered.
2. It is equally important to insist upon the patient looking all
the time into the centre of the telescope; otherwise an astigma-
tism will appear which is not a central astigmatism, and is conse-
quently not the astigmatism we wish to ascertain.
Hardy's ophthalmometer, in which the mires are
stationary and the prisms are movable, is said to be
a more accurate instrument, but I have personally failed
to confirm this.
The Sutcliffe keratometer, which is a one-position
ophthalmometer, is said to be a very reliable instrument.
A full description will be found in the Ophthalmoscope,
April, 1909.
(c) The Ophthalmoscope. — Ophthalmoscopically, astig-
matism is revealed by observing that all parts of the
fundus are not in focus at the same time, no matter what
lens we turn into position.
Estimation of the Amount of Astigmatism hy the Oph-
thalmoscope. — Bearing in mind the fact that vertical
vessels are seen through the horizontal meridian, and
horizontal vessels through the vertical meridian, we
focus, for instance, the vessels passing horizontally from
the disc to the macula, and find the weakest concave or
ASTIGMATISM 13 1
strongest convex glass that gives us the best picture;
this will, of course, give us the refraction of the meridian
at right angles to this— viz., the vertical. We then focus
the vessels that pass up or down from the disc, and
estimate thus the refraction of the horizontal meridian,
and the difference between the two meridians is the
measure of astigmatism.
When the principal meridians are not horizontal and
vertical, we focus vessels passing obliquely — say up and
out from the disc — and afterwards those passing down
and out, and so on.
The patient must be under a cycloplegic, and the
observer's accommodation must be relaxed, which intro-
duces the personal element and causes this method to be
rarely used by ophthalmologists in preference to retin-
oscopy or the ophthalmometer.
It is important to remember that the vessels to be
observed should be those situated near the macula,
because we are estimating the refraction of the central
part of the dioptric system.
It is hardly necessary to add that this method is not
delicate enough for the diagnosis of small errors of
astigmatism.
The Ophthalmoscope by the Indirect Method. — This is
of little use in diagnosing low errors. When the error
is pronounced, the optic disc appears oval, and its
elongation is in the meridian of least curvature. When
the focus-glass is withdrawn from the eye, if the aerial
image remain the same size in one meridian but become
smaller in the other, the case is one of simple hyperopic
astigmatism; and if the image become larger, it is
a case of simple myopic astigmatism. In compound
hyperopic astigmatism the image becomes smaller in
both meridians, but more so in one ; and the reverse, of
course, in compound myopic astigmatism. In mixed
astigmatism, on withdrawing the focus-glass, the disc
appears to become relatively larger in the direction of
132
THE REFRACTION OF THE EYE
the maximum, and relatively smaller in the direction of
the minimum meridian.
3. Subjective Methods.— (^) Methods based on the
fact that when astigmatism is present lines running in
different directions are not all clearly seen at the same
Clock-face.
The fan, or " rising sun."
" Confusion letters," such as E and Z.
Pray's letters.
The clock-face (Fig. 66) SLud fan (Fig. 62), if observed
by a non-astigmatic eye at rest, will be seen to have the
lines all equally black; but in astigmatism the vertical
lines, for instance, will appear black, while the horizontal
are gray, or the oblique down and in, black, and the
oblique down and out, gray.
ASTIGMATISM 133
It should be borne in mind that the meridian of the
eye which corresponds to the darkest Hnes is the meridian
of greatest ametropia; thus, a patient who requires
- 1 cyl. axis horizontal to make all the lines appear
equally dark has simple myopic direct astigmatism, and
sees the vertical lines darkest before correction. The
patient's eyes must be under a cycloplegic, and concave
or convex spherical glasses may have to be placed in
front of the eye to correct any general ametropia present,
otherwise the whole chart may be out of focus.
This method is not delicate enough for very low
degrees of astigmatism, and, in fact, is rarely used.
Confusion Letters. — There are certain letters which
astigmatics often confuse, such as D and O or U, E and
Z, S and B, and when a patient on using Snellen's types
makes these mistakes we suspect astigmatism.
Fray's Letters are letters printed v^th stripes running
in different directions ; the patient selects the letters that
appear darker than the others, and the direction of the
stripes in the selected letter or letters corresponds to the
meridian of greatest ametropia (Fig. 67) .
(6) The Chromo-aberration or Cobalt-blue Test, based
on the principle that violet or blue rays, being more
refrangible than red, are brought to a focus sooner
(Chromatic Aberration) .
Cobalt-blue glass contains a great deal of red, and
allows only blue and red rays to pass. Such a glass of
suitable thickness is mounted in a trial frame placed
before the eye to be examined, the other eye being
excluded. A clear round point of light should be looked
at from a distance of 4 to 6 metres. When the eye is
emmetropic, the light appears violet; when hyperopic,
the light appears blue in the centre, surrounded by a red
ring ; and when myopic, red in the centre, surrounded by
a blue ring. If astigmatism be present, the light appears
oblong, vertically, or horizontally, in different charac-
teristic shapes. Scientifically, this is a most interesting
134 THE REFRACTION OF tHE EYE
test, but its weakness consists in the fact that it is purely
subjective, and that the surgeon is entirely dependent on
the patient's description,
(c) Examination of the Patient before Snellen*s Types—
(i) With the Stenopaic Slit (Fig. 58). — This is an opaque
disc to fit into the trial frame, and through the centre
there is a slit about i milHmetre broad. On looking
/6S'
W B
go" /OS' fS°
1iili.11 i/
US'
<55^
rs
Fig. 67.
through this sht the patient sees only rays passing
through the meridian corresponding to the slit ; all other
rays are excluded, and the glass in front of this slit
that gives the best vision represents the refraction of
this meridian. The slit is then turned round at right
angles, and the refraction of the other meridian is taken.
The difference between the two meridians is the amount
ASTIGMATISM 135
of astigmatism, and the value of the cylinder that will
have to be employed to correct the defect.
For instance, when, with the slit vertical, | is read,
and convex glasses make vision worse, this meridian is
emmetropic. On turning the slit round, when a +i
glass is required to get ^, this meridian is hyperopic,
the astigmatism is i d, and is corrected by a cylinder
+ 1 axis vertical.
This method is not very accurate, as much depends
on the width of the slit, and better and quicker methods
have superseded it ; but sometimes in a difficult case of
mixed astigmatism it is of assistance.
(2) With Cylinders. — This method is too wearisome to
be used by itself, but when we have ascertained the
refraction by the ophthalmometer and retinoscopy, we
always employ it as a final test.
Treatment. — All those engaged in refraction work
should observe two golden rules:
First Rule : Always suspect the presence of astig-
matism.
Second Rule : Never be satisfied that astigmatism
is eliminated unless the examination has been
made under a cycloplegic in all under forty or
forty-five years of age.
There is no refractive error in which cycloplegics are of
such paramount importance as in astigmatism of a small
amount.* The ciliary muscle has formed a bad habit
* The following table shows the relative frequency of small
errors :
500 consecutive refractions = 1000 eyes.
•12 164
'Under '5
Under i d -[
Astigmatism ■{ ['5 and over
•25 306 V 542
•37 72 j
•5 108 \
•62 14 V 192
1-75 70 J
I D and over ..... 132
No astigmatism ........ 134
Ophthalmometer correct = 982. ^^^^
136 THE REFRACTION OF THE EYE
(of which the patient is often quite unconscious), and
only gives up this habit when forced to do so by being
paralyzed.
Cylindrical lenses correct regular astigmatism of the
cornea, and when the error is small, do the work that
the ciliary muscle has been doing at so great a cost to
the nervous system. When the error is large, certainly
when it is over '75 d, the ciliary muscle cannot correct
the defect, and consequently makes no attempt to do
so ; but the greatest care must be exercised in giving the
exact cylinder that corrects the defect, because, if a
small portion is left uncorrected, the ciliary muscle can
do the rest of the work and strain results. For instance,
by retinoscopy we find an eye with the horizontal
meridian showing +4, and the vertical +2. We find
that a cylinder +2, axis vertical, and a sphere +1, gives
f , and when the effects of the cycloplegic have passed
off, we give, say, cylinder +2 axis vertical. Had we
been a little more careful we should have found that
the best result was obtained by a cylinder +2*25 axis
vertical, and this '25 we have omitted to correct is cor-
rected by the lens, and we introduce eyestrain which
did not exist before.
The estimation of astigmatism is now made entirely
by (i) the ophthalmometer,* (2) the shadow test, and
(3) the final trial of glasses before Snellen's types placed
at 6 metres from the patient.
Armed with the knowledge of the refraction of the
principal meridians and the direction of the axes, the
final test is very easy. The difference between the two
meridians represents the strength of the cylinder, and
the spherical glass is represented by the refraction of
the weakest meridian. Thus, when the horizontal
meridian is - 4, and the vertical - 6, we take a cylinder
- 2, place its axis to correspond with the least ame-
* This instrument is of the utmost value in correcting small
errors; it was correct in 982 cases out of 1,000 (see table, page 135).
ASTIGMATISM 137
tropic meridian — that is, horizontal — and combine it
with a spherical glass - 4.
In mixed astigmatism the selection of the glasses is a
little more complicated.
As mentioned before, the difference in refraction of
the two meridians equals the amount of astigmatism.
When the horizontal is +3, and the vertical -2, 5 d
is the amount of the astigmatism. Now, we can use
either a concave cylinder and convex sphere or vice
versa, and sometimes it is as well to try both kinds, as
one may be preferred. Let us in this case take a - 5
cylinder, and place it in the trial frames with the axis
horizontal {i.e., at right angles to the myopic meridian),
and correct the hyperopia with a +3 sphere. When
the patient recovers from the cycloplegic, this sphere
will have to be reduced to +2, or even +i*5. As a
general rule it is best to choose the cylinder that corrects
the most defective meridian. For instance, the vertical
meridian is - 1, and the horizontal +4. Here select
a +5 cylinder, set vertically, combined with a -i
sphere, which may have to be strengthened to - 1*5
when the cycloplegic has passed off.
The optician should be instructed to set the concave
glass next the eye, in order to obtain a periscopic effect.
The rule in mixed astigmatism is: The cylinder
represents the difference between the two meridians,
with its axis at right angles to the meridian whose sign
it corresponds to, and the spherical glass should be the
value of the meridian whose sign is the opposite to the
cylinder.
In testing the patient under a cycloplegic 6 metres
from Snellen's type with the trial glasses, we may have
to add or subtract from the various glasses, sphericals
or cylindricals, to get the best result, and the best
position of the axis of the cylinder may be a few degrees
different from what the retinoscopy or the ophthal-
mometer indicated; but we must remember that the
138 THE REFRACTION OF THE EYE
subjective examination must always have the " last
word," the objective examination being our guide, and
never our master.
Another important rule to be observed in the treat-
ment of astigmatism is that it should be fully corrected,
and when the patient has recovered from the cycloplegic
neither the power nor the axis of the cylinder should be
altered, although we may have to deduct from or add
to the spherical lens. The exceptions to this rule are
few, and are as follows : (i) In children under seven years
of age, when the astigmatism is less than '5, no cylinder
need be ordered if no symptom of eyestrain be present ;
(2) in patients over, say, 40 years of age, who have never
had their astigmatism corrected and who require a high
cylinder, as this latter may give discomfort at first, a
10
^
^30
Fig. 68.
slight reduction may be made in the power, but the full
correction should be given as soon as possible later.
In examining patients for astigmatism, atropine
should be previously used up to the age of 25, and even
up to the age of 35, if a low error is suspected; hom-
atropine is sufficient for older patients; after 50 no
cycloplegic is necessary.
In ordering cylinders be careful to indicate accurately
the axis. If this is done by simply writing down the
degrees, a mistake may occur, as there is no uniformity
at present in the numbering of the trial frames or pre-
scription forms. The International Ophthalmic Con-
gress at Naples, 1909, suggested that the nasal ex-
tremities of the horizontal line should be zero, the
temporal extremities 180°, and the vertical line 90°, but
this has not yet been universally adopted. The simplest
IRREGULAR ASTIGMATISM
139
method is to indicate the axis in degrees from the
vertical or the horizontal, as in Fig. 68, or on an optician's
form, or, better still, on a form engraved or stamped on
the paper, as in Fig. 69.
Note. — So important is it, when treating small errors
of astigmatism, to correct even the smallest amount,
that the trial case should be fitted with cylinders repre-
senting fractions of •12 up to i d — i.e., in addition to
•25, '50, '75, there should also be •12, 'Z7> '62, and '87
cylinders.
Irregular Astigmatism. — Physiological irregular astig-
matism is present in all lenses. It is due to separate
sectors of the lens having a different refractive power,
and is infinitesimal in amount. It is this condition
Fig. 69.
which causes a bright star to appear as a radiating
figure instead of a bright point.
Sometimes the refractive power of the separate
sectors of the lens is so great that several images of a
point are formed, and it is in this way we get monocular
polyopia in incipient cataract.
Irregular Astigmatism in the Cornea may be consider-
able, and is generally the result of disease, such as ulcers,
nebulae, wounds, and conical cornea. It is due to a
difference in curvature in different parts of the same
meridian, and often produces distortion of objects, which
regular astigmatism rarely does.
Irregular astigmatism is diagnosed by —
I. The Ophthalmometer. — ^This is the readiest and the
easiest method. The reflection of one or both mires is
140 THE REFRACTION OF THE EYE
distorted, and this distortion and the relative positions
of the mires vary irregularly in different positions- of
the arc.
2. Retinoscopy. — There is no definite shadow, or, if it
be present, it behaves in an irregular manner, and glasses
produce no definite and regular effect.
3. Placido's Disc (Fig. 70). — This is a round disc
supported by a handle. The disc is about 7 or 8 inches
in diameter, and has painted on one side alternate
concentric rings of black and white, and has a small hole
in the centre. The patient stands with his back to the
light, and the disc is held by the surgeon a short dis-
tance from the patient's eye in such a manner that an
image of the rings is thrown on to the cornea to be
examined, the patient being directed to look at the
IRREGULAR ASTIGMATISM I4I
centre. Looking through the central hole, the surgeon
sees a diminutive image of the rings on the cornea, and
if the latter is normal the rings are round and evenly
separated. In regular astigmatism the image appears
elliptical, the long axis corresponding with the meridian
of least curvature; but when irregular astigmatism is
present, the rings are " crinkled " and distorted.
Treatment. — Place before the eye in a trial frame an
opaque disc with a small central opening (stenopaic
disc), shift the position of the opening, and also place
up different lenses. If vision be improved, prescribe
stenopaic spectacles, with the lens if indicated; and if
the opening be eccentric, specify the amount of de-
centring.
In conical cornea this treatment is sometimes bene-
ficial, but, unfortunately, in most cases the spectacle
treatment of irregular astigmatism is useless.
As regular astigmatism may also be present, it is
worth while making the above examination with a
stenopaic slit, turning it round in different positions to
see if any improvement results with or without the addi-
tion of a convex or concave spherical lens; and if any
improvement results with a lens, the prescription of the
equivalent cylinder may improve vision.
If the defect is caused by a corneal nebula, and
especially if the patient is young and the opacity not
too dense, considerable improvement sometimes follows
the use of the yellow oxide of mercury ointment, a small
piece of which should be rubbed into the cornea by
massage through the upper lid, about twice a week.
CHAiPTER IX
PRESBYOPIA
The Influence of Age upon the Accommodation. — In
quite early youth the crystalUne lens is practically a
small bag of semifluid jelly, and accommodation takes
place by its being squeezed by the action of the ciliary
muscle in such a manner that its antero-posterior
diameter is enlarged (see page 31). So great is this
" squeezability " (if I may use the term) in the very
young, that an accommodative power of 20 d can often
be recorded. As age advances, a hardening process
or sclerosis goes on in the lens as in all the other tissues
of the body, and so its " squeezability " becomes less
and less until a point is reached when the near point
of accommodation, which represents the fullest accom-
modative power, has so far receded that the normal eye
requires assistance in the shape of a convex lens in
order to see near objects distinctly. This is presbyopia.
At the age of 10 years the average emmetrope's near
point is 7 cms. from the eye, and his far point being at
" infinity," we see that his amplitude of accommodation
is 14 D (in Fig. 71, in the first column on the left, there
are 14 divisions between p and ;'), whereas at the age
of 30 his near point has receded to 14 cms., and his
amplitude of accommodation is then only 7 D — -that is,
in twenty years he has lost half of his accommodative
power.
The same happens whatever the refractive condition of
the eye. For instance, a hyperope of 4 d (Fig. 72) at
142
PRESBYdPtA
143
Near
Point.
Cms.
7 ••
fO fS 20 25 30 3S
Age.
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Fig.
71-
Showing the range of accommodation of an emmetrope at
different ages.
the age of 10 has his near point 10 cms. from the eye,
and p = '-^;^ = -W = iOD,r is negative, and
a = 10 - ( - 4)
= 10 + 4
= 14 D.
Again, at 30 we see (Fig. 72) that ^ = 3 d, p being
now 33 cms. from the eye, and
a = 3 + 4
= 7D.
* The numerals above represent years, those on the left
dioptres. The line p p represents the curve of the punctum.
proximum, and the line r r that of the punctum remotum.
144 THE REFRACTION OF THE EYE
70 IS 20 25 30 3S 4^ ^ SO S6 60 66 70 76 80
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Fig. 72.
Showing the range of accommodation of an uncorrected hyperope
of 4 D at different ages.
A myope, say, of 3 d (Fig. 73) has his near point, at
the age of 10, 6 cms. from the eye, and ^ = 17 d.
a = 17 - 3
= 14 D.
^ At the age of 30 we see by the diagram that p = 10 D,
for p = 10 cms., and R is still 33 cms. on the positive
side; hence
a = p - r
= 10-3
= 7D.
\\'hatever the static refraction of the eye, r remains
stationary till about the age of 55, when we see that in
PRESBYOPIA
145
all three diagrams it begins to curve downwards, show-
ing that the emmetrope becomes hyperopic, the hyper-
ope more so, and the myope less so. This is called
acquired hyperopia. A point is finally reached when p
and r unite — in other words, when all accommodation
ceases ; this is about the age of 75 ; but in emmetropia and
Age.
/O /S 20 26 30 35 <^ ^i SO SS €0 €S 70 7S 80
:i
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Fig. 73.
Showing the range of accommodation of an uncorrected myope
of 3 D at different ages.
hyperopia the positive part of accommodation — viz.,
that employed for near objects — ceases at an earlier age.
In emmetropia p is seen to cross the zero line between
the ages of 60 and 65 (Fig. 71) (some accommodation is
still left, but it is employed in correcting the " acquired
hyperopia "), and in hyperopia even earlier. The
greater the degree of hyperopia, the earlier will p cross
10
146
THE REFRACTION OF THE EYE
the zero line. In the case of a hyperope of 4 d (Fig. 72)
this happens between the ages of 40 and 45 — that is,
a hyperope of 4 D, when he reaches the age of 42,
although he has some amplitude of accommodation,
has to make use of it entirely for distance; on the
other hand, all myopes of more than 3 D can make
use of all their accommodative power for near work
(Fig. 73).
These conclusions were arrived at by Bonders from a
number of observations made and recorded, and the
diagrams are the results of the averages of these observa-
tions; but we now know that he under-estimated the
accommodative power between the ages of 10 and 45,
the average power being about i'5 more. This mistake
was probably due to a majority of his patients having
some latent hyperopia which he was unaware of, as he
did not make his patient " emmetropic " before testing
the accommodative power.
Duane, in 1909, examined the accommodative power
of 600 patients whom he had previously made emme-
tropic {Jour, of Amer. Med., vol. lii., page 1992), and he
gives the following tables :
Age.
10
15
20
25
30
35
40
45
50
55
60
Minimum.
Mean.
Maximum
II-2
14
17-5
IO-5
13-4
i6-5
9-1
II-5
14-2
8-2
IO-3
12-9
6-8
8-5
iO'6
5-6
7
8-8
4-8
6
7-5
3
3-8
4'7
1-4
1-8
2.3
0.9
1-3
1-6
0.9
I'2
1-5
Following on the same lines, I have examined the
accommodative power of over 2,000 patients with their
refractive error fully corrected, and the chart (Fig. 74)
shows the average curve of my results.
PRESBYOPIA
147
/<? /s 2 a 2s ^(p3s «%? -PS S(? ss eo €6 /-a
I
/2
\
//
\
^0
A
»v
\
.
(<?
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7
6
>
s.
\
V
N
V
4
3
Z
-/
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s.
N
s,
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X
Fig. 74.
My table is as follows :
Age.
7-10
10-15
20
25
30
35
40
45
50
55
60
65
Minimum.
9
7
6
5-5
4-5
4
2-5
2
I
0-75
0-50
0*50
Mean.
Maximum.
13-5
18
12
18
IO-5
14
9
13-5
7-5
12
6-5
10
5-5
8.5
4-25
7
3-5
6
2-5
5
1-75
4
I
3
The loss of elasticity of the lens is accompanied by
greater firmness and increase in size, and, in later years.
148 THE REFRACTION OF THE EYE
by a loss of homogeneousness and transparency both
of the lens and vitreous, which is such a striking con-
dition in youth. The lens reflects more light, and, by
oblique illumination, often gives a false idea of cataract.
At the age of 45 fevery individual has, more or less,
about 4 D range of accommodation. From our formula
a = p - r we get p = a + r — i.e., p = 4 + r. When
the person is an emmetrope we can ignore r; therefore
p = 4. Now P, the punctum proximum, is '-^^;
therefore P = ub^ = 25 cms. That is, the average
emmetrope at the age of 45 cannot read nearer than
25 cms., or 10 inches. This may be taken as the
presbyopic point.
Donders fixed the presbyopic point at 22 cms., the
point to which, he said, the average individual's near
point had receded at the age of 40.
As a matter of fact, there is no fixed presbyopic point.
An individual may be said to have arrived at the
presbyopic period when he finds that he cannot do his
near work for any length of time without discomfort
or some symptom of strain. A long- armed man who
does very little near work may not require glasses until
the age of 50, whereas a seamstress, with the same re-
fraction and of the same age, may have had to take to
glasses five years earlier.
Some writers object to the term " presbyopia," and
would expunge it from ophthalmology, giving us nothing
in its place. We must have some term to express that
condition in which, as the result of the increase of years,
the range of accommodation is diminished and the vision
of near objects is interfered with.
The recognition of presbyopia is not difficult. When
a patient complains that he has to hold his book, when
reading, further away than he has been accustomed to,
that this is more especially so by artificial light, that the
figures 3, 5, and 8 become confused, and that n and u are
difficult to distinguish, and at the same time asserts
PRESBYOPIA 149
that his distant vision has not altered, we may be almost
certain we are dealing with a presbyope.
When a patient whose near point has receded, say, to
33 cms. attempts to read or work at that distance for
any length of time, symptoms of eyestrain will be sure
to supervene. It is a fact that we get from everyday
experience, that the full power of a muscle can be exer-
cised only for a very short time without fatigue. A
person whose near point is at 33 cms. is using the whole
power of his ciliary muscle in order to focus an object
at that distance on his retina, and fatigue of the muscle
will very soon ensue. This fatigue causes the muscle to
relax, it cannot contract to its full extent; vision then
becomes hazy, and becomes distinct again only when
the object has been further removed from the eye. At
the same time the patient will probably complain that,
after reading some little time, headache comes on, and
the eyes begin to water; these temporary symptoms of
eyestrain will pass into chronic symptoms in time, and
the red, irritable-looking, watery eyes of middle-aged
people are often due to this cause.
Treatment. — Two classes of patients come for treat-
ment. One class simply requests glasses for reading
(some of these have been to an optician and have
themselves selected a weak spherical glass which proves
unsuitable) ; the other class comes complaining of some
symptom of eyestrain, and they may or may not be
aware that they require glasses.
A cycloplegic is rarely necessary (except in isolated
cases where our results are unsatisfactory or we wish
to examine the lens for cataract), because ciliary spasm
is very unlikely to be present, and there is no latent
hyperopia, it having all become manifest. We first
ascertain the distant correction, and then the near
point of distinct vision and the most comfortable
distance for reading or working. Suppose distant vision
is normal and the punctum proximum is 28 cms., and
150 THE REFRACTION OF THE EYE
the distance at which the patient wishes to read is
33 cms., his ampUtude of accommodation is -^ = 3*5 d ;
to avoid fatigue he must not use the whole of this, but
must keep about J in reserve. Let him have i*5 .d in
reserve; this leaves him 2 D available accommodative
power, and, as he requires 3 d, we supply the deficit by-
giving him +1 D glasses.
The treatment, simple as it appears, is not always
successful, because the accommodative power varies
with the individual, and when it is low a greater reserve
has to be left. We generally find that an emmetrope
of about 48 requires a reading glass of +1 D, and an
additional +1 for every five years.
Perusal of the table on page 147 shows what a wide difference
there is in the accommodative power of different individuals;
when the accommodative power is lower than normal it will be
found that the individual is older, and looks older, than his age,
and vice versa. Premature presbyopia, which is really pre-
mature senility, is provoked by various conditions, but intestinal
stasis is perhaps the commonest cause. A small error of refrac-
tion uncorrected not only tends to lower the power of the ciliary
muscle by the constant drain on its energy, but also tends to
hasten the sclerosing processes in the lens (see page 163).
When ametropia is present, the distance correction
must be ascertained, and the presbyopic glasses added
to it; thus, if the distance glass is +2, and an addition
of +1*5 is required for reading, the reading glass is
+3-5; if the distance correction is - 4, and an addition
of -I- 2 is required for reading, the reading glass is - 2 .
When astigmatism is present, the correcting cylinder
must, of course, be added.
It was the custom until quite recently for presbyopes
who had fairly normal distant vision to be px^ovided with
reading glasses only, but we now recognize the impor-
tance of correcting all errors of refraction, especially at
this period of life, so that unless the patient is an emme-
trope he will require glasses for distance as well as near
work. This double correction is best prescribed in the
PRESBYOPIA 151
form of " bi-focal '* spectacles or pince-nez (see page 17),
where the upper part is used for distance and the lower
for reading. These bi-focals are now made so that the
division between the two portions is invisible,* and it is
not an exaggeration to assert that they have completely
revolutionized the treatment of presbyopia.
They should be prescribed for all patients who have
the slightest error of refraction in addition to their
presbyopia, if they show any sign of eyestrain or nerve
waste. On the other hand, those who live an open-air
life, whose distant vision is practically normal, and who
^i^joy good health, may be allowed to have reading
glasses only.
As the bi-focals have to be used for writing as well as
reading, it is very important that the lower section be
not too strong. It is rarely necessary at any time of
life to wear a stronger addition than 3 d ; should the
patient want a stronger reading glass for the evening, it
must be prescribed as a separate glass. As an alterna-
tive to bi-focals, the reading " addition " may be
mounted in pince-nez or " hook fronts," to be worn in
front of the distance glasses when reading. When
so-called " music " glasses for near work at arm's length
are required, they should be +1 or -hi'5 D weaker than
the reading glasses.
Fatigue of the Accommodation. — We must be careful
to distinguish between presbyopia and ordinary physio-
logical fatigue of the ciliary muscle, which may come
on after prolonged use of the eyes in the normal, or
may be associated with general muscle fatigue, as in
neurasthenia. As we have seen, presbyopia is not
necessarily due to weakness of the ciliary muscle, but
to the sclerosing of the lens.
* The optician must take great care in fitting and centring
bi-focals ; the division between the glasses should never be higher
than the margin of the lower lid, and full allowance should always
be made for convergence — thus, if the patient converges 3 mms.,
then each reading portion should be decentred in i'5 mms.
CHAPTER X
ANISOMETROPIA
Anisometropia is a condition in which the refraction
of the two eyes is different. A difference in refraction
in the two eyes is more often met with than absolute
equaUty, and in astigmatism it is very common to find
a difference of *2^ or •50 between them.
It was formerly the practice only to recognize aniso-
metropia when, to produce the maximum of visual
acuity in the two eyes, different glasses were required;
but the increased knowledge we have now of the effects
of eyestrain has taught us that the smallest amount
must not be neglected, and, further, that the smaller
the amount of anisometropia, the more important is it
to correct it.
Every possible combination may exist:
1. One eye may be emmetropic, and the other ame-
tropic.
2. Both eyes may be ametropic —
{a) The same variety of ametropia, but unequal
in degree;
{b) Different varieties of ametropia, one eye
myopic and the other hyperopic ; this variety
is sometimes called " antimetropia."
When one eye is astigmatic and the other hyperopic
or myopic, the astigmatic eye has generally the same
form of ametropia as the non-astigmatic eye, and very
often one of its meridians has the same amount of
ametropia.
152
ANISOMETROPIA 153
Except when it is the consequence of an operation,
loss of lens, etc., anisometropia may be regarded as con-
genital, and attributable to the unequal development of
the eyes.
The difference of the refraction of the two eyes may be
very great, and is then often associated with marked
asymmetry of the face; a difference of lo dioptres has
been recorded.
Varieties of Anisometropia. — There are three varieties
of anisometropia:
1. Simultaneous binocular vision exists.
2. The eyes are used alternately.
3. One of the eyes is permanently excluded.
I. Simultaneous Binocular Vision — Tests for
Binocular Vision — {a) The Prism Test. — The patient
fixes an object at a distance, and a strong prism, base
in or out, is placed before one eye, the other being
uncovered; at the moment of interposing the prism, if
the eye make a movement towards the apex of the
prism, binocular vision exists.
(h) Snellen's Coloured Glass Test* — " Friend " Test. —
This apparatus consists of a frame, hung up before the
window, in which letters of coloured glass, alternately
green and red, are placed. The patient standing in
front of them, at a distance of 4 or 5 metres, is provided
with a spectacle frame into which one red and one
green glass is placed, the colour of these glasses being
of the same intensity as that of the letters. Only
the red letters can be seen through the red glass, and
the green letters through the green glass, so that each
eye, separately, only sees half the letters. The letters
in the frame may be made to spell a word, such as
FRIEND, so arranged that the letters f, i, and N are
red, and R, e, and d green. If the patient, having had
* This test was introduced into England from Bonders'
clinique in 1881 by the author.
154 THE REFRACTION OF THE EYE
any ametropia corrected, sees all the letters and spells
the word "friend," we know he has binocular vision;
but if, e.g., he only spells f i n, we know that he is using
the eye with the red glass in front of it, and that the
other eye is excluded from vision.
For binocular vision to exist in anisometropia the
difference in refraction between the two eyes — that is,
the degree of anisometropia — must be small, although
cases have been recorded where it has amounted to 6 d.
Under these circumstances, although the magnitude and
acuteness of the images in the two eyes are unequal,
they overlap and help each other.
If each ciliary muscle could act independently, the
anisometrope could very often correct each eye by a
separate and independent accommodation in each eye;
but it is generally believed that the same effort of
accommodation is made on both sides, with the result
that only one image is sharp. At the same time, it
should be noted that when the anisometropia is of low
degree, some patients may have the power of producing
asymmetrical accommodation to a limited extent. If we
place a convex glass of +»5o in front of one eye and a
concave glass of - *$o in front of the other, and thus
produce an anisometropia of i d, and attempt to read
with the glasses, a very distinct feeling of strain is ex-
perienced; but whether this strain is brought about by
actual asymmetrical accommodation, or only by the
attempt to produce it, it is difficult to say.
Treatment. — The treatment in these cases varies con-
siderably. In quite young patients, unless the difference
between the eyes is very great, the full correction of each
eye should be ordered in accordance with the plans stated
in the previous pages. In older patients we must try
the binocular effect before prescribing glasses. The full
correction in each eye is especially indicated when
marked symptoms of eyestrain exist, and in these cases,
although the treatment may be complained of at first.
ANISOMETROPIA 155
it is wise to insist upon it, and in many cases the dis-
comfort produced at first by the glasses disappears in a
few weeks, and eventually the symptoms of eyestrain
disappear. On the other hand, patients who have lived
many years without having their anisometropia cor-
rected, have become so accustomed to the difference
between the eyes that the removal of this difference not
only confers no benefit, but proves irksome, and they
may complain of dazzling, giddiness, and headache.
Place the proper correction in front of each eye, and
try the binocular effect; if this is unsatisfactory, take
off a little from the stronger glass or add a little to the
weaker, or do both. Sometimes we find that the patient
prefers the same correction in both eyes, and that this
correction is weaker than that required by the more
ametropic eye. For instance, suppose the right eye
is emmetropic and the left hyperopic to the extent of
2 D, we try first a plane glass in front of the right and
+2 in front of the left; if this causes discomfort, we
may try a +»5 in front of the right and +i«5 in front
of the left. If this is still uncomfortable, we try 4-1 in
front of both eyes. No definite rule can be laid down;
each case should be treated according to its require-
ments and the patient's sensations.
It is a good plan when the correction has been found
under a cycloplegic to deduct a little more from the
more ametropic eye and a little less from the other.
For instance, supposing under atropine the refraction
is, right eye +2, and left eye -i-3'5; we give +i«25
for the right, and +2 for the left. When the difference
between the glasses is great, the patient should be
taught to turn his head when looking to the right or left,
and not his eyes, so that he is always looking through
the centre of his glasses. If he does not look through
the centre of his glasses, a prismatic effect is produced;
this, of course, will produce an artificial heterophofia
and cause strain. Mr. W. A. Dixey has suggested that
156
THE REFRACTION OF THE EYE
this prismatic effect on looking eccentrically can be
avoided by reducing the margin of the stronger lens to
the power of the weaker one in myopic patients, where
the difference in the two eyes is marked and binocular
v'sion exists. For instance, the right eye is -2. and
the left - 6 ; the outer margin of the - 6 lens is reduced
to -2, as in Fig. 75.
2. The Eyes are used alternately. — One eye is
emmetropic or slightly hyperopic, and is used for
distance, and the other myopic, and used for near work.
If eyestrain be not present, the patient may prefer to
have no glasses. Although he has lost binocular vision,
Cz
=a
Fig. 75.
he has gained other advantages; in some cases he can
entirely dispense with muscular effort, his ciliary muscle
and internal recti being rarely used. If eyestrain be
present, we must prescribe glasses after ascertaining the
binocular combination that suits best. We sometimes
find that if, for instance, both eyes are hyperopic, the
patient prefers the same glass in both eyes corresponding
to that required by the most ametropic; this latter eye
will then be used for distance and the other for near
work. In the same manner, if both eyes are myopic,
we give each the correcting glass of the weaker.
In all these cases the cylinder should never be altered.
ANISOMETROPIA 157
except in rare instances where there is a great difference
in the astigmatism of the two eyes, and then the stronger
cyhnder may have to be reduced.
3. One Eye is permanently excluded from
Vision. — ^When the difference between the eyes is
great, the more defective eye is Httle used, and tends
to become amblyopic, if it is not so already. In such
cases we must give each eye its proper correction, and
instruct the patient to practise the amblyopic eye by
totally excluding vision with the good eye by covering
it with a patch for a certain time every day. In " am-
blyopia ex anopsia," occurring in young patients with
convergent strabismus, this treatment, patiently perse-
vered in, is often most satisfactory.
The defective eye may never take its proper place in
binocular vision, but in some cases it may become very
useful, especially if any damage or disease should affect
the good eye; and, moreover, the cosmetic effect which
sometimes occurs is considerable, for, if treated in time,
the strabismus which so often appears in these cases
may be prevented.
Adults with only one useful eye may be allowed to
wear a monocle, provided that the correction does not
include a low-power cylinder. If a cylinder is required,
the glass must always be placed in the proper position :
and if a high cylinder is used, the patient can tell at once
if the axis of the cylinder has been properly adjusted.
But in a cylinder of '25 or '5 it will be impossible for
him thus to judge the correctness of the axis, even
though he thinks he is putting the glass into its proper
position. In such cases, if a monocle is insisted upon,
the cylinder should be omitted.
Monocles should be mounted in a frame with a bracket,
in order that the glass may be removed from contact with
the skin and lashes.
Presbyopia and Anisometropia. — When presbyopia is
present with anisometropia, we should, whenever pes-
158 THE REFRACTION OF THE EYE
sible, prescribe bi- focal glasses. The lower reading
section must be found by the same kind of trial as we
made for the distant correction. It does not follow that
the same addition is given to each eye. For instance,
one eye has a hyperopia of 2 and the other of 4, and the
patient is 52; we should try +4 and +6 for reading,
or, if this be not comfortable, +4»5 and +5 '5, or +5
in both eyes.
When one eye is permanently excluded, and the re-
maining eye is ametropic and presbyopic, as in aphakia
after cataract extraction, reversible spectacles are useful ;
the distant correction is on one side and the reading
on the other. When the patient is walking, the distant
correction is in front of the " good " eye and the reading
glass in front of the useless eye, and when he wishes to
read he reverses the spectacles, and so brings the read-
ing-glass in front of the good eye, the bridge being
made to fit the nose in either position.
When the difference between the two eyes is marked, and
simultaneous binocular vision exists, although the distance cor-
rection is accepted, discomfort may come with near work. This
is due to the production of an artificial hyperphoria. Let us
suppose the right eye to have a myopia of i'5, and the left a
hyperopia of the same amount. When reading and looking, say,
6 mm. below the optical centres, the patient is looking through
a prism 1° base down in front of the right eye and 1° base up
in front of the left. (A lens i d, decentred 8*7 mm., produces a
prismatic effect of i°.) The difficulty can be overcome by
cementing on the lower portion of each lens the necessary correct-
ing prism. In the above case a prism 1° base up in front of the
right eye and 1° base down in front of the left will correct the
hyperphoria.
When a presbyopic correction is also required, the prismatic
effect may be obtained by decentring, or by making the prism
convex to the amount required.
CHAPTER XI
APHAKIA
Although, to be quite correct, aphakia denotes an
eye without a lens, the latter having been purposely
removed by operation, or accidentally lost through a
perforating wound or ulcer, we generally include under
aphakia, conditions in which the lens is more or less com-
pletely dislocated (as in the old operation of " couch-
ing "), so that rays of light passing into the eye do not
intercept any portion of it.
The absence of the lenticular images, found by holding
a light near to the eye (see page 31), and a tremulous
iris, are characteristic signs of this condition.
A strong convex glass must be placed before the eye
to obtain distinct distant vision, unless the original
condition of the eye was one of high myopia. A convex
glass of II or 12 d, placed about 13 mm. in front of the
eye, is about the equivalent of the crystalline lens ; but,
" as the refracting system of the eye is now entirely
different from that of an ametropic eye, the tests of
visual acuteness are no longer comparable with that of
normal eyes. The retinal image in an aphakic eye is
I •33 times larger than in a normal eye. Hence vision
of f in a corrected aphakic would really only correspond
to a visual acuteness of A in a normal eye " (Percival).
As all accommodation is lost through the removal of
the lens, the convex glass has to be increased in strength
for near work, according to the distance of the near
work from the eye ; thus, if 33 cms. is the spot required
159
l60 THE REFRACTION OF THE EYE
at which to read, + 3 must be added; and if 25 cms. is
the distance, + 4 must be added. After cataract ex-
traction there is almost always a large amount of inverse
astigmatism, and a + cylinder of i to 2 d, placed
horizontally, or nearly horizontally, will probably be
required. As this inverse astigmatism is generally
fairly high immediately after the operation, and gradu-
ally diminishes during a period of two or three months,
it is wise not to prescribe the cataract glasses until
this period has elapsed.
When a cylinder is required, the glass is best given
in the sphero-toric form (see page 16).
When the original condition of the eye was one of
ametropia, the following formula is a fairly accurate
guide of the glasses required after operation (Percival) :
-,25 +D
1(T0
2
where D is the original refraction of the eye and D'
is the approximate glasses required; thus, if D = - 15,
then
TA/ 25 + (-15) 25-15 10 ,
2-i^f 2+-I5 2-15 ^ ^'
and if the original refraction was a hyperopia of 6,
^, 25+6 31 31
2-Tw 2 --06 1-94
This formula is calculated for the position of the cor-
recting lens D being 13*7 mm. in front of the eye; if
this distance is increased, the convex glass is propor-
tionately decreased in strength. For practical purposes,
if we halve the amount of the original error and add
to it + II, we shall find that this is approximately the
glass required. According to this, if the patient had
* This sign represents approximate equivalency.
APHAKIA l6l
originally a myopia of 22, he probably requires no dis-
tance glass after the removal of the lens.
The most convenient form of glass is the bi-focal (see
page 16) . Some patients prefer the reversible form (see
page 158).
If the correction be not satisfactory, an examination
of the patient in the dark room with focal illumination
and a strong lens, will probably reveal an opaque cap-
sule, which will require needling before any good result
is obtained with glasses.
IX
CHAPTER XII
EYESTRAIN
Eyestrain, as we now understand it, extends over a
far wider field than it did even twenty-five years ago,
and the list of troubles which more or less depend upon
its presence grows larger every year. It includes, of
course, the old " asthenopia " that Bonders, writing
in 1858, considered was due to hyperopia, and the
" muscular asthenopia " which Von Graefe, a few years
later, attributed to strain of the internal recti muscles.
The word " asthenopia," by its derivation, denotes
weak sight, which, in the large majority of cases, does
not exist; and even if we take the broader meaning,
implying tired sight, the word is equally inappropriate.
It is therefore best to restrict the term " asthenopia "
entirely to retinal fatigue — i.e., retinal asthenopia —
with which we are not here concerned.
Eyestrain may be defined as a symptom or group of
symptoms produced by the correction or attempt at
correction by the ciliary muscle of an error of refraction
or a small amount of anisometropia, or as a want of
balance between the external muscles of the eye. Where
gross errors exist, either in the refraction or the aniso-
metropia or in the muscular equilibrium, the patient
cannot correct, and consequently makes no attempt
to correct, the defect, and eyestrain is not produced.
The smaller the error, the more likely is eyestrain to be
present, and also, unfortunately for the patient, the
more likely is it to be overlooked.
162
EYESTRAIN 163
The symptoms of eyestrain may be grouped under
three headings :
1. Manifestations on the eye and Uds.
2. Peripheral irritation.
3. Nerve exhaustion.
1. Manifestation of Eyestrain on the Eye and Lids. —
Eyestrain means an increased demand for work on the
part of the cihary or external muscles, which determines
an increased flow of blood to these parts; if this is
constant, congestion is liable to follow, and with it pain.
With the parts thus rendered specially receptive to
infective germs, there is little difficulty in understanding
that inflammation should ensue, and thus we find that
blepharitis, conjunctivitis, corneal ulcers, phlyctenulae,
iritis, cyclitis and glaucoma may have eyestrain as one
of the predisposing causes. There is also little doubt
that cataract may be started by the irregular contrac-
tion of the ciliary muscle in correcting low errors of
astigmatism, and that the correction of these errors,
with the consequent disappearance of the eyestrain, will
stay the progress of this disease.
2. Peripheral Irritation —
(a) With pain.
(b) Without pain.
Peripheral Irritation with Pain. — In the same manner
that a decayed molar may produce neuralgia by peri-
pheral irritation, so eyestrain may produce headaches,
or any other form of peripheral irritation. Headache
is by far the commonest symptom of eyestrain. So
common is it, indeed, that no physician should attempt
to treat a patient for constantly recurring headaches
unless the existence of eyestrain has been eliminated by
proper correction under a cycloplegic.
The position and character of the headache form no
guide to the cause, for ocular headache may be of any
possible variety. It may simply amount to a slight
164 THfi REFRACTION OF THE EYE
aching over the eyes or at the back of the orbit. It may
be a frontal headache. Sometimes it originates and
remains Hmited as a vertical or occipital pain, or it
may originate in the brow, and pass to the vertex and
occiput, and pass down the spine. It may be unilateral
as a typical hemicrania, and may be indistinguishable
from a true migraine attack.
There is no rule as to its position; this varies with
the individual. In some it is superficial, akin to neu-
ralgia; in others, deep-seated.
It may be a dull, heavy ache, difficult to localize
accurately, or it may be a sharp, shooting, lancinating
pain, that seems to originate in a tender spot on the
scalp or forehead. Some describe the pain as an opening
and shutting of the skull, others as if a nail were being
driven into the vertex.
But the commonest form of eyestrain headache is a
pain over one or both brows, often termed " brow ague."
Again, the time of the headache varies; it may be a
permanent headache or periodic, and it may appear to
have no direct association with excessive use of the
eyes.
Early-morning headache is a very common form of
ocular headache, and this astonishes most patients, as
they imagine that the night's rest should have removed
the possibility of this.
To explain the periodicity of the headaches we have
to remember that the cause of the headache may be
multiple. There may be two, or even three, factors
present. A patient who suffers from a periodical head-
ache may have eyestrain,- a gouty or other diathetic
tendency, and at times, added to these, some special
nerve depressant, such as worry or trouble.
Now, each of these factors, or possibly any two, may
not suffice to cause a headache; all three must be
present, and only at such times as all three are present
is the headache there. If the eyestrain be removed by
EYESTRAIN 165
correcting the error, the other causes, save under ex-
ceptional conditions, will never succeed in producing
the pain.
It is important to remember that with this headache
there is often associated nausea, and even vomiting.
The so-called biliotis headache of the " old school " is
generally an ocular headache.
The intimate relation of the nerve-supply of the ocular
muscles with the fifth nerve, and the association of the
latter with the sympathetic and pneumogastric, explain
the method of origin of the symptoms.
The ciliary muscle and the external eye muscles,
when strained, demand an increased supply of blood,
which in time leads to congestion. This in its turn will
not be limited to the strained part, but will spread to
other parts of the eye, causing the watery, red eye
already alluded to. The pain of " fatigue " is probably
due to this congestion, and accounts for the tender
eyeballs so common in migraine.
Liveing says that the nerve-storms in migraine have
their point of departure or principal focus in the optic
thalami, and that the normal course is from above
downwards to the nuclei of the vagi, and from before
backwards in the sensory tract, thus explaining the
peculiar visual phenomena, such as teichopsia and other
symptoms of ophthalmic migraine.
Of course, there are cases of true migraine, when the
" point of departure " is from above downwards, but a
very large percentage of cases labelled " migraine " are
purely ocular in origin; and if the error of refraction is
corrected, or the muscular equilibrium re-established,
the symptoms disappear.
The connection of the fifth nerve with the sympathetic
enables us to understand how the peripheral irritation
of eyestrain can pass to the dura mater, pia mater, and
sensory layer of the brain cortex.
Gowers says: " If the sensory cells of the cortex, in
l66 THE REFRACTION OF THE EYE
which the cranial and intracranial sensitive structures
are represented, are the most readily influenced of all
the sensory cells, we can understand that headache
should result from vascular repletion."* Everyone
knows from experience that a headache increases some-
times to an alarming severity when the head is lowered —
i.e., when more blood flows to the head.
As we have already seen, although in eyestrain the
peripheral irritation, if it takes the above form, is always
or almost always present, it may not manifest itself
as a headache unless there is a general increase of blood-
pressure. If the blood-pressure be lowered by general
treatment, the headache will disappear. But this does
not mean that the headache was not ocular in origin.
It simply means that one of the factors causing the
pain has been removed, and the other, or others, are not
sufficient to cause it.
It is therefore a good rule never to attempt to treat
a headache as a migraine without previously eliminating
eyestrain.
Peripheral Irritation of Eyestrain unaccompanied by
Pain. — The chief types of this form of eyestrain are
epileptic attacks and choreiform movements of the facial
muscles.
It is important to remember that it is not the error
of refraction that causes the peripheral irritation, but
the unconscious correction of the error by the patient.
When the defect is great, no attempt is made to correct
it, as the ciliary muscle can only correct low degrees of
astigmatism; hence there is no eyestrain.
It is only a cycloplegic that will reveal this uncon-
scious correction; hence no eyes can be reported as
normal unless carefully examined under atropine or
homatropine.
The removal of eyestrain does not, in the strict sense
of the word, cure epilepsy any more than it cures a
* " Nervous System," vol. ii., page 795.
EYESTRAIN 167
headache, but by removing the eyestrain we remove
one of the causes, and frequently the only cause, that
determines an attack. An epileptic attack and some
forms of headache only differ in degree; they are
" nerve-storms."
The foregoing remarks apply equally to those so-called
choreiform movements, tics, and habit-spasms which are
so distressing to children and young adults, which take
the form of spasmodic involuntary twitching of the
facial muscles.
Constant blinking of the eyes in children should
always make one suspicious of the presence of eyestrain.
In all such cases it is most important to eliminate eye-
strain. Such cases are purely functional, and it is not
too much to say that a pair of suitable glasses will
immediately alleviate, and not infrequently cause a
complete cessation of, the symptoms.
Vertigo as a symptom of peripheral irritation may
exist, but its commonest manifestation is in connection
with diplopia.
Nausea and vomiting have already been mentioned
in connection with ocular headache; they may exist
without headache, and apart from diplopia and vertigo,
and may be due to eyestrain. Their presence is ex-
plained by the intimate association of the fifth with the
vagus.
As an instance of the variety of form which peripheral
irritation, through eyestrain, may assume, hyperhidrosis
must be mentioned.
3. Nerve-Power Waste; Nerve-Exhaustion; Neuras-
thenia; Brain-Fag. — This is a manifestation of eyestrain
that is as common as it is subtle. Subtle it must be,
as every possible cause has been cited as the origin of
the various groups of symptoms which are exhibited,
except the eyes. It has not been sufficiently recognized
that in a large majority of those cases called neuras-
thenia the real trouble is a constant " nerve-power
1 68 THE REFRACTION OF THE EYE
leakage " or waste of nervous energy, and in a large
number of cases eyestrain is the cause.
This " nerve-waste " may exist in a person of robust
nervous temperament without much, if any, harm, but
in one whose nervous conditions are unstable it must
in time show itself. Even perfectly robust individuals
showing no symptoms of eyestrain may manifest it
when under altered conditions, such as after shock of
any kind, or lowering of the general vitality from any
cause. ■
A person with a low degree of astigmatism, or with
anisometropia, or want of balance of the ocular muscles
during all his waking hours is sending down impulses to
the eye to correct the defect, and when he starts on
near work he starts with a big deficit, and further strain
results. This must mean great waste of nervous energy.
Writers on neurasthenia agree that the ages when the
disease is most liable to show itself are between 20 and
49, and these are exactly the ages when this form of
eyestrain is most manifest.
Again, those most affected are almost invariably people
who are engaged in constant near work, such as engine-
fitters, post-office and bank clerks, teachers, journalists,
and professional men and women.
Insomnia is a very prominent symptom of eyestrain,
and so a vicious circle is started, eyestrain producing,
among other troubles, insomnia, and insomnia in its
turn aggravating the patient's condition because the
all-important restorative is wanting. The extremely
depressing effect of insomnia is a matter of common
knowledge, and the depression caused by it has led
hundreds to suicide.
The physician who is called upon to treat a so-called
" functional nerve disorder," and fails to eliminate the
element of eyestrain, fails in his duty both to himself
and to his patient, for there is no functional trouble that
may not be due to eyestrain,
EYESTRAIN 169
The depression attending " nerve- waste " may lead to
the alcohoUc habit, and the irritabiHty so often present
in those suffering from functional nerve disorders often
induces the sufferer to resort to sedatives, such as
morphia.
If the " nerve- waste " is arrested, the depression or irri-
tabihty is removed, and the drug habit is more easily
overcome.
Once we allow that this nerve-power waste may exist,
we recognize that there seems to be no limit to the
various ailments which may, perhaps gradually, and
often imperceptibly, follow.
Dyspepsia and constipation, and other disorders of
the alimentary tract, may be caused by eyestrain.*
The dyspepsia may be, as we have seen, a direct reflex
irritation from the strained eyes through the pneumo-
gastric, and it may also be the result of nerve-power
waste; for if this waste causes depression, and the due
amount of nerve energy for digestion is not available,
then the various digestive processes are interfered with,
not only in the stomach, but throughout the digestive
tract.
It has been suggested that eyestrain is present in
patients suspected of being in the so-called pretuber-
culous stage, and that by removing the eyestrain the
advent of tubercle may be prevented.
This is quite consistent with the above theories, and
with the experience of many.
The patient suffering from nerve-power waste has a
low resisting power to all disease germs, and is in what
one might call a " pre-germ stage," so that he is more
liable to any infection than the normal person.
There can, therefore, be little doubt that the correc-
tion of eyestrain takes a very prominent place in pre-
ventive medicine.
* " Aberrant Dyspepsias," by Leonard Williams. {The
fiospital, Decernber 12, 1908.)
170 THE REFRACTION OF THE EYE
The chief reason why the eyes are so seldom suspected
of being the cause of neurasthenia, brain-fag, and the
different varieties of functional nervous troubles, is that
the majority of patients have either good sight or they
are already wearing glasses which apparently correct
their refractive errors.
The enormous importance of correcting eyestrain has
been abundantly shown during the war in the number
of soldiers suffering from shell-shock, and neurasthenia
following on head injuries. The loss of nerve energy
consequent on an accident renders it imperative to
arrest any further leakage, and the correction of a low
error of astigmatism or anisometropia, or, in bad cases,
where testing is impossible, the bandaging of one or
both eyes, has led to immediate improvement in the
whole condition of the sufferer. {Note. — It ought never
to be forgotten that credit for forcing the attention
of the profession to the importance of eyestrain belongs
to Gould of America, who brought out his fascinating
Biographic Clinics in 1903. Dr. Harwood, in an
admirable paper read at the Oxford Ophthalmological
Congress in 1917, maintains that the real cause of the
troubles grouped as eyestrain, is an instability of the
normal tonic contraction of one or both ciliary muscles,
and that this instability is produced when, as Weir
Mitchell puts it, the brain is sensitized by disease.)
CHAPTER XIII
HETEROPHORIA
It has been shown (page 44) that in ideal binocular vision
the visual axes are parallel when the eyes are at rest,
E R (Fig. 76, A), and that when the eyes accommodate
for a point p, both eyes converge to that point; that
in normal vision, if we destroy the possibility of fusion,
that convergence lags behind accommodation, and
D R
R D
o R
Fig. 76.
instead of converging for p, the visual axes are in the
d rection e a (Fig. 76, B), the difference between a and
p being the " fusion supplement." We have also seen
that if the position of rest is one of divergence (Fig. 76, C),
A is further removed from p, and the " fusion supple-
ment " is larger. Now, this position of divergence at
rest is caused by " muscular insufficiency," in this case
of the internal recti muscles. This means that the
171
172 THE REFRACTION OF THE EYE
internal recti are insufficient to produce parallelism
without active muscular contraction which the demand
for binocular vision necessitates during all the waking
hours; hence the muscles are never at rest, and when
the necessity for convergence arises, the interni start
with a deficit of power. The constant using up of part
of the convergence power fatigues the internal recti
muscles, and the positive part of the amplitude of con-
vergence will be found very much diminished.
The amplitude of convergence may be quite up to the
average, but if the positive part of it is too small, this
indicates the presence of insufficiency of the internal
recti and the liability to eyestrain. Take a patient
who has 3 m.a. of latent divergence, and whose con-
vergence near point is 14 cms. : we have —
ca = 7 - ( - 3)
= 7+3
= 10 m.a.
10 m.a. of amplitude of convergence would be quite suffi-
cient if it were all positive, but only 7 are positive, and
only from J to J of this power should be used for any
length of time, which means that only about 2 m.a.
should be used, which would be useless for near work.
Remove the insufficiency — that is, remove his latent
divergence — and he will not only have 10 m.a. of positive
convergence power, but even more, for the constant
fatigue of the internal recti, produced by the work of
overcoming the latent divergence, will be removed.
" Insufficiency " of a muscle implies that the muscle
is relatively weaker than its opponent, so that in static
and dynamic vision extra impulses have to pass to that
muscle in order to produce perfect fusion.
An insufficient muscle is not necessarily weak. It is
insufficient because its opponent through spasm or pre-
ponderance {Uebergewicht, as Graefe called it) is "too
sufficient"; hence in the case of insufficiency of the
HETEROPHORIA 173
interni, although the muscles may be insufficient for
convergence, they may be perfectly able to take their
part in the associated movements of the eyes to either
side.
When a muscle is absolutely weak, it is wiser to
employ the term "inefficiency." Thus, convergence
inefficiency means inability in the internal recti to act
normally, irrespectively of their opponents. A patient
suffering from convergence inefficiency appears to have
no power to supply a fusion supplement.*
Insufficiency may be present in any of the muscles.
If the visual axes at rest are convergent, the external
recti muscles are insufficient to produce paralleUsm
(Fig. 76, D). In this case binocular distant vision,
which should be perfect rest to the eyes, necessitates the
constant contraction of the external recti, and they are
liable to become fatigued and to cause eyestrain. During
convergence, also, we may find latent convergence, the
eyes converging to a point nearer than p (Fig. 76, D),
and the necessity for fusion demands contraction of the
external recti to overcome this overaction of the interni :
hence fatigue. And so with the other muscles. If one
muscle by spasm or preponderance over its opponent
prevents the eyes from assuming the normal position
when at rest, there is liability to fatigue and eyestrain;
whether it manifests itself or not (there are a large
number of cases of insufficiency which never produce
any symptoms) depends entirely on the amount of
insufficiency and the nervous condition of the indi-
vidual.
When we consider that on the relative strength of the
muscles of the eyeball depends the position of the eye,
and that the smallest amount of preponderating strength
or the slightest amount of weakness of one muscle will
* Landolt calls the eyestrain produced by the first variety
(insufficiency) " peripheral motor asthenopia, " and that produced
by the second variety (inefficiency) " central motor asthenopia."
174 THE REFRACTION OF THE EYE
cause a displacement or a tendency to displacement
{i.e., a latent deviation) of the eyes, we can only wonder
that the condition of parallelism of the visual axes in
distant vision is so constantly found. The secret is, that
the desire for binocular vision, obtained by the fusion of
the two images, acts as an unconscious stimulus to the
weaker muscle, and masks the relative weakness. If
binocular vision be impossible through the sight of one
eye being very much inferior, then the stimulus is absent,
and the eyes assume a divergent or convergent position
of rest, which becomes manifest, and is then a squint.
In other words, the heterophoria passes into a hetero-
tropia. We must be careful not to use the term " in-
sufficiency " in connection with squint; it is quite
unnecessary, and causes a great deal of confusion to
apply the term to, for instance, an atrophied internal
rectus in an old divergent strabismus. An insufficiency,
if unrelieved, may pass into a squint, but they are two
distinct things.
In estimating insufficiency of a muscle we must
beware of attaching too much importance to one ex-
amination; a muscle may be insufficient at one time
and not at another.
The following terms, suggested by Stevens of New
York, are now in common use in designating the different
forms of insufficiency (see plate, page 45).
Orthophoria = visual axes parallel, and lying in the
same horizontal plane.
Heterophoria = visual axes not parallel or not in the
same horizontal plane; divided into:
1. Exophoria. The eyes tend to turn out; insuffi-
ciency of the interni.
2. Esophoria. The eyes tend to turn in: insuffi-
ciency of the externi.
3- Hyperphoria. One eye tends to be on a higher
level than the other, due to insufficiency of the
superior or inferior rectus.
EXOPHORIA 175
4. Insufficiency of the oblique muscles —
(a) Hyperesophoria, a tendency up and in.
{b) Hyperexophoria, a tendency up and out.
5. Cyclophoria.
I. Exophoria {Insufficiency of the Internal Recti — Con-
vergence Strain). — " Convergence Insuflaciency " is a
latent external squint, overcome for the time by the
strong desire for single vision.
Strain of the internal recti is essentially dependent
upon binocular vision, and persons who have not the
advantage of binocular vision, by a compensation of
Nature cannot suffer from this trouble.
Tests for Insufficiency of the Interni. — It has been
shown that the Maddox test is the best and simplest.
If we find latent divergence for distance, or latent
divergence of more than a metre angle at J metre, or
both, we can positively assert that the interni are
insufficient, and we can confirm this by ascertaining the
amplitude of convergence, the ordinary working distance
of the patient, and the reserve power of convergence.
Except in neurasthenic insufficiency, the estimation of
the adducting power of the interni by prisms is not of
much use. We may get an adducting power of 30°, and
yet if the externi are " preponderating " we shall have
insufficiency present.
Although exophoria may be associated with uncor-
rected high hyperopia, due to the patient approaching
his work very near the eyes in order to obtain large
retinal images, it is in myopia that we generally find
this condition, where the error is uncorrected, or only
partially corrected. The excess of convergence over
accommodation, and also the excessive convergence,
must sooner or later cause fatigue of the internal recti.
Treatment. — In the majority of cases, when the want
of muscle balance is small, the correction of the error
and the constant wearing of the glasses will in a short time
176 THE REFRACTION OF THE EYE
remove the exophoria. One of the advantages of wear-
ing the concave glasses for near work is that the work
must be held some distance from the eyes, which of
course means less strain to the convergence ; and as this
is associated with a restoration of the harmony between
the accommodation and convergence, which always
means less strain, one is not surprised at the good result
of this treatment.
If exophoria still persists after some months of this
treatment, a prism, base in, should be prescribed with
the glasses, or, if the concave glass is fairly strong, the
prism effect may be obtained by decentring the glass
out (see Fig. 77).
When prisms are ordered, it is unwise to give the full
correction. Suppose, for instance, our patient shows
an exophoria with the Maddox distance test which is
corrected by a 4° prism, base in, before one eye, if we
give a 2° prism, base in, before each eye, we shall be
helping the muscles too much, and give the patient no
chance of improving; we should in this case prescribe
a 1° prism, base in, before each eye.*
It should never be forgotten that the prism treatment
is not a curative treatment ; we are treating the symptom,
and not helping the condition to disappear.
Even when prisms are ordered, the patient should be
enjoined to use them as special glasses to be worn only
for NEAR work.
2. Esophoria (Insufficiency of the External Recti). —
When latent convergence is demonstrated for distance,
we say that we have " insufficiency " of the external
recti- — that is, in the position of rest the externi are
relatively weak. If there be no manifest convergence,
the visual axes assume a parallel condition; but to
maintain this, constant active contraction of the external
* The use of prisms is limited to about 4° in front of each
eye, as any stronger prism would make the glasses too
heavy.
ESOPHORIA
177
recti must take place when distant vision is used, in
order to prevent diplopia. Latent convergence is very
common, but, in the majority of cases, gives rise to no
symptoms. The explanation is, that the latent devia-
tion is slight, and in our civiHzed state active use of the
Fig. 77.
P, Object looked at ; P', apparent position.
eyes is mostly associated with a necessary convergent
condition. When the latent convergence is excessive,
we may get symptoms of eyestrain.
Test for Insufficiency of the Externi. — ^When the
Maddox test reveals latent convergence for distance or
for J metre, it is present.
12
178 THE REFRACTION OF THE EYE
The abducting power of the normal extern! is equal
to a prism of 7° or 8°.
Insufficiency of the externi is generally associated with
hyperopia.
We have seen that in hyperopia, accommodation is
required in excess of convergence, and, unless the two
can be dissociated, the hyperope must converge to a
nearer point than is necessary. This over-contraction of
the interni causes latent convergence for distance, and
also for the near point (Fig. 76, D), the external recti
become insufficient, and extra impulses must pass down
to these muscles in order to obtain " fusion vision."
This may lead to fatigue and eyestrain, which, how-
ever, often disappear, owing to the development of a
convergent squint and loss of binocular vision (see
page 91).
Treatment. — By putting the hyperope into glasses we
re-establish the harmony between convergence and
accommodation, remove the spasm of the internal recti,
and consequently also the insufficiency of the externi,
restoring the balance of all the muscles.
One of the pleasantest things to do in ophthalmic
work is to cure a squint, or a tendency to squint, by
simply giving the patient glasses. If parents would
only realize that, in a large number of cases, this can be
done by bringing the child early enough — i.e., before
the latent squint has become manifest, or possibly during
the early period of the manifest squint — there would be
fewer squints.
It is very seldom that we have to resort to prisms in
the treatment of esophoria, but, should it be found
necessary, the prism must be placed base out, and if
decentring is substituted, convex glasses must be
decentred outwards (see Fig. 78). (See page 208.)
Note. — Method of finding the Amount of Decentring necessary to
produce the Effect of a given Prism in a given Lens (Ward Holden) .
— Take 8-7 mm. as the distance a lens of i d most be moved
to produce the effect of a prism 1°, as the unit, multiply 8*7 mm.
HYPERPHORIA I79
by the number of the prism whose effect is required, and divide
the product by the number of the lens in dioptres. Thus the
effect of a prism 3° in a lens of 7 d is obtained by decentring that
lens to the extent of — mm. =.V7 mm.
3. Hyperphoria {Insufficiency of the Superior or
Inferior Rectus). — This condition is revealed by the
Maddox test with the glass rod vertical (see page 48).
Fig. 78.
P, Object looked at ; P', apparent position.
Treatment. — A small amount is often present where an
error of refraction is present, and generally disappears
in a few weeks, after correction. It is generally asso-
ciated with astigmatism and anisometropia.
l80 THE REFRACTION OF THE EYE
If prisms have to be resorted to, they must be placed
base up or down, according to the condition existing.
A hyperphoria is often induced when reading through
glasses correcting anisometropia; for the treatment see
page 158.
4. Hyperexophoria — Hyperesophoria (Insufficiency of
the Oblique Muscles). — This is the result of the superior
oblique of either eye being too strong for its inferior,
or vice versa. The parallelism of the vertical meridians
of the corneae is maintained by the equilibrium of these
muscles; hence, when one muscle is weaker, excessive
work must be put upon it in order to preserve the
parallelism, and eyestrain results.
The Maddox rod test reveals the heterophoria.
We must hope that the correction of the refractive
error will remove the want of balance, and, if necessary,
we must use prisms set obliquely; but some cases, un-
fortunately, appear to be incurable.
5. Cyclophoria. — This is a rare form of heterophoria,
and is due to the turning of one or both eyes round an
antero-posterior axis. If the Maddox rods in a frame
are placed exactly horizontal before one eye, and the
streak of light is seen by the patient as being more or
less sloping, instead of perfectly vertical, cyclophoria
is present. When present, it is generally found asso-
ciated with oblique astigmatism. (See Maddox,
" Ocular Muscles," 1907, page 236.)
Treatment. — Extreme care should be taken to correct
completely the errors of refraction, and very special care
should be taken in ascertaining the exact angle for the
cylinder, and also in ascertaining that the optician has
rigidly carried out instructions. As a rule, this suffices
to remove the trouble. Prisms are of no use.
Heterophoria in Emmetropia. — Although rare, this
condition may exist, and may be due to —
I. Excessive or prolonged convergence.
This generally produces at first an esophoria, probably
HETEROPHORIA l8l
due to slight spasm of the internal recti, and later,
when fatigue sets in, an exophoria. It is difficult to
say whether this form of heterophoria produces any defi-
nite symptoms, as the eyestrain that may be present is
more likely due to the strain of the accommodation.
2. Weakness of certain muscles produces " ineffi-
ciency," caused by general debiUty, especially noticed in
those recovering from a severe illness.
3. Congenital defect. One of the external muscles
may be attached to the eyeball too far forward, or its
opponent too far back.
Treatment. — In the healthy, rest of the eyes is the
obvious treatment, if symptoms of eyestrain appear;
and if an excessive amount of fine near work (such as
miniature painting) has to be done, short periods of
work should alternate with outdoor exercise and some
other form of rest to the eyes.
When the heterophoria is the result of defective or
enfeebled muscles, we must first of all improve the
general health by enjoining outdoor exercise and atten-
tion to the bowels, and by the administration of tonics,
and then, remembering that absolute rest will only tend
to increase the weakness, we must commence regular
exercise of convergence (if we are dealing with " con-
vergence insufficiency ") for short periods, gradually
increased, and forbid the patient to use the eyes for
near w^ork except at these times. To carry out this
treatment in young subjects the use of a cycloplegic is
most helpful. Forced or prolonged efforts of conver-
gence would only help to increase the fatigue of the
internal recti, and therefore we must commence with
very short efforts, and increase those efforts very slowly.
Orthoptic training or gymnastic exercises can be, with
very great benefit, extended to the extrinsic muscles.
For strengthening the internal recti we employ properly
regulated convergence for a short time at different
periods of the day, the time to be gradually increased
1 82 THE REFRACTION OF THE EYE
as the muscles increase in strength, measured, of course,
by the tests previously cited. For strengthening the
external recti we must employ prisms with their base
in. The best plan is to provide the patient with a
square prism — say 2° — and tell him to practise fusion
several times a day ; and when this is accomplished with
ease, we can gradually increase the strength of the prism
until the " insufficiency " disappears.
As a last resource, weak prisms, with their base in,
may be ordered for convergence insufficiency. With their
help the convergence effort is diminished, but all hope
of the muscles regaining their normal condition must
be abandoned, unless the prisms are only used tem-
porarily while the general condition is being improved.
Prisms employed in this way never cure " insufficiency " ;
they only relieve it.
Treatment by Tenotomy. — In a few special cases
tenotomy has succeeded when the milder treatment has
failed. The cases where it is likely to be beneficial
are patients with marked exophoria possessing an ample
range of convergence. A tenotomy of one or both
external recti, and in bad cases advancement of an
internal rectus, will remove the exophoria; and by this
setting free the whole of the convergence power, by
turning the " negative " convergence into " positive,"
the symptoms of eyestrain are removed.
Landolt gives a very good example of such a case
(Fig. 79, a). Before the operation there was 3 m.a. of
divergence, and the convergence near point was 14 cms.
with eyestrain. In this case
ca = Y/ - ( -3) = Y/- + 3 = 10 m.a.,
but only seven of these lo-metre angles of convergence
power were available; tenotomy of the external rectus
removed the divergence, the whole of the 10 m.a. became
available, and the eyestrain disappeared (Fig. 79, a').
Fig. 79, h, illustrates another case which was cured
MALINGERING 183
by tenotomy; here the result was not so perfect as in
the first case (Fig. 79, a).
Fig. 79, c, is an example of what Landolt calls neuras-
FiG. 79.
thenic insufficiency, when the amplitude of convergence
is exceedingly small, and when tenotomy can do no good.
Malingering. — A malingerer will sometimes pretend that one
eye is blind, in order to escape war service, or to obtain compen-
sation after injury. Malingering is sometimes a symptom of
hysteria. It may be detected by —
1. The Prism Test. — Both eyes being uncovered, and directed
to a distant object, a prism, base up or down, is placed before
the " blind " eye. Diplopia is complained of if the eye is not
blind, and the fraud is exposed.
If a 6° prism is held before the " blind " eye, base out, this eye
will move in, at the moment of placing the prism, in order to
avoid diplopia, if it is not blind.
2. The "Friend " Test (see page 153) . — The malingerer does not
know that with this test he can read only half the letters with
one eye. If he reads the whole word, he is detected.
3. Bishop Harman's Diaphragm Test. — 'This instrument is not
only useful for detecting malingering, but is also a very ready
test of binocular vision and its defects. (For description, see
The Ophthalmoscope, vol. viii., p. 495.)
CHAPTER XIV
STRABISMUS
Squint. — A heterophoria, or latent squint, may at any
time become heterotropia, or manifest squint, if the
necessary muscular effort to preserve parallelism cannot
be made or maintained. Thus, insufficiency of the
internal recti in myopia, produced by excessive con-
vergence, may become temporarily or permanently a
divergent squint, and we have already seen how a con-
vergent squint develops when the patient cannot use his
accommodation in excess of his convergence in hyper-
opia (page 91).
Varieties of Squint. — (i) Concomitant, in which the
squinting eye moves with its fellow, and always deviates
to the same degree from the correct position.
(2) Paralytic, when the movement of the squinting
eye is restricted by paralysis of the muscle.
We shall deal in these pages only with the first variety.
Forms of Concomitant Squint — (i) Convergent. — In-
ternal squint (esotropia).
If the squinting eye is not amblyopic, there is homony-
mous diplopia. In Fig. 80, r, the right eye is fixing the
object o, but l, the left eye, which is squinting in, re-
ceives the image of o at m', which is on the nasal side
of the macula m; hence the left eye projects o to the
left or same side.
2. Divergent strabismus. — External squint (exotropia) ,
If the squinting eye is not amblyopic, there is heterony-
mous or crossed diplopia. In Fig. 81, R, the right eye, is
184
STRABISMUS 185
fixing the object o, but l, the left eye, is squinting out,
and receives the image of o at m', which is on the tem-
poral side of the macula m; hence L projects o to the
right or opposite side.
Fig. 80. — Homonymous Diplopia.
3. Vertical strabismus (hypertropia), in which the
visual axis of one eye is deviated upwards.
These three different forms of concomitant squint
may be —
1. Constant, in which one eye is always the squinting
eye. This condition is also called monolateral.
2. Alternating, in which either eye can fix, the fellow
•86
THE REFRACTION OF THE EYE
squinting; in these cases the vision of both eyes is
generally equally good.
Squints may also be periodic or intermittent. When
the squint is developing — for instance, when the hetero-
phoria is passing into a heterotropia — the latter may be
M' M M
Fig. 8i. — Heteronymous Diplopia.
manifested only after fatigue, or when certain constitu-
tional conditions are present.
Maddox puts the case very clearly, thus: When the
breadth of diplopia is greater than the breadth of fusion
power, no effort can unite the images, and this is (i) a
constant squint. When they are almost equal, the
images may be united by a great effort for a short time ;
STRABISMUS 187
this is (2) a periodic squint. When the breadth of
diplopia is considerably less than the breadth of fusion
power, the images are easily united, and this is (3) a
latent squint (heterophoria) . The difference between
the three is merely a question of degree.
Angle Gamma. — We must be careful to distinguish between a
real squint and an apparent squint. We may have in hyperopia
an apparent divergent squint, and in myopia an apparent con-
vergent squint, due to the visual and optic axes not coinciding.
When the optic axis passes through the fovea it coincides with
the line of vision or line of sight, and also with the line of fixation ;
but the exception to this is the rule, and an angle is formed by
the line of fixation m o with the axis a a' (Fig. 82). This angle
is called the angle gamma (o m a). (The angle oka [Fig. 82]
made by the line of vision and the optic axis may be considered
identical with the angle o m a, and is sometimes called the angle
gamma.)
The angle gamma is positive, as in Fig. 82, when the fovea is
on the outer side of the optic axis, and it is generally positive in
emmetropia and hyperopia, and in some cases of hyperopia the
angle is so great (amounting even to 10°) that it gives the eyes
an appearance of divergence (see Fig. 83) — an apparent divergent
squint; the eyes, although looking at and fixing the point o,
appear divergent in the direction a o'.
The angle gamma is negative when the fovea (f, Fig. 84) is on
the inner side of the optic axis — that is, between the optic axis
and the optic nerve. In some cases of myopia this is so marked
as to give the eyes the appearance of convergence (see Fig. 84) —
an apparent convergent squint ; the eyes, although looking at the
point o, appear to converge in the direction of a o'.
The angle alpha (o x e. Fig. 82) is the angle formed by the axis
which passes through the most curved part of the cornea (the
summit) with the line of vision. It is spoken of as positive when,
as in Fig. 82, the anterior portion of the corneal axis is situated
on the outer side of the line of vision, and negative when it is on
the inner side. Generally the axis of the cornea very nearly
coincides with the optic axis, so that for all practical purposes
the angles gamma and alpha mean the same thing.
The etiology of Concomitant Squint.— Any cause
which disturbs the muscular equilibrium may be the
starting-point of a squint, so that any of the causes of
heterophoria (see page 174) may be the factors in the
causation of a squint; but the chief are —
I. The Accommodation Theory. — We have already seen
how, in hyperopia (page 91), when the patient has
i88
THE REFRACTION OF THE EYE
Fig. 82. — A Schematic Figure to show Angles a and 7.
(After Landolt.)
A a', Optic axis; k, nodal point; m, centre of rotation; c, centre
of cornea; b b, base of cornea; e l, corneal axis; f, fovea
centralis; o, point of fixation; k o, line of vision; m o,
line of fixation ; o x e, angle a ; o m a, angle -y.
STRABISMUS
189
Fig, 83. — Apparent Divergent Strabismus due to a Large
Positive Angle Gamma.
Fig. 84. — Apparent Convergent Strabismus due to a
Negative Angle Gamma.
I go THE REFRACTION OF THE EYE
to use his accommodation in excess of his convergence,
a convergent squint is developed, if he cannot dissociate
the two; he has to choose between indistinct binocular
vision and clear monocular vision with a squint, and he
chooses the latter.
Again, in myopia he has to use his convergence in
excess of his accommodation; "insufficiency" of the
interni develops, and in time an external squint is
manifest.
2. Anatomical Peculiarities — The Muscle Theory. — A
broad face with a large interpupillary distance means
the necessity for greater convergence; a narrow orbit
with a long myopic eye prevents freedom of movement
of the eye, and consequently causes greater strain and
the necessity for greater muscular effort in convergence.
The external rectus may be inserted into the eye too
far forward or the internal rectus too far back, or vice
versa.
Any of these conditions, especially if associated with
ametropia, may be strong factors in the causation of a
squint.
3. Non-Development of the Fusion Sense. — At birth
the eyes move independently of each other, and hence
new-born children often squint. As they begin to take
notice of surrounding objects they develop the power
of fusion, and the two images which fall on the maculse
are thus fused by the brain, the centre being known as
the " fusion centre," or centre for binocular vision.
Any cause which reduces the visual acuity of one eye
tends to develop a squint, especially if a heterophoria or
latent disturbance of equilibrium pre-exist. Among the
commonest causes may be mentioned corneal opacities,
cataract, and intra-ocular diseases. Thus, a patient
with heterophoria has an attack of keratitis in one eye ;
corneal opacities result, which lower the visual acuity of
this eye, and a manifest squint develops.
The amblyopia may be congenital.
STRABISMUS
191
Note. — This amblyopia, which causes, or helps to
cause, a squint, must not be confused with the ambly-
opia which is the result of the squint, and which develops
through the non-use of the eye. When a squint
develops, diplopia must occur at first, and t ) g t rid of
A
Ppimany PoaitLO/i
N 5
Screen
B
5econdariy PoftiCion
Fig. 85.
this diplopia, the brain refuses to recognize the image
from the squinting eye; this form is called amblyopia
ex anopsia.
Worth maintains that the essential cause of squint is a
defect of the fusion faculty, and that this defect alone
is sufficient, even if there be no error of refraction.
192 THE REFRACTION OF THE EYE
The Diagnosis and Measurement of Concomitant
Squint. — Let us proceed to measure a convergent squint.
Make the patient fix an object, say, a couple of metres
from the eyes, taking care to place the object midway
between the two eyes. Let us suppose that the left eye
fixes the object and the right eye squints inwards: we
note the external margin of the cornea of both eyes by
making a small ink spot on the lower lid; let s be this
mark on the right eye (Fig. 85) and n' on the left. We
now cover the left eye with a screen, and tell the patient
to fix the object again; this he does with the right eye,
and we notice a marked excursion of this eye; we now
note the position of the external margin of the cornea N.
The distance n s is the primary deviation.
When the left eye is covered and the patient is fixing
with the right eye, if we look behind the screen we
notice that the left eye makes a distinct incursion ; and if
we mark on the lid the two positions of this eye, we get
the distance s' n' as representing the secondary devia-
tion, which is equal to the primary deviation. This
enables us to diagnose very easily between a concomitant
and a paralytic squint, for in the latter the secondary
deviation is always very much greater than the primary.
We measure a divergent squint in the same manner.
When the squinting eye is blind, we must use a
strabismometer (Fig. 86), and read off on the scale the
amount of the squint in millimetres.
A much more reliable method is to measure the
amount of squint by the perimeter (Fig. 87). Placing
the patient so that the squinting eye is opposite the
fixation-point f, and with both eyes uncovered, we
direct him to look at a distant point d, the squinting
eye and f and d being all in one line. We now hold a
small candle or match at the fixation-point F, and
gradually move it along the arc, which is placed hori-
zontally, looking directly behind the candle for its
image reflected on the cornea of the squinting eye.
STRABISMUS 1 93
When the image is in the centre of the pupil we read
off the position of the candle on the arc, and the degree
mark represents the angle of the strabismus.
Treatment. —
1. The correction of the error of refraction.
2. The education of the fusion sense.
3. The education of the amblyopic eye.
4. The readjustment of the muscles by operation.
Convergent Strabismus. — Almost 80 per cent, of
patients suffering from convergent strabismus are hyper-
FiG. 86.
opes, and the defect is manifested very early in life —
in fact, when the child begins to use his eyes for near
vision, looking at picture-books, etc.; the majority of
such patients develop a squint about the age of three.
The squint, as a rule, develops slowly, and the parents
are not the first to notice it. They generally assign as
the cause an illness, such as measles, or the imitation of
a squinting companion.
The first treatment is to put the eyes under atropine
for at least a week, and in slight cases the squint entirely
13
194
THE REFRACTION OF THE EYE
disappears when the eyes are fully under a cyclop' egic.
The refraction is then estimated by retinoscopy, and the
full correction, less i d, is ordered in large oval or
circular spectacles, to be worn always. For instance,
suppose under atropine retinoscopy at i metre gives
STRABISMUS 195
f- 4, then the full correction is +3, and the glasses
iven would be +2. With an intelligent quiet child
rpectacles may be ordered at three years of age, with
instructions, of course, for their removal when the child
is romping.
If the atropine removes the squint, as a rule the
glasses will also do so.
Be careful to insist upon the constant use of the glasses,
and inform the parents that this treatment will probably
have to be persevered in for years.
If the squinting eye is amblyopic, the child should
be made to practise it by placing an opaque clip on the
glass of the good eye, or, what is better still, especially
with the very young, by bandaging up the good eye
and forcing the bad eye to be used for a short time
every day — for instance, at mealtime. In many cases
this treatment results in great improvement of vision
(see page 96) .
Another useful method of forcing the amblyopic eye
to work, is to instil atropine into the good, or fixing,
eye only ; as the accommodation of this eye is paralyzed,
it can only be used for distant vision, and for near vision
the amblyopic eye must be used, and is then forced to
work for a considerable period of the day. Some sur-
geons consider that this treatment takes the place of, and
is better than, excluding the fixing eye with a bandage.
The use of atropine in both eyes (except for the pur-
pose of estimating the error of refraction), which is
sometimes advocated when the child is considered to be
too young for glasses, is to be strongly condemned.
As an essential cause of convergent squint may be a
defective development of the fusion faculty, the or-
thoptic training is very important, and cannot be begun
too early.
Worth has devised an instrument called the amblyo-
scope, which is on the principle of the stereoscope : each
eye looks through a separate tube, and a different part
196 THE REFRACTION OF THE EYE
of the whole picture is placed before each tube, and so
arranged that when both eyes are being used these two
portions are fused into one picture. These tubes are
not fixed together, but can be separated to any angle,
and thus adjusted to the particular squint (see Trans.
Ophth. Sqc, vol. xxi., page 245). The picture before
each tube can be separately illuminated, and so, by
lowering the illumination of the picture before the
better eye, or increasing that before the worse eye,
binocular fusion vision may be easily obtained in very
bad cases, and consequently Worth's amblyoscope is
far superior to the numerous cheaper stereoscopic
instruments.*
The above treatment should be given a good trial as
long as the smallest improvement is shown; but when
no improvement is taking place, and especially when the
squinting eye is becoming amblyopic or more amblyopic,
we must resort to operation.
Squint operations are always best done under cocaine
and adrenalin, because, when the patient is conscious,
we are able to judge accurately the amount of adjustment
necessary ; but when we have to operate on quite young
children, a general anaesthetic is necessary.
In slight convergent strabismus, division of the
internal rectus may be sufficient; if not, we must ad-
vance the external rectus of the same eye; and lastly,
in rare cases, division of the other internal rectus may
be necessary.
The younger the patient, the more likely is the treat-
ment of atropine (in the fixing eye) and glasses to succeed
and the operation to be unnecessary ; but in older sub-
jects, squints are rarely cured except by tenotomy or
advancement, or both.
While the spectacle treatment is being tried, the
* Worth considers that the training of the fusion sense cannot
be begun too early, and that it is rarely of any use after the age
of 6. The surgeon should himself give the child the " lesson"
with the amblyoscope once a week for at least six weeks.
STRABISMUS 197
refraction should be carefully estimated under atropine
at least once a year.
Divergent Strabismus. — This condition is rarely seen
in young children, and, as a rule, develops at puberty or
later — in fact, when myopia is progressing. By stop-
ping undue convergence, and so helping to restore the
fatigued internal recti, we may cure slight cases at the
onset by putting the patient into glasses; but it is
very uncertain, and in the majority of cases, when once
the " insufficiency " has passed into a squint, nothing
short of an operation is any good. This consists
in dividing the external rectus, and, when this is not
sufficient, in advancing the internal rectus. Training
the eye, and so trying to reduce the amblyopia, should
also be resorted to. When the squint is not very
marked, give the glasses and the orthoptic training
a fair trial of, say, six months before resorting to
operation.
After squint operations it is most important for the
patient to continue to wear the correction, and also to
persevere with the stereoscopic training. He should be
warned that, unless this is done, there is a danger of
further trouble — for instance, a convergent strabismus
that has been corrected may develop into a divergent
strabismus, etc.
In oldish patients with an old convergent squint,
where the squinting eye is not amblyopic, and when an
operation is refused or deemed inadvisable, if diplopia
is present, prisms base out must be ordered with the
correction.
CHAPTER XV
CYCLOPLEGIA, CYCLOPLEGICS, AND CILIARY SPASM
Cycloplegia. — Cycloplegia, or paralysis of the ciliary
muscle, may be due to —
1. Drugs, such as atropine.
2. Systemic poisons — diphtheria, influenza,
syphilis, etc.
3. Disease of the nervous system, concussion of
the brain, etc.
Cycloplegics. — A cycloplegic is a drug which tem-
porarily paralyzes the ciliary muscle, and by its use we
are enabled to estimate the refraction of the eye at rest.
Cycloplegics are also mydriatics — that is, they paralyze
temporarily the sphincter iridis, and cause dilatation of
the pupil.
The only cycloplegics we need concern ourselves with
in refraction work are atropine and its derivative, horn-
atropine. These drugs paralyze the sphincter iridis and
the oculo-motor nerve-endings in the ciliary muscle;
consequently the pupil is dilated and accommodation
power is reduced, or (when the full action of the drug is
obtained) lost, leaving the eye adjusted for its far point
and in a state of rest.
Atropine is the stronger drug, and should be used,
when practicable, in all young subjects where the ampli-
tude of accommodation is very great. In children under
16 years the full effect is obtained only after two days'
use; in older patients complete cycloplegia may occur
in a few hours.
198
CYCLOPLEGics tgg
The effect of the drug does not begin to pass off for
thirty-six hours, and the accommodation power is not
fully restored until a week or ten days have elapsed.
The pupil is restored to its normal size about the same
time, sometimes a little earlier.
Homatroplne has the same general effect as atropine,
but differs in that its full effect on the pupil and ciliary
muscle manifests itself more promptly, and disappears
much more rapidly, than atropine ; but it is not so com-
plete a paralyzer of the ciliary muscle as atropine, and in
young people whose accommodation is very active it is
not to be relied on. On the other hand, in most people
over 25 years of age it paralyzes the muscles quite
enough for all practical purposes when used in suffi-
ciently strong doses and combined with cocaine.
Cocaine favours the absorption of the drug by render-
ng the outer epithelial layer of the cornea and the
conjunctiva more pervious.
As a general rule, the full effect of homatropine is
obtained in an hour. This effect begins to pass off in two
hours, and the whole effect has disappeared in twenty-
four or twenty-six hours. The restoration of the pupil
to its normal size takes a few hours longer.
Between the ages of 16 and 20 the selection of the
cycloplegic must depend on the time that the patient can
give up to the examination. Always try and obtain con-
sent for the use of atropine in such cases, as its effect is
more rehable; but where only one day can be spared,
the surgeon will have to be content with homatropine.
In young subjects at school who cannot give up the
time to atropine cycloplegia, homatropine should be
exhibited two or three times at intervals of half an hour
before the refraction is estimated; but if a satisfactory
result is not obtained, atropine will have to be used.
One great advantage of homatropine is that it rarely, if
ever, produces toxic symptoms.
The Form in which Cycloplegics should be used.— It
200 THE REFRACTION OF THE EYE
is impossible to know when using drops or solutions
how much of the drug is absorbed and how much is
wasted, and they do not keep well. Atropine in solution
is liable to produce toxic symptoms by passing down the
tear passages into the throat.
On the other hand, ophthalmic " tabloids " and discs
have been brought to such a state of perfection that
they form the most scientific, efficient, and safe method
of administering the drugs.
The most useful tabloids are: atropine -^l^ gr., and
homatropine with cocaine Jq gr- each.*
Tabloids or discs should dissolve quickly when placed
on the inner surface of the lower lid, and should cause
little or no irritation or pain.
Atropine may also be used in the form of an ointment,
the pure alkaloid being dissolved in vaseline, the propor-
tion being gr, iv. of atropine to the ounce of vaseline.
This should be put up in a small tube, and a small
quantity placed on the inside of the lower lid, by means
of a clean glass rod, morning and late afternoon. It
should not be used on the day the examination is to be
made, as the presence of the vaseline may interfere with
the tests.
Cycloplegics are rarely necessary over the age of 45.
The accommodative power is considerably reduced by
that time, and any latent hyperopia that may have been
present has become manifest (see Presbyopia, page 149).
Never use a cycloplegic should there be any suspicion
of glaucoma or a tendency to glaucoma.
Make it a rule never to use atropine in patients with
high hyperopia over 25 years of age, and then there
need be little fear of inducing a glaucomatous attack,
because homatropine is very speedily and efficiently
counteracted by eserine, and if homatropine has been
used, and any suspicious symptoms arise, a tabloid of
eserine (^^ gr.) will allay all anxiety.
* Burroughs, Wellcome and Co., tabloids " B " and " W."
CILIARY SPASM 201
Cycloplegia following Diphtheria. — When the accommodation
is paralyzed after diphtheria, it generally occurs in young sub-
jects, is bilateral, and may follow a most insignificant attack of
the disease. The patient is in the same condition as an old
person who has lost all accommodation, and the treatment is
practically the same. If emmetropic, reading glasses only are
necessary, and the weakest convex glasses with which reading is
possible should be prescribed in order to encourage the ciliary
muscle to act, and these glasses should be changed for still weaker
ones as the power returns to the muscle. When an error of
refraction is present, bi-focal glasses for distance and near vision
must be ordered (see Presbyopia, page 150).
As dilated pupils from iridoplegia very often coexist with
cycloplegia, considerable improvement in vision may result from
the use of a drop of eserine in the eyes every morning.
When cycloplegia results from any other cause, the aforesaid
treatment should be adopted, combined with the internal adminis-
tration of iodide of potassium or strychnine, etc., as may be
indicated.
Spasm or Cramp of the Ciliary Muscle. — ^This is the
opposite of cycloplegia, and occurs in two forms: (i) A
temporary spasm, soon passing off with rest; (2) a
permanent spasm, referred to in the previous pages as
spasm of accommodation, and generally associated with
hyperopia in young people (see page 94), and producing
an apparent myopia. Both forms are the result of
strain of the ciliary muscle, and are, with rare excep-
tions, cured by the use of a cycloplegic and the correc-
tion pi the refraction error.
There is a form of spasm of the accommodation which Leslie
Paton calls " functional spasm." In the case he cites {Trans.
Ophth. Sac, vol. xxxvii., p. 370) a lady with a small amount of
myopic astigmatism at times acquired a spasm of accommodation
of 9 or 10 D, accompaniedby" cramp of convergence." This con-
dition is seen in neurasthenic patients and is produced by eye-
strain. De Schweinitz ("Diseases of the Eye," 8th edition, p. 122)
says: " Spasm is prone to occur in individuals of neurasthenic
condition, and is a frequent symptom of hysteria, often associated
with cramp of convergence."
This form of spasm may occur in quite old patients, even up
to 45. Most meticulous care should be employed in ascertaining
the refractive error, and the proper correction will in time effect
a cure.
All these forms of spasm are exaggerations of the tonic spasm
that exists in normal conditions and which disappears under the
action of a cycloplegic.
CHAPTER XVI
METHODS OF EXAMINATION— NOTE-TAKING
Methods of Examination. — The room should, if possible,
be sufficientty long tv allow the patient to be seated
6 metres, or 20 feet, from the type. When this length
is not obtainable (even diagonally), reversed types must
be used and hung over the patient's head behind him,
and opposite should be a mirror, on which the type is
reflected. The distance should be so arranged that
the distance between the patient and the mirror and
between the mirror and the type together measure
6 metres.
Apparatus Required. — The distant type should be
Snellen's type, and several boards, with a different
arrangement of letters, should be used, and changed as
necessity arises, or they may be arranged in a box form
and rotated by a cord by the surgeon from where he is
standing. Dixey's test types* (Fig. 88) are arranged so
that only one line of type is displayed at a time, the
different lines being turned into position as required
by a cord, as in the box form of type. The change of
type prevents the patient from learning the arrange-
ment of the letters.
The type must be well and evenly illuminated, prefer-
ably by artifical light, and, if possible, in a dark part
of the room, so that the difference between a bright and
dark day has little or no effect on the record.
* Dixey, 3, New Bond Street, W.
202
APPARATUS
203
A dark room, although desirable, is not absolutely
necessary; the whole consulting- room can be darkened
with a blind or curtain, or a dark corner can be curtained
off. Absolute darkness is not a sine qua non.
The lighting should, if possible, be electric, and
ground-glass lamps should be used, or the special high
candle-power lamp made for eye or throat work, which
is mostly ground glass with a small portion clear.
Failing the electric light, the " incandescent " is,
perhaps, the best form of gas illumination.
The reading type should be kept clean in a cover.
Both forms of type are figured at the end of the book.
The Trial Case. — This contains pairs of concave and
convex spherical and cylindrical lenses and prisms.
The spherical lenses should be numbered in intervals of
•12 to I, '25 to 4, '5 from 4 to 8, and in intervals of
I from 8 to 20.
The cylindrical lenses should be numbered in intervals
of •12 to I, '25 from i to 3, *$ from 3 to 6, and intervals
of I from 6 to 8.
204 THE REFRACTION OF THE EYE
The prisms should be from i° to 12°, or 14°.
The lenses should be as thin as possible, and they
should all be mounted in thin frames, with a small flat
handle on which the number is engraved.
When the surgeon does not wish to start with so
expensive a set of lenses, he can manage very well with
a set consisting of the following glasses :
Sphericals, convex and concave : 30 pairs each from
•12 to 20.
Cylindricals, convex and concave: 18 pairs each
from •12 to 6.
Prisms from 1° to 12°.
This set in a suitable case with stenopaic discs,
adjustable trial frame, etc., can be supplied by Messrs.
Hamblin, 5, Wigmore Street, at a reasonable cost.
Trial Frame. — Get a really good trial frame, regard-
less of cost. One of the best is made by Curry and
Paxton. It should be light (made of aluminium),
capable of being adjusted to fit any patient, and of being
correctly centred. It should have a screw for rotating
the cylinder (which can, if necessary, be used by the
patient), for by this means we insure much greater
accuracy in obtaining the correct angle of the cylinder.
It should be so arranged that where two lenses are
used — i.e., a sphere and a cylinder — they should almost
touch, and the back glass should be as close to the eye
as the lashes will permit. There are many bad trial
frames on the market, the worst example being the
rigid one supplied in most trial cases, which is practically
useless.
Besides the above contents of the trial case, there
should be several " blanks " to block off vision, a steno-
paic slit, a pin-hole disc, and neutral tinted glasses of
various shades.
Refraction Ophthalmoscope. — It is false economy to buy
a cheap one. Get a good instrument to start with, and
it will last a lifetime (see page 61).
EXAMINATION OF PATIENT
205
Ophthalmoscopic Mirrors. — A plane and a concave
mirror are wanted, and these may be procured in one,
each mirror serving as the cover of the other (see page 83) .
Focus-glass. — A large focus-glass, as described on
page 62.
Maddox Apparatus. — The test-board, near test-type,
rod and prism (see page 45).
Perimeter. — McHardy's recording perimeter is the
best, but is expensive; there are many almost as useful,
and much cheaper.
The Ophthalmometer is an expensive instrument, but
is really indispensable (see page 126).
Cycloplegics. — Tabloids of homatropine and cocaine,
aa gr. -gV; atropine, gr. ojxri and eserine, gr. u^.
Fig. 89.
Prescription forms for spectacles are supplied by most
opticians, or they can be engraved or stamped on the
surgeon's own paper, as illustrated in Fig. 89.
Mark the axis by writing the degree as indicated at
page 138.
Note-Making. — It is very useful in recording notes of
patients to have some scheme. Fig. 90 is a suggested
form when using the card system, but if the surgeon
wishes more voluminous notes, and especially if he
prefers a bound book, the ophthalmic case-book, pub-
lished by Pulman, of Thayer Street, W., will meet all
his wants.
The Systematic Examination of the Patient. — After
recording name, age, history, and symptoms, the patient
is placed in the chair opposite the type, and the trial
206
THE REFRACTION OF THE EYE
EXAMINATION OF PATIENT 207
frame is adjusted. With one eye blocked, the visual
acuity of each eye is separately recorded, and also
roughly the effect of concave or convex glasses. There
is no necessity to waste much time over the first examina-
tion if a cycloplegic is going to be used. On the patient's
return under atropine or homatropine, take him straight
into the dark room, examine carefully with the ophthal-
moscope, and then ascertain the refraction by retinos-
copy. If you have an ophthalmometer, measure the
astigmatism. Then take the patient back to the type
examination, find out the glass or combination of glasses
that give best vision, record this, and let him return for
a final visit when the effect of the cycloplegic has passed
off, when you order the correction. When the patient
cannot return for a third visit, you must order the
glasses according to the rules laid down in the previous
chapters.
Of course, when a cycloplegic is not used, the examina-
tion is completed in one visit.
Remember that in old patients hyperopia is often
present and absolute, and a weak convex glass will often
improve vision from /^ to i.
Always note and record the spectacles that have been
previously worn.
CHAPTER XVII
SPECTACLES
Spectacles. — The treatment of errors of refraction can-
not be considered to be complete unless the optician
has accurately made up the prescription, consequently
it is very important for the surgeon to check the glasses
and their fit ; if practicable, this should never be omitted.
The optical centre of the glass should coincide with the
visual axis. Distance glasses should be centred for
distance, and near-work glasses for the point at which
they are intended to be used. Glasses for constant use
should be centred for a point between these two.
Glasses have a prismatic effect if decent red. A con-
vex glass may be said to consist of two prisms with the
bases in contact. If the glasses are too wide apart,
the patient looks through the inner side of the glass,
which has the same effect as looking through a prism
with its base outwards (see Fig. 78) ; consequently the
convergence effort will have to be increased. If the
glasses are too close together, we have the same effect
as a prism with its base inwards, and the convergence
effort will be diminished, the accommodation being in
excess. Concave glasses may be said to be two prisms
with their apices in contact, and the effect of their being
out of the centre is the reverse of that of convex lenses
(see Fig. 77). When these results are not desired —
that is, when the lenses are not purposely decentred —
it is easy to understand how badly-fitting spectacles
may be worse than useless.
208
SPECTACLES
20^
To check the centring of the glasses, draw a thick
vertical ink line on a piece of paper, hold the lens at a
slight distance from it, and move it from side to side;
when the lens is convex, the line seen through the lens
will appear to move in the opposite direction ; when con-
cave, in the same direction. In Fig. 91 a convex glass
has been moved to the right, and the line seen through
the lens has moved to the left. By moving the lens
from side to side the neutral part will be found where the
lines are continuous ; this is the optical centre for lateral
movement. Mark this point as a short vertical line on
Fig. 91.
the lens with ink or a special grease pencil; now turn
the lens round through 90^ and proceed as before;
where the two lines meet is the optical centre of the lens,
and this should correspond with the centre of the pupil
when the eyes are adjusted for the distance for which
the glasses are ordered.
If a prismatic effect is desired and ordered, then the op-
tical centre should be decent red as explained on page 178.
The plane of the glasses should be perpendicular to
the visual axis when in use, hence reading glasses should
be tilted forwards.
The best form of bridge is the saddle-bridge ; it should
be flat in order not to indent the nose, and should fit the
14
210 THE REFRACTION OF THE EYE
nose accurately. It is not unusual to find that the
lower elbows (Fig. 92, b) are touching the sides of the
nose, but that the upper arch (Fig. 92, a) is not in
contact. If the bridge is not well made, the spectacles
will slip.
Sometimes, however carefully the bridge is adjusted,
it indents the nose in young children, and perhaps tends
to interfere with the development of the nose ; Hamblin's
" scholar's frame " remedies this, as the bridge is lifted
off the nose by side clips, the glasses having the appear-
ance of a combined spectacle and pince-nez.
The glasses should be as near the eyes as the lashes
will permit; 13 to 14 mm. is the average distance; but
when the lashes are very long, this distance will have to
be increased. It is most important to remember that
Fig. 92.
the lashes must not touch the glass ; when the lashes are
very long, periscopic lenses should be ordered.
Concave glasses are weakened and convex glasses
strengthened by removing them further from the eyes,
and vice versa.
The sides of the frame should touch the temples and
pass behind the ears in all spectacles made for children,
and preferably in every case where the glasses are to be
worn always. The portion behind the ear should fit
comfortably, so that the wearer is hardly conscious
of it.
Under no circumstances should anyone but an em-
metropic presbyope be allowed to wear folders. Folders
never fit, are rarely correctly centred, and tend to
become bent, so that one or .both glasses, are oblique
SPECTACLES 211
to the plane of the eyes, and one is often nearer the eye
than the other.
A cylindrical lens becomes more cylindrical, and a spherical
one sphero-cylindrical, by being placed obliquely before the
eyes.
Pince-nez should be rigid and light. If they are worn
always, they should, preferably, be frameless. The clips
which keep pince-nez in position on the nose should be
made of malleable material, so that they can be shaped
to the sides of the nose, and the surface next the nose
should be rough. It is most important that all glasses
should "sit" horizontally; especially is this the case
when cylinders are worn.
Monocles may be allowed in cases of monocular
amblyopia (see page I57)-
Children should always have spectacles, and not pince-
nez, and the frames ought to be strong and light.
Crookes*S Glass. — As far as we know the infra-red rays are
of no use to us for visual purposes, but, bearing in mind the possi-
bility that their continued action maybe harmful to the eye, it is
advisable, in selecting an anti-glare glass, to choose one that cuts
off the infra-red rays, but not at the expense of the all-important
screening off of the ultra-violet ones. At the same time, if such
a glass has a slight neutral tint a small amount of luminosity
is cut off and renders it more useful against glare.
After an enormous number of experiments extending over some
years. Sir William Crookes has succeeded in finding such a glass,
or, rather, many such glasses. Messrs. Chance Bros, have put on
the market two glasses, Crookes " A " and Crookes " B."
Crookes " A " is practically No. 187, and its composition is:
Fused soda flux . . . . . . 83-0
Cerium nitrate, crystallised . . . . i7«o
But as it is difficult commercially to get a pure cerium salt, there
is present a small amount of didymium, which gives the glass a
pale pink tint when viewed edgeways. This glass cuts off 27 per
cent, of the heat rays, practically all the ultra-violet rays (the
limit being \ 3650), and transmits 99 per cent, of the light, and
is of the greatest value in refraction work, especially in the
correction of myopia, and the extra cost above ordinary glass
is trifling.
212 THE REFRACTION OF THE EYE
Crookes " B " has a slightly darker tint, and corresponds to
No. 197. Its composition is:
Fused soda flux
Cerium nitrate, crystalUsed
Nickel sulphate, crystallised
Cobalt sulphate, crystallised
Uranoso-uranic oxide < .
. 79-00
. 20-50
0-30
0*05
0-I5
lOO-OO
It is opaque to ultra-violet rays of shorter wave length than
X 3700, and cuts oh 41 per cent, of heat rays. It is transparent
to 45 per cent, incident light.
It is not to be recommended for general use, but is especially
indicated when very great glare or heat is encountered.
It is sometimes very useful in high myopia.
Final Note. — Patients ought always to be reminded
that the treatment of their error of refraction is by no
means permanent. Changes will take place. Young
patients should be re-examined at least once a year
(see page 98), older ones every two or three years.
CHAPTER XVIII
ILLUSTRATIVE CASES
Simple Hyperopia and Anisometropia. — Miss L. aged 21, com
plained of headaches, especially after doing any near work.
V.=f+-50 Hm. B.E.
Under atropine :
R.V. A+i-75=f. L.V.^V+i-50=|.
No astigmatism.
Ordered +'75 sph.+ '50 sph.
To be worn especially when doing near work.
Her headaches disappeared.
Simple hyperopia without astigmatism or anisometropia
rarely gives rise to symptoms in young people unless the error
is great.
Simple Myopia— Heterophoria. — Mr. S., aged 25, clerk. SufEers
from chronic conjunctivitis, which has been getting worse lately.
Has worn glasses for distance, but never for near work.
V. = <A-3-5=f B.E.
His accommodation near point is 9 cms.
,.. a=i^-3-5
= 11-3-5
= 7-5
His convergence near point is 18 cms. He has i m.a. latent
divergence for distance, and i'5 m.a. for \ metre (Maddox).
...ca=:SS^-(-i)
= 5-5+1
= 6'5 m.a.
He was ordered - 3*5 B.E., to be worn always, and he was
specially instructed never to approach his work nearer than
33 cms. Some months later he returned, showing great improve-
ment. There was no heterophoria for distance, and only '5 m.a.
for J metre.
213
214 THE REFRACTION OF THE EYE
Hyperopic Astigmatism — Epilepsy. — Master A. B., aged lo,
has had epileptic attacks for some years, with only slight benefit
from medical treatment.
V. =1 in both eyes.
Under atropine :
R E / + '^5 cyl. (axis vert.) \ _ ,
• '\+ 1 sph. / ^'
T p / +-50 cyl. (axis vert.)\_g
The cylinders were ordered for constant use. A report, re-
ceived eight months later, stated that the boy had been perfectly
well since wearing the glasses.
Hyperopic Astigmatism (Low). — Miss N., aged 24, has of late
been suffering intensely from headaches, always aggravated by
near work,
V.=^B.E. NoHm.
Under homatropine :
V -«6 / + '25 cyl. (axis vert.) \ _ « ^ p
^~^\ + -50sph. f-s^-^-
The ophthalmometer shows -25 direct astigmatism.
Ordered 4- '25 cyl. (axis vert.). B.E.
for constant use.
The wearing of this correction completely cured the patient.
Hypermetropic Astigmatism (Very Low) — Anisometropia — Eye-
strain. — Private S., suffering from shell-shock, aged 22.
Great lassitude; thoroughly ill — headache, insomnia, no or-
ganic lesion.
V. =f in both eyes.
Under atropine :
RV - 6/ + -i2cyl.t\6 LV-f + '^5cyl.\i = |^
^•^•-T5| + .62sph. /«• ^•^•\ + -50sph. P-
The following glasses were ordered for constant use :
R-E-{t:;'s^h.''''""'"''* L.E, + .25cyl.V
and in a few weeks he was quite well.
Astigmatism— Anisometropia.— Miss B., aged 30, complains of
" dreadful headache " and insomnia.
R.V. «. L.V. f .
Under homatropine the ophthalmometer showed:
R.E. '5 direct astigmatism.
L.E. I 'S direct astigmatism.
Retinoscopy :
ILLUSTRATIVE CASES 215
R.E. L.E.
+ 1'
+ 2-5
+ 1 +1
K.Y.^+ -5 cyl. (axisvert.)=t.
L.V.1% + 1.25 cyl. (axis vert.) =|.
The following glasses were ordered :
For constant use :
/ + -5 cyl. (axis vert.)
J ^ r+ 1-25 cyl. (axis vert.)
Note. — On returning for the final examination when the effects
of the cycloplegic had passed off, it was found that the patient pre-
ferred '75 off the left and only -50 off the right. When possible,
it is always best to take a little more off the stronger glass, with
the view of lessening the difference in the correction of the two
eyes, at least in one meridian.
Simple Myopic Astigmatism.— Mr. P., aged 29. Has had
migraine for some time.
V.=f B.E.
Under homatropine : ophthalmometer shows -5 direct astig-
matism, and vision =1 c - '5 cyl. (axis horizontal) B.E. On re-
covering from the cycloplegic, - '25 sph. added to the cyl.
improved each eye separately, but binocularly the cyl. only gave
him f , and these glasses were ordered for constant use, and some
weeks afterwards the patient reported that the migraine had
disappeared.
Myopic Astigmatism (Very Low).— Miss N. B., aged 30, has
suffered for some years from " nerves," and is now complaining
of dyspepsia. Vision in both eyes was § part, and the ophthal-
mometer showed inverse astigmatism '12 in both eyes.
Under homatropine :
, f-'i2 cyl. (axis vert.)\ _ ^ ^
r^\+-50sph. ^—^o.tL.
The minus cylinder was given her to wear always, and sbi
months later she reported that she was perfectly well and the
dyspepsia had entirely disappeared.
Myopic Astigmatism (Compound). — Miss B., aged 28. Ha
uever worn glasses. Complains of dreadful headaches.
V.^-^;^"^ and -4=^ B.E,
2l6 THE REFRACTION OF THE EYE
Under homatropine the ophthalmometer showed :
R.E, I inverse astigmatism.
L.E. '5 inverse astigmatism.
R V / ~ ^ ^y^- ^^^^ vert.)-! _„
• X-asph. f-^
L V / - -5 cyl. (axis vert.) \ _ ^
•'•\-3.5sph. /-«•
These glasses were ordered for constant use, and the patient
reported that the headaches disappeared.
Note. — The above is also an example of anisometropia, and it is
very commonly found in those cases, as above, where the differ-
ence is not great, that in one axis the refraction is the same in
both eyes. Here the horizontal meridian is - 4 in both eyes.
Mixed Astigmatism. — Miss H., aged 23, complains that her
vision is very bad, and that the eyes are very painful. Never
worn glasses.
V. =-^ in both eyes.
Under homatropine the ophthalmometer showed 2*5 astigma-
tism oblique in both eyes.
Retinoscopy :
X X
4-2 --5 -1+2
45
^•^•l-i-5sph. /-Tir-
40
^•^•\-2sph. /-r^-
These glasses were ordered for constant use.
In cases of which above is an example visual acuity is gene-
rally poor, but great improvement results a few months after
wearing the correction.
Mixed Astigmatism (Low) and Anisometropia and Eyestrain. —
Miss T. P., aged 18, had had headaches and spinal pain for four
years.
R. andL.V.=f.
Under atropine :
The following glasses were ordered for constant use:
^ ^ / + -37 cyl- (axis vert.). j ^ / + -25 cyl. (axis vert.).
■^" •\ - -12 sph. •^*\-«i2sph,
ILLUSTRATIVE CASES 217
She immediately began to improve, and when last heard of was
feeling better every week.
Simple Presbyopia. — Mr. H., aged 53. He says his distant
vision is good, but that the glasses he uses for near vision (+1)
do not give him the help they did, and after reading a short time
the print becomes confused, and he has to rest a short time before
resuming reading.
V.=f. No Hm. B.E.
He reads D=0'5 well c. + 2 sph., and the ophthalmometer
shows no astigmatism. +2 were ordered for all near work.
Simple Hyperopia and Presbyopia. — Mr. E., aged 58. Has a
considerable amount of conjunctivitis, from which he has been
suffering for a year. Is using -f 2'$ sph. B.E.
R.V.^«^+ 1.25=1.
L.V.^+1.5 =t.
The ophthalmometer shows no astigmatism. Reads D=0'5
well c + 4 sph.
He refused glasses for distance, and was ordered + 4 sph. for
all near work.
Note. — He preferred the same glass in both eyes.
Simple Myopia and Presbyopia. — Mr. O., aged 52.
R.V.
L.V.
;}=A-3=f.
The ophthalmometer shows no astigmatism. Reads D=0'5
well c - i'5 sph.
He was ordered the following glasses:
The patient's muscle test was normal, and he had no symptom
of eyestrain.
Hyperopic Astigmatism and Presbyopia. — Miss E., aged 52,
complains of constant headache.
R V _ «/ + '75cyl. (axis vert.) "i_
^•^•-^*\+i.5 sph. /"*•
The ophthalmometer shows astigmatism as under:
R.E. '75 direct.
L.E. .50 direct and oblique.
With + 2'5 added to above she read D =0'5 well.
Bi-focal glasses Qrd^red fgr constant use, with correction as
ftbpve,
2X8 THE REFRACTION OF THE EYE
Some months later patient reported that the headaches had
entirely disappeared .
Note. — It will be noticed that the ophthalmometer gave in the
left eye a lower correction than that which the patient preferred.
This constantly occurs in oblique astigmatism, and it is probably
due to the presence of a slight amount of static lenticular astigma-
tism, which, of course, the ophthalmometer cannot recognize.
Myopic Astigmatism and Presbyopia. — Mr. B., aged 46, com-
plains of twitching of the eyelids and a strained feeling about
the eyes after reading,
anil- '75 cyh (axis vert.) j _,
L.V.J-*75sph. / "
The ophthalmometer showed inverse astigmatism -75, He
reads easily D=0'5 with -1- '75 cyl, (axis horizontal).
The following glasses were ordered :
Distance: r _ .75 cyl. (axis vert.)
^•^•\--75sph.
Reading : ^^ ^ ^^^ ^^^ ^^^.^ j^q^.^^.) .
He preferred to have two pairs of glasses, and not bi-focals.
Spasm of Accommodation.— Master L., aged 14. Has had
some trouble with the eyes for some time, with headache after
near work. He saw an optician, who tested his vision (without
atropine), and the boy chose - 3 sph., which were ordered him.
Under atropine :
T^ V « / + '25 cyl. (axis horiz.) \ _ „
^^^"^ I + -50 sph. j -^'•
T \T c / + '12 cyl. (axis horiz.) \ _g
• •^M + -50 sph, /-o-
The ophthalmometer showed -25 inverse astigmatism for the
R.E,, and '12 for left. The cylinders were ordered for constant
use, and in a few weeks the spasm entirely disappeared.
Note. — This case shows the danger of giving glasses, especially
concave glasses, to young people, without using a cycloplegic.
The eyestrain had produced spasm of the ciliary muscle, which
masked the real condition, and made him appear to be m5^opic.
High Myopia (with Astigmatism) (Progressive ?) .—Miss B„
aged 17. Has been wearing for some time the following glasses:
- 2 cyl. (axis horiz.) 1 -□ ^
-10 sph. I^-^-
Originally these helped her considerably in distant vision, but
now they are only useful when reading, and even when using the
glasses she holds her book 12 cms. from the eyes.
\
ILLUSTRATIVE CASES SIQ
Under atropine : lo
RV.{ : \i^ll *^} = A one letter.
2
L-V-{:!5Vp''h:'^}=A one letter.
The ophthalmometer shows:
R.E. I '5 astigmatism, direct and oblique.
L.E. 2'0 astigmatism, direct and oblique.
Retinoscopy :
R.E. L.E.
-17 V^/
-19 -17
The atropine correction was ordered for constant use, and the
patient was strictly enjoined never to approach her work nearer
than 33 cms.
Considering the high myopia, the fundus of both eyes was fairly
normal, although marked crescents were present. Three years
later with the same glasses she read the whole of ^^ and D=0'5
well at 33 cms. The eyes were again put under atropine, and
the refraction was found to be practically the same as at the
first examination.
Note. — This case is a marked example of the importance of
preventing undue convergence in high myopia, and also illus-
trates the admirable result of fully correcting the error.
Aphakia. — Mrs. P., aged 58. Right eye: Advanced cataract.
Projection good. Left eye: Commencing cataract; vision = j^.
Right cataract extracted, and five months later, after needhng,
vision was :
+ 13 sph.
4- 2 cyl,
^•)
These glasses were ordered to be used for a short time every
day, and later on, when the cataract in the left eye had advanced,
+ 3 was added for reading and the glasses ordered to be worn
as bi-focals.
Myopic Astigmatism — Esophoria. — Master S. C, aged 14.
Vision = f in both eyes. Under atropine vision was:
R.V.Apart{;;|3%^y{,-.(---^*)}=t.
L.V.Apart--75cyl. •j = t.
220 THE REFRACTION OF THE EYE
The ophthalmometer showed '5 inverse astigmatism in the
right eye, and in the left ^ys inverse astigmatism, slightly oblique.
The Maddox test showed 1-5 metre angles of esophoria. He had
been wearing 4° base out in both eyes, with no good result,
sympto«is being headache and general malaise. The foUowmg
glasses were ordered for constant use:
K.E. - '50 cyl, (axis vert.) .
L.E. -.75cyl.l/
He had been in the habit of approaching his work very near
to the eyes, and no doubt this produced a spasm of the internal
recti, and the trouble had been kept up by eyestrain. He re-
covered completely in a few weeks.
Muscle Strain— Convergence " Insufficiency "—Myopia— Aniso-
metropia.— F. L., aged 31, a clerk, suffering from slight chronic
conjunctivitis, complains of headache, chiefly frontal, coming on
after work; says that lately the headache appears before he has
been two hours at work.
"H
L. < ^\ - I D.
His convergence near point is 18 cms,; if he is told to look at
the tip of a pen at this distance, and any attempt is made to bring
it nearer, the right internal rectus is seen to suddenly give way,
the right eye turns considerably outwards, and diplopia super-
venes. Examined by the Maddox test, he shows ^ m.a., latent
divergence for distance, and i m.a., which soon becomes 2 m.a.,
for I metre. He was given a mixture of iron and strychnine, and
advised to take a fortnight's rest, and on returning at the end of
that time the following great improvement was noted: c P —
8 cms. Latent divergence for distance = ^ m.a., and at | metre =
} m.a. His range of convergence had increased from 6 m.a. to
12 m.a. He was advised to return to business, and was ordered
the above correction for constant use, and told not to approach
his work nearer than 25 cms. He eventually became completely
well.
Convergent Concomitant Strabismus. — Master E., aged 8. Has
marked convergent strabismus, which is alternating, although he
most frequently fixes with the left eye.
L.V;}=t+i-Hm.
Under atropine for a fortnight the squint entirely disappeared,
and +2 =1 B.E. He was given + i sph. for both eyes for con-
stant use in circular frames.
A year later the atropine correction was only i '5, and -t- '75 was
ordered; eighteen months later the atropine correction was only
-I- I, and, as the squint had then entirely disappeared, glasses
were discontinued.
CHAPTER XIX
VISION TESTS FOR THE SERVICES
I AM indebted to the heads of the various departments
of the pubUc services for their courtesy in supplying
me with these revised standards.
COMMISSIONS IN THE REGULAR ARMY AND SPECIAL
RESERVE.*
(a) Squint, or any morbid condition of the eyes or of the lids
of either eye liable to the risk of aggravation or recurrence,
will cause the rejection of the candidate.
(6) The examination for determining the acuteness of vision
includes two tests: one for distant, the other for near,
vision. The Army test-types will be used for the test for
distant vision, without glasses, except where otherwise
stated below, at a distance of 20 feet; and Snellen's Opto-
typi for the test for near vision, without glasses, at any
distance selected by the candidate. Each eye will be ex-
amined separately, and the lids must be kept wide open
during the test. The candidate must be able to read the
tests without hesitation in ordinary daylight.
(c) The standards of the minimum acuteness of vision with
which a candidate will be accepted are as follows :
Standard I.
Right Eye. Left Eye.
Distant vision: V. =f. V. =|.
Near vision: Reads o, 6. Reads o, 6,
Standard II.
Better Eye. Worse Eye.
Distant vision: V.=f. V., without glasses = not below
-^^\ and, after correction
with glasses = not below ^-^.
Near vision: Reads o, 6. Reads i.
* These regulations were received from the War Office in
November, 1917.
221
222 THE REFRACTION OF THE EYE
Standard 111.
Better Eye. Worse Eye.
Distant vision: V., without V., without glasses = not belovf
glasses = not below ^\; and, ^^•, and, after correction,
after correction with glasses with glasses = not below /^j .
= not below f.
Near vision: Reads o, 8. Reads i.
{d) In Standard III., the standard for the test for distant vision,
without glasses, for officers of the Special Reserve, will be
not below -^.
{e) Inability to distinguish the principal colours will not be
regarded as a cause for rejection, but the fact will be noted
in the proceedings and the candidate will be informed.
(/) The degree of acuteness of vision of all candidates for
commissions will be entered in the proceedings in the
following manner :
(i.) Candidates whose vision fulfils the requirements of
Standard I. :
" Vision, normal."
(ii.) Candidates whose vision does not fulfil the require-
ments of Standard I. :
V.R. = ; with glasses = ; Reads
V.L. = ..... ; with glasses = ; Reads
{g) No relaxation of the standard of vision will be allowed.
ROYAL AIR SERVICE.
" Vision (including colour perception) must be normal."
NAVY (OFFICERS AND MEN).
To determine the acuity of vision, Snellen's letter types are
to be used, and care is to be taken that the proper distances are
carefully marked off, either on the floor or on the wall of the
room. Should the room not be sufficiently long for the 6-metre
card to be used, the 5-metre card may be substituted, and both
these cards are to be supplied, as well as the proper size of types
for testing near vision.
The colour sense is to be determined by means of Holmgren's
wool-test, and care is to be taken that when the wools become
dull from use they are to be renewed.
Officers. — A candidate must have no defect of sight; he must
be able to read without glasses f by each eye separately, and the
near type at the distance for which it is marked. Squint, or any
defective action of the eye muscles, any disease of the eye, or
any imperfection in the colour sense, disqualifies.
VISION TESTS FOR THE SERVICES
223
Men :
Rating. Vision Required.
Air Service . . . . . . Vision, f one eye, f the other.
Colour sense must be normal.
Wiremen, Armourer's Crew, Long Service. — Vision, f both
and Shipwrights eyes. Colour sense must be
normal.
Hostilities only. — Vision, ^^ both
eyes.
Blacksmiths, Coopers, Plumb-
ers, and Painters . . . . Vision, f both eyes.
Boys . . . . . . . . Vision, f both eyes. Colour sense
must be normal.
Carpenter's Crew
Electrical Artificers . .
Engine- Room Artificers
Vision, f both eyes. Colour sense
must be normal.
Long Service. — Vision, f both eyes.
Colour sense must be normal.
Hostilities only. — Vision, y% both
eyes. Colour sense must be
normal.
Vision, f one eye, ^ the other.
Colour sense must be normal.
Officers' Stewards and Cooks, Colour sense not essential. Glasses
Boy Servants, and Ships' allowed. Vision, ^\ both eyes.
Cooks
Royal Marine Bands (Buglers,
R.M.L.I., R.M.A., and
Band Boys)
Royal Marine Artillery
Royal Marine Light Infantry
Vision for Buglers: f one eye,
tV the other. Colour sense
must be normal.
Vision for Band Boys: ^ both
eyes. Colour sense not essen-
tial.
Long Service. — Vision, ^ one eye,
^ the other. Colour sense
must be normal.
Hostilities only. — Vision, ^f^ both
eyes. Colour sense not essen-
tial. Glasses allowed.
Long Service. — Vision, § one eye,
^ the other. Colour sense
must be normal.
Hostilities only. — Vision, ^ both
eyes. Colour sense not essen-
tial. Glasses allowed.
224 THE REFRACTION OF THE EYE
Rating. Vision Required.
Seamen . , . , . . Special Service. — Vision, | one
eye, y\ the other. Colour
sense must be normal.
Hostilities only. — Same standard.
Sick- Berth Attendants . . Vision, ^ both eyes. Colour
sense must be normal. Glasses
allowed.
Stokers . . . . . . Vision, y\ one eye, ^ the other.
Colour sense not essential.
Any defect of vision must be due to errors of refraction which
can be corrected to normal by glasses, and vision without glasses
must in any case be not less than /^ with each eye, and the can-
didate must also be able to read d =o, 6 of Snellen's test types.
Note. — For distant vision, D=f is considered normal;
for near vision, ability to read d = '6 at any distance chosen
by the candidate.
Assistant Clerkships in the Navy. — Short-sighted candidates,
in other respects fit, are especially considered ; a moderate degree
of refraction error would not disqualify, provided the eyes are
in other respects normal.
APPOINTMENTS UNDER THE GOVERNMENT OF INDIA.
The Ecclesiastical, Education, Geological Survey, Agricultural,
Indian Finance, Customs, Civil Veterinary, and Other Depart-
ments not specially provided for in the following pages.
1. A candidate may be admitted into the Civil Services of
the Government of India if ametropic in one or both eyes, pro-
vided that, with correcting lenses, the acuteness of vision be not
less than | in one eye and f in the other ; there being no morbid
changes in the fundus of either eye.
2. Cases of myopia, however, with a posterior staphyloma,
may be admitted into the Service, provided the a,metropia in
either eye does not exceed 2-5 d, and no active morbid changes of
choroid or retina be present.
3 . A candidate who has a defect of vision arising from nebula
of the cornea is disqualified if the sight of either eye be less than
1% ; and in such a case the acuteness of vision in the better eye
must equal |, with or without glasses.
4. Squint or any morbid condition, subject to the risk of
aggravation or recurrence, in either eye, may cause the rejection
of a candidate. The existence of imperfection of colour sens*-
will be noted on the candidate's papers.
f..'
VISION TESTS FOR THE SERVICES 22$
The Departments of Forest, Survey, Telegraph, Factories, and for
Various Artificers*
1. If myopia in one or both eyes exists, a candidate may be
passed, provided the ametropia does not exceed 2-5 d, and if with
correcting glasses, not exceeding 2*5 d, the acuteness of vision
in one eye equals f and in the other f , there being normal range
of accommodation with the glasses.
2. Myopic astigmatism does not disqualify a candidate for ser-
vice, provided the lens or the combined spherical and cylindrical
lenses required to correct the error of refraction do not exceed
— 2'5 d; the acuteness of vision in one eye, when corrected, being
equal to |, and in the other eye f , together with normal range
of accommodation with the correcting glasses, there being no
evidence of progressive disease in the choroid or retina.
3. A candidate having total hypermetropia not exceeding
4 D is not disqualified, provided the sight in one eye (when
under the influence of atropine) equals f , and in the other eye
equals f , with + 4 d or any lower power.
4. Hypermetropic astigmatism does not disqualify a candidate
for the Service, provided the lens or combined lenses required to
cover the error of refraction do not exceed 4 d, and that the sight
of one eye equals | , and of the other f , with or without such lens
or lenses.
5. A candidate having a defect of vision arising from nebula
of the cornea is disqualified if the sight of one eye be less than y\.
In such a case the better eye must be emmetropic. Defects of
vision arising from pathological or other changes in the deeper
structures of either eye which are not referred to in the above
rules may exclude a candidate for admission into the Service.
6. Squint or any morbid condition, subject to the risk of aggra-
vation or recurrence, in either eye, may cause the rejection of a
candidate. The existence of imperfection of colour sense will
be noted on the candidate's papers.
Public Works Department and Superior Establishments,
Railway Department.
1. If myopia in one or both eyes exists, a candidate may be
passed, provided the ametropia does not exceed 3 '5 d, and if
with correcting glasses not exceeding 3-50 the acuteness of vision
in one eye equals f , there being normal range of accommodation
with the glasses.
2. Myopic astigmatism does not disqualify a candidate, pro-
vided the lens, or the combined spherical and cylindrical lenses,
required to correct the error of refraction does not exceed 3*5 d;
the acuteness of vision in one eye, when corrected, being equal
to f , and in the other f , together with normal range of accommo-
* Artificers engaged in map and plan drawing may be con-
sidered separately, and this standard relaxed if it appears to te
desirable.
15
22 6 THE REFRACTION OF THE EYE
dation with the correcting glasses, there being no evidence of
progressive disease in the choroid or retina.
3. A candidate having total hypermetropia not exceeding 4 d
is not disqualified, provided the sight in one eye (when under
the influence of atropine) equals f , and in the other eye equals f ,
with +40 glasses, or any lower power.
4. Hypermetropic astigmatism does not disqualify, provided
the lens or combined lenses required to cover the error of refrac-
tion do not exceed 4 d, and that the sight of one eye equals f , and
the other f , with or without such lens or lenses.
5. A candidate having a defect of vision arising from nebula
of the cornea is disqualified if the sight of that eye be less than
•j^. In such a case the better eye must be emmetropic. Defects
of vision arising from pathological or other changes in the deeper
structures of either eye which are not referred to in these rules
may exclude a candidate.
6. Squint or any morbid condition, subject to the risk of
aggravation or recurrence, in either eye, may cause the rejection
of a candidate. Any imperfection of the colour sense is a dis-
qualification for appointment to the Engineering Branch of the
Railway Department, or as Assistant Superintendent in the
Trafiic Department. In all other cases a note as to any imperfec-
tion of colour sense will be made on the candidate's papers.
The Indian Medical Service and the Police Department.
1. Squint, or any morbid condition of the eyes or of the lids
of either eye liable to the risk of aggravation or recurrence, will
cause the rejection of the candidate.
2. The examination for determining the acuteness of vision
includes two tests — one for distant, the other for near, vision.
The army test-types will be used for the test for distant vision,
without glasses, except where otherwise stated below, at a dis-
tance of 20 feet; and Snellen's Optotypi for the test for near
vision, without glasses, at any distance selected by the candidate.
Each eye will be examined separately, and the lids must be kept
wide open during the test. The candidate must be able to read
the tests without hesitation in ordinary daylight.
3. A candidate possessing acuteness of vision, according to
one of the standards herein laid down, will not be rejected on
account of an error of refraction, provided that the error of
refraction, in the following cases, does not exceed the limits
mentioned — viz.: (a) In the case of myopia, that the error of
refraction does not exceed 2*5 d; [h) that any correction for
astigmatism does not exceed 2*5 d; and, in the case of myopic
astigmatism, that the total error of refraction does not exceed
2-5 D.
4. Subject to the foregoing conditions, the standards of the
minimum acuteness of vision with which a candidate will be
accepted are as follows:
VISION TESTS FOR THE SERVICES 22/
Standard I.
Right Eye. Left Eye.
Distant vision : V.=|. V.=f.
Near vision: Reads o, 6. Reads o, 6.
Standard II.
Better Eye. Worse Eye.
Distant vision: V. =f. V., without glasses = not below
^; and, after correction
with glasses = not below ^.
Near vision: Reads o, 6. Reads i.
Standard III.
Better Eye. Worse Eye.
Distant vision: V., without V., without glasses = not below
glasses = not below -^f\ and ^; and, after correction
after correction with glasses with glasses = not below ^.
= not below f .
Near vision: Reads o, 8. Reads i.
N.B. — In all other respects candidates for these two branches
of the Service must come up to the standard of physical require-
ments laid down for candidates for commissions in the army.
The Indian Pilot Service, and Candidates for Appointments as
Guards, Engine-drivers, Signalmen, and Pointsmen on
Railways.
1. A candidate is disqualified unless both eyes are emmetropic,
his acuteness of vision and range of accommodation being perfect.
2. A candidate is disqualified by any imperfection of his
colour sense.
3. Strabismus, or any defective action of the exterior muscles
of the eyeball, disqualifies a candidate for these branches of
The Indian Marine Service, including Engineers and Firemen.
1. A candidate is disqualified if he has an error of refraction
in one or both eyes which is not neutralized by a concave or by
a convex i d lens, or some lower power.
2. A candidate is disqualified by any imperfection of his colour
sense.
3. Strabismus, or any defective action of the exterior muscles
of the eyeball, disqualifies a candidate for this branch of service.
228 THE REFRACTION OF THE EYE
Special Duty.
Candidates for special duty under Government must possess
such an amount of acuteness of vision as will, without hindrance,
enable them to perform the work of their office for the period
their appointment may last. In all cases of imperfection of
colour sense a note will be made on the candidate's papers.
HOME CIVIL SERVICE.
There is no fixed standard. The candidate is referred to " a
competent medical adviser, ' ' leaving him to apply whatever tests
he may deem suitable, and whatever standard the particular
situation may require.
A candidate is considered unfit if he has any serious defect in
vision. A moderate degree of ordinary short sight corrected by
glasses would not, as a rule, be regarded as a disqualification;
but candidates for the Customs Outdoor Service are liable to
disqualification for any defect of vision. Candidates for some
other appointments of a special character would be rejected for
colour blindness, but for the Covenanted Civil Service of India
and for ordinary home appointments it is not by itself a dis-
qualification.
PRISON SERVICE.
Candidates are expected to have " normal vision " in both
eyes, and any slight departure from normal vision is considered
on its merits in accordance with the duties which the candidate
would be required to perform if appointed.
THE METROPOLITAN POLICE SERVICE.
Candidates are required to have "normal vision" in both
eyes, without glasses. The ordinary test-types are used, and
the range of accommodation is sometimes, but not uniformly,
tested. Candidates who show only slight deviation from the
** normal " standard are considered on their merits.
ENGLISH RAILWAYS.
No uniform standard. Each company has its own standard.
Every engine-driver should have normal colour perception;
and, without glasses, vision should be at least ^ in each eye.
VISION TESTS FOR THE SERVICES
229
en
1
Vision with glasses
(corrected vision)
counts.
2i
>
en
.d
li
en y
en
<u
"to
.d
"^e/5
li
■J)
•>
Vision without glasses
counts. For home
service, garrison ser-
vice, and garrison
service abroad, glas-
ses are allowed with-
in unspecified limits.
J
Ul
>
1
•E
a
in
1
1
1
i
<=>
o.d
.S d
H*
a;
d
+->
.Sd
Ul S
3
^*
.So
H«
U ncor r e cted
vision must be
I in better eye,
^(j- in worse eye.
The better eye
may be the left.
1
\ in better eye. Other
eye may have mini-
mal vision. For
Landsturm vision
= \. If one eye has
vision = ^ the other
may be blind.
Group I, J in each
eye. Group 2, ^ in
one ; ^ in other.
d
II
H«
No correction allowed
for general service.
Uncorrected vision
must be ^ in each
eye, or 4 in the right
eye with ,V in the
left.
<
1
1
Ui
z
c
.a
8
1
Above 6 D. no limit if
standard of corrected
vision is attained.
Above 7 D. no limit if
standard of corrected
vision is attained.
1
No amount specified,
but according to
vision required high-
est amount possible
is about 2-5 D. in
better eye and 3'5D.
in worse eye.
1
6-5 D. For Land-
sturm no limit
if standard of
corrected vision
attained.
Q
No amount speci-
fied, but accord-
ing to vision
required highest
amount possible
is about 2*5 D.
Germany
<
H
D
<
<
H
1— 1
Great
Britain*
BIBLIOGRAPHY
I HAVE to acknowledge my indebtedness to the follow-
ing books and papers for much valuable material :
" Accommodation and Refraction of the Eye ": Donders.
" The Refraction and Accommodation of the Eye ": Landclt.
" Refraction of the Eye " : Hartridge.
" Refraction and how to Refract ": Thorington.
" The Eye, its Refraction and Diseases ": Gibbons.
"Refraction": Druiff.
" Kurzsichtigkeit " : Hirshberg.
" Anomalies de la Refraction ": Fromaget and Bichelonne.
" Squint " : Worth.
" Convergent Strabismus ": Holthouse.
"The Ocular Muscles": Maddox. 2nd edition.
" Ophthalmological Prisms ": Maddox.
" Prismatic Combinations ": Percival.
" Prescribing of Spectacles ": Percival.
" Examination of the Eye ": Snell.
" Physiologic Optics ": Tscherning.
" Elementary Ophthalmic Optics ": Parsons.
" Textbook of Physiology' ": Michael Foster.
" Light " : Lewis Wright.
" Treatise on Practical Light ": R. S. Clay.
" Curiosities of Light and Sight ": Shelford Bidwell.
" Hygiene of the Eye " : Cohn.
" Glaucoma ": Priestley Smith.
" Textbook of Ophthalmology ": Fuchs. 3rd edition.
" Diseases of the Eye " : Fick.
" Diseases of the Eye ": Berry.
" Diseases of the Eye ": Mayou. 2nd edition.
" Diseases of the Eye " : Swanzy.
230
BIBLIOGRAPHY 23I
" System of Diseases of the Eye ": Norris and Oliver.
" Diseases of the Eye " : De Schweinitz. 8th edition.
" Diseases of the Eye ": Noyes.
" Handbuch der Augenheilkunde " : Graefe and Saemisch.
" Precis d'Ophthalmologie " : Morax.
" Functional Nervous Diseases ": Stevens.
" Diseases of the Nervous System ": Gowers.
" The Diseases of the Nervous System " : Ross.
" Lectures on Nervous Diseases ": Ranney.
" The Eye in General Disease " : Knies.
" Minor iVIaladies " : Leonard Williams.
"Headache and other Morbid Cephalic Sensations": Harry
Campbell.
"Megrim and Sick Headaches": Liveing.
"Headaches": Day.
Transactions of the Ophthalmological Society.
The Ophthalmoscope.
The Ophthalmic Review.
British Journal of Ophthalmology.
Graefe' s Archiv f. Ophthalmologie.
Knapp's Archives of Ophthalmology.
Transactions of the A merican Ophthalmological Society.
A merican Journal of Ophthalmology.
Annals d'Oculistique.
INDEX
ABDUCTING power of external
rectus, 178
Aberration, chromatic, 133
in bi-focal lenses, 18
spherical, 18
of crystalline lens, 34
Absolute hyperopia, 90, 93
Accommodation, 29
absolute, 38
amplitude of, 34, 88, loi, 142
and convergence, 38, 50, 52,
lOI
asymmetrical, 123, 154
binocular, 38
centre of, 42
cramp of, 94, 105, 201, 218
diminution of, 142
far point of, 34, 85, 100
fatigue of, 151
influence of age upon, 142
in astigmatism, 123
in hyperopia, 87
in myopia, 100
in presbyopia, 142
iris in, 23
loss of, 142, 148
mechanism of, 31
muscle of, 33
near point of, 31, 34, 88, loi
paralysis of, by cycloplegics,
region of, 37
relative, 38
spasm of, 94, 105, 201, 218
theories of, 33
theory of squint, 187
Acquired hyperopia, 92, 145
Acuity of vision, 25
in absolute hyperopia, 92
Acuity in astigmatism, ii6, 118
in myopia, 105
record of, 26, 207
Adducting power of internal
rectus, 175
Age, influence of, upon accom-
modation, 142
Ague, brow, 164
Air, Royal, Service, vision tests
for, 222
Alpha, angle, 187
Alternating strabismus, 185
Amblyopia, 157, 190, 191
as a cause of squint, 191
ex anopsia, 157, 191
Amblyoscope, Worth's, 195
Ametropia, 24
axial, 115
curvature, 116
Amplitude of accommodation,
34, 88, loi, 142
of convergence, 50, 182
Anatomical conditions associated
with myopia, 103, 104
with strabismus, 190
Angle alpha, 187
gamma, 95, 103, 105, 187
of convergence, 48
of deviation, 5
of five minutes, 26
of incidence, 57
of one minute, 25
of prism, 5
of reflection, 57
of refraction, 5, 7
of strabismus, 192
of vision, 25
Anisometropia, 152, 214, 216
asymmetry of face in, 153
232
INDEX
233
Anisometropia, binocular vision
in, 153
varieties of, 153
Annular muscle of Miiller, 23, 95
Anterior linear focus, 117
principal focus, 20
Antimetropia, 152
Aphakia, 16, 92, 159, 219
Apparatus for vision testing, 202
Apparent myopia, 89, 94, 109,
218
strabismus, 95, 105, 187, 189
Army, vision tests for, 221
Artificial light in eye-work, 63,
66, 202, 203
Asthenopia. See Eyestrain
Astigmatic clock, 132
fan, 122, 132
headache, 122, 163
hyperopia, 119, 214
myopia, 119, 215
surface, images formed by,
117
Astigmatism, 115
accommodation in, 123
asymmetric, 120
asymmetrical accommoda-
tion in, 123
compound hyperopic, 119
myopic, 119, 215
corneal, 121, 125, 126, 139
crystalline, 121, 125
diagnosis of, 125
direct, 119
form of images in, 117
homonymous, 120
hyperopic, 119, 203
inverse, 119, 160
irregular, 139
lenticular, 121
measurement of, 72, 125
mixed, 119, 137, 216
myopic, 119, 215, 216
oblique, 73, 80, 119, 131, 216
physiological, irregular, 139
regular, 121
regular, 118
seat of, 121
simple hyperopic, 119
small errors of, relative fre-
quency of, 135
symmetric, 119
Astigmatism, tests for, 125
transient, 121
treatment of, 135
Asymmetric astigmatism, 120
Asymmetrical accommodation in
astigmatism, 123
in anisometropia, 154
Asymmetry of face in anisome-
tropia, 153
in astigmatism, 125
Atrophy, choroidal, in myopia,
107
Atropine, 97, iii, 135, 198
tabloids or discs, 200
Attachments of eye-muscles, 40
Axial ametropia, 115
hyperopia, 91
myopia, 100
Axis, major, of cornea, 187
of cylinders, 13, 136, 138
of rotation of eye-muscles, 40
optic, 19, 22, 187
principal, 9
secondary, 9
visual, 187
Band of light in retinoscopy, 80
Base of prism, 3, 5
Bi-concave lens, 9, 11
Bi-convex lens, 8, 10
Bi-focal lenses, 16, 151, 201
Bilious headache, 165
Binocular accommodation, 38
in anisometropia, 154
vision, 42
loss of, in hyperopia, 91
tests for, 153
Blepharitis, 92, 163
Blinking and eyestrain, 167
Brain-fag, 167
Bridge of spectacles, 209
Brow-ague, 164
Camera, photographic, 19
Capsule of lens, ^3
Tenon's, 39
Cardinal points, 20
Cases, illustrative, 213
Cataract, 125, 163
caused by eyestrain, 125, 163
extraction, glasses after, 159
234
THE REFRACTION OF THE EYE
Cataract, increase of density of
lens, causing myopia, loi
monocular polyopia in, 139
Centrad, 6
Central motor asthenopia, 173
Centre, oculo-motor, 42, 43
of accommodation, 42, 43
of internal rectus, 42, 43
of rotation, 188
optic, 43
optical, of lenses, 9, 22,
208
pupillary, 42, 43
Centring of lenses, 208
method of checking. 209
Changes in fundus in myopia,
107, 113
Chart of accommodation,
author's, 147
Bonders', 143, 144, 145
Choreiform movements of facial
muscles, 166, 167
Choroidal atrophy in myopia,
107
Chromatic aberration, 133
Chromo-aberration test, 133
Ciliary muscle, 32, ^;^
fatigue of, 151
in hyperopia, 33, 95
in myopia, ^3) 112
processes, 3^
spasm of, 94, 105, 201,
218
Circles, diffusion, 30, 105, 107
Circular muscle of Miiller, 33,
95
Civil Service, vision tests for
Home, 228
Indian, 224
Clock-face, 132
Cobalt-blue glass test for astig-
matism, 133
Cocaine, 199
Combination of lenses, 15
Compound hyperopic astigma-
tism, 119, 214
myopic astigmatism, 119,215
Concave lenses, 9, 11, 16, 99, 176,
203, 208
mirror, 57, 59, 82
Concomitant squint, 184
Confusion letters, 133
Confusion of images in astigma-
tism, 118
Congenital defect of eye-muscles,
i8i
tendency to myopia, 104
Conical cornea, loi, 141
Conjugate foci in lenses, ij
in mirror, 58
Conjunctivitis and eyestrain,
92, 163
Constant squint, 185
Convergence, 39, 48
amplitude or range of„ 50,
172
and accommodation, 38, 52,
90, lOI
angle of, 48
insufficiency of, 53, 105, 172,
175, 220
latent, 44, 177
positive part of amplitude
of, 172
punctum proximum in, 51
remotum, 50
relation between, and accom-
modation, 38, 52, 90, lOI
Convergent strabismus, 91, 95,
184, 187, 193, 220
Convex lenses, 8, 10, 203, 208
Cornea, 21, 31, 121
conical, loi, 141
in hyperopia, 95
ulcers of, 163
causing astigmatism, 139
Corneal astigmatism, regular,
115, 121, 123
irregular, 139
axis, 187
Cramp of accommodation, 94,
105, 201, 218
Crescent in myopia, 107
Crookes's glass, 211
Crossed diplopia, 184
Crown glass, index of refraction
of, 4
Cupping of myopic disc, 107
Curvature, ametropia of , 116
hyperopia, 92
Customs Outdoor Service, vision
required for, 228
Cyclophoria, 180
Cycloplegia, 198
INDEX
235
Cycloplegics, 198
Cylindrical lenses, 12,
204
axis of, 12, 136
testing, 14
16, 135,
Dark room, 202
Decentring lenses, 178
Desks in schools, height of, no
Detachment of retina in high
myopia, 108, 113
Deviation, angle of, in prisms, 5
latent, of muscles, 44, 105,
172
primary, in strabismus, 192
secondary, in strabismus,
192
Diaphragm test, Harman's, 183
Diffusion circles, 30, 107
Dioptre, 13
prism, 6
table of, and inches, 14
Dioptric apparatus of the eye, 19
system, 13
Diplopia, 183
heteronymous, 184, 186
homonymous, 184, 185
Direct ophthalmoscopic exam-
ination, 66
Disc, optic, 66, 95, 106
cupped or " dragged," in
myopia, 107
shape of, in astigmatism, 65
in myopia, 106, 107
Disc, pin-hole, 18, 141, 204
Placido's, 140
stenopaic, 141, 204
Discission of lens in high
myopia, 114
Dissociation between accommo-
dation and convergence, 38,
54, 90, 102
Distance, test type for, 26, 202
Distant vision, Maddox test in,
relation of eyes in, 44
Divergence, latent, 105, 171, 175
Divergent strabismus, 105, 184,
197
Dixey glasses for anisometropia,
155
test types, 202
Donders' formula for accommo-
dation, 34
for convergence, 50
on astigmatism, 123
on convergence in myopia,
103, 108
on presbyopia, 146, 148
Dot and line test (Graefe's), 54
Dynamic lenticular astigmatism,
121
equilibrium of ocular mus-
cles, 48
Dynamometer, Landolt's, 52
Dyspepsia due to eyestrain, 169,
Elasticity of lens, diminution of,
with age, 142, 147
Electric ophthalmoscope, 61
Elongation of eyeball, 25, 100
Emmetropia, 24, 35, 64, 68, 79,
142, 180
Epilepsy, relation between eye-
strain and, 166, 214
Erect image in high hyperopia,
65
in spherical lenses, n
Eserine, 200, 205
Esophorio, 176, 219
Esotropia, 184
Examination, methods of, 202
of cornea, 63
of patients, 205
ophthalmoscopic, 63
Exercises, orthoptic, 181
Exophoria, 175
Exotropia, 184
External rectus muscle, 39
abducting power of, 178
insufficiency of, 176
tenotomy of, 182, 197
Eye, normal, optic properties of,
19
standard, 21
Eyestrain, astigmatism and, 122,
136
blepharitis from, 92, 163
brain-fag and, 167
choreiform movements from,
^67 . . . ^
conjunctivitis and, 92
definition of, 162
236
THE REFRACTION OF THE EYE
Eyestrain, dyspepsia from, 169,
epilepsy and, 166
headache and, 122, 163
heterophoria and, 173
hyperopia and, "6"], 93
insomnia and, 168
migraine and, 164
nerve exhaustion and, 167
neurasthenia and, 167
presbyopia and, 149
shell shock and, 170
Face, asymmetry of, in aniso-
metropia, 153
Facultative hyperopia, 90, 92
Fan, 122
Far point of accommodation, 34
in hyperopia, 85
in myopia, 100
of convergence, 50
Fatigue of accommodation, 151
P^ocal distance of lenses, 10
illumination, 63
interval of Sturm, 118
Focus, anterior, 20
conjugate, in convex lenses,
II
in concave mirrors, 57
linear, in astigmatism, 117
posterior, 20
principal, 10, 11
virtual, 11
Focus-glass, 62
Folders, 210
Formation of images by astig-
matic surfaces, 117, 118
in indirect ophthalmoscopy,
64
in lenses, 10
in the eye, 22
Frames, spectacle, 210
scholar's, 210
Franklin lenses, 17
Frequency, relative, of different
forms of ametropia, 25
of small errors of astig-
matism, 135
Fundus in myopia, 107
in astigmatism, 131
in direct method of oph-
thalmoscopy, 67
Fundus in indirect method of
ophthalmoscopy, 64
Fusion sense, 191, 196
supplement, 54, 102, 171
Gamma, angle, 95, 105, 187
Glass, focus, 62
Crookes's, 211
Glasses. See Lenses
coloured, Snellen's, 43, 153
dark or tinted, 113, 204
Glass-rod test, 44
Glaucoma caused by eyestrain,
163
in hyperopia, 98
caused by atropine, 98, 200
Gould's biographic clinics, 170
Graefe's explanation of con-
vergence of covered eye,
42
dot and line test, 54
Hamblin, scholar's frame, 210
trial case, 204
Harman's diaphragm test, 183
Harwood, Dr., on eyestrain, 170
Headache, bilious, 165
in astigmatism, 122
ocular, 163
Helmholtz's theory of accom-
modation, 33
ophthalmoscope, 58
Hemicrania, 164
Hereditary tendency to myopia,
104
Hess, :i2>
Heteronymous diplopia, 184
Heterophoria, 171, 213
cause of, 171
eyestrain and, 173
tenotomy for, 182
tests for, 44, 175, 177
varieties of, 174
Heterotropia, 174, 184
High myopia, 107, 112, 113, 212
discission of lens in, 114
treatment of, 112
Homatropine, 97, 112, 199
Home Civil Service, vision tests
for, 228
Homonymous astigmatism, 120
diplopia, 184
INDEX
237
" Hook-fronts," 151
Hygiene, ophthalinic, no
Hyperesophoria, 180
Hyper exophoria, 180
Hyperopia, 25, 85, 213
absolute, 90, 93
accommodation in, 35, 88
acquired, 92, 145
angle gamma in, 95, 187
axial, 91
causes of, 91
curvature, 92
diagnosis of, 95
esophoria and, 178
estimation of, 95
exophoria and high, 175
eyestrain in, 92
facultative, 90, 92
index, 92
latent, 90
length of eye in, 91
manifest, 89
physical signs in, 95
punctum proximum in, 35,
88
remotum in, 35, 85
relative, 91
symptoms of, 92
total, 90
treatment of, 95
varieties of, 91
Hyperopic astigmatism, 119, 214
Hyperphoria, 158, 179
Hypertropia, 185
Illumination, focal, 63
in retinoscopy, 78
of dark room, 202
of test types, 203
Images crossed in diplopia, 184
erect, 65, 95
form of, in astigmatism,
117, 118
formation of, in the eye, 22
in indirect method, 64
in lenses, 11
in emmetropia, 64
in hyperopia, 65, 95
in myopia, 65
inverted, 64
on cornea, 31
on crystalline lens, 31
Images, retinal, size of, 23
virtual, 11
Inch system of measuring
lenses, 13
compared with dioptres, 14
Incidence, angle of, 57
Incipient cataract, monocular
polyopia in, 139
Index hyperopia, 92
of refraction, 3
Indian Civil Service, vision
tests in, 224
Marine Service, vision tests
in, 227
Medical Service, vision
tests in, 226
Pilot Service, vision tests
in, 227
Police Service, vision tests
in, 227
Railways, vision tests in,
225, 227
Indirect method of ophthalmo-
scopic examination, 63
advantages of, 65
Infinity, punctum remotum at,
in accommodation, 35
in convergence, 50
Influence of age upon accommo-
dation, 142
Initial convergence, 54
Insomnia, 168, 214
Insufficiency of convergence,
53> i7i> 175. 181, 220
of eye-muscles, 102, 171,
172, 175, 176
Intermittent squint, 186
Internal rectus, 39, 41
adducting, power of, 175
centre of, 43
insufficiency of, 53, 102,
175
tenotomy of, in strabismus,
196
Interval, focal, of Sturm, 118
Inverted image, 64, 65
Iris, 32, 33
constrictor of, 42
in accommodation, 33
Irregular astigmatism, 139
of cornea, 139
of lens, 139
238
THE REFRACTION OF THE EYE
Jaeger test type, 28
J aval's ophthalmometer, 126
Lagging of convergence behind
accommodation, 52
Landolt on convergence, 42, 56
on "motor asthenopia," 173
on tenotomy for hetero-
phoria, 182
Landolt' s ophthalmq- dynamo-
meter, 51, 52
Latent convergence, 47, 176
deviation, 44, 171
divergence, 47, 52, 175
hyperopia, 90
Length of eyeball, 21
Lens, crystalline, 30
absence of, in aphakia, 159
capsule of, 3;^
changes in, during accom-
modation, 30, 31, 32
discission of, in high myo-
pia, 114
elasticity of, 33
diminished by age,
142, 147
images on, 31
irregular astigmatism of,
139
regular astigmatism of, 121,
123
spherical aberration in, 18
Lens, magnifying for examina-
tion, 62
Voigtlaender's, 63
Lenses, bi-concave, 8, ii, 15, 99,
176, 203
bi-convex, 8, 10, 15, 86, 96,
203
bi-focal, 16, 151, 161
centring of, 208
combination of, 15
concavo-convex, 9
Crookes's, 211
cylindrical, 12, 16, 135, 203
decentred, 176, 178, 208
Franklin, 17
meniscus, 9
neutralizing, 14
numeration of, 13
periscopic, 15, 137
plano-concave, 9
Lenses, plano-convex, 9
spherical, 9
sphero-cylindrical, 15
testing, 14, 209
tinted, 113, 204
toric, 16, 160
Lenticular astigmatism, 121,
123, 139
Letters of test types, 26
Pray's, 134
Light, velocity of, i
artificial, 63, 203
Linear focus in astigmatism,
117
Liveing and " nerve storms,"
165
Luxe bi-focals, 17
Macula, 72
changes in, in myopia, 107
Maddox test for muscles, 44, 54
Malignant myopia, 108
Malingering, 183
Manifest hyperopia, 89
Marple mirror, 62
Marple's skiascopes, 83
Measurement by "direct
method," 66
of astigmatism, 72, 130
of hyperopia, 70, 95
of myopia, 69, 106
Mechanism of accommodation,
31
Media, refracting, of the eye,
20
Meniscus, 9
Meridians, principal, in astig-
matism, 116, 119, 136
Meridional asymmetrical ac-
commodation, 123
Metre angle, 49
Metrical system of measuring
lenses, 13
Migraine, 165
Mires of ophthalmometer, 127,
129
Mirror, Marple's, 62
Mirrors, centre of curvature of,
concave, 57, 82, 205
plane, 57, 82, 205
reflection from, 57
INDEX
239
Mixed astigmatism, 119, 137,
216
Monocles, 157, 211
Monocular polyopia in cataract,
139
vision, 43, 157
Monolateral strabismus, 185
Morton's ophthalmoscope, 60
Motor asthenopia, central, 173
peripheral, 173
centres, 42, 43
Miiller, annular muscle of, 33, 95
Muscae volitantes, 105
Muscle, ciliary, ^^
spasm, 94, 105, 201, 206
strain, 172
Muscles of eyeball, 39, 40
attachments of, 40
axis of rotation of, 40
congenital defect of, 181
equilibrium of, 48
Muscular inefficiency, 173
insufficiency, 172
Mydriasis, causing spherical
aberration, 18
Mydriatics, 199
Myopia, 25, 99, 213, 217
accommodation in, loi
apparent, 89, 94, 109, 218
axial, 100
causes of, 104
ciliary muscle in, ^;i, 112
detachment of retina in,
108, 113
diagnosis of, 105
exophoria and, 175
formation of image in, by
indirect ophthalmoscopy,
65
full correction of, io8, 109,
219
hereditary tendency to, 104
high, 107, 112, 218
discission of lens in, 114
treatment of, 112
influence of age on, loi
length of eyeball in, loi, 104
malignant, 108
measurement of, by ophthal-
moscope, 69, 106
ophthalmoscopic appearances
in, 107
Myopia, optic disc in, 107
posterior staphyloma in,
100, 107
presbyopia and, 144
progressive, 108, iii
punctum proximum in, 35,
lOI
remotum in, 35, 99
refractive, 100
region of accommodation
in, 38
retinoscopy in, 74
shape of eyeball in, 103
symptoms of, 105
treatment of, 108
visual acuteness in, 105
vitreous opacities in, 105
Myopic astigmatism, 119, 215,
218
crescent, 107
Nagel's metre angle, 49
Navy, vision tests for, 222
Near point of accommodation,
3i> 34, 88, loi
convergence, 51
Negative aberration, 18
angle alpha, 187
gamma, 105, 187
Nerve centres, ocular, 42
optic, 43
power waste, 167
Neurasthenia, 167
Neurasthenic muscular insuffi-
ciency, 175, 183
Neutralizing lenses, 14
Nodal point, 20, 21, 25
Normal vision, 27
relation of eyes in, 44
Note-book, 205
Note-taking, 205
Numbering of lenses, 13
of prisms, 5
Oblique astigmatism, 80, 119,.
126
Obscurations of vision, 94, 149
Ocular headache, 163
muscles. See Muscles
Oculomotor centre, 42, 43
Operative treatment of hetero-
phoria, 182
240
THE REFRACTION OF THE EYE
Operative treatment of myopia,
114
of strabismus, 196
Ophthalmic discs or tabloids,
200
hygiene, no
Ophthalmo-dynamometer, 52
Ophthalmometer, Hardy's, 130
Meyrowitz's, 126
Ophthalmoscope, 58
electric, 61
Morton's, 60
qualities of a good, 59
Ophthalmoscopic examination,
63
in astigmatism, 130
in hyperopia, 95
in myopia, 106
Optic axis of lenses, 14, 19
centre, 43
disc, 65, 95, 106, 107
Optical centre of lenses, 9, 19,
208
Optics, I
of reflection, 57
Orbit, shape of, in myopia, 103
Orthophoria, 47, 174
Orthoptic exercises, 181
Paralysis of accommodation by
cycloplegic, 95, 109, 193, 198
Perimeter, 192
Periodic squint, 186
Periscopic lenses, 15, 137, 210
Photographic camera compared
to eye, 19
Physiological astigmatism, 121,
139
Pilot Service, Indian, tests for,
227
Pince-nez, 211
Pin-hole disc, 18, 141, 204
Placido's disc, 140
Plano-concave lens, 9
convex lens, 8
Point, far. See Punctum re-
motum
near. See Punctum proxi-
mum
of reversal in retinoscopy,
78
Points, cardinal, 20
Points, nodal, 20
principal, 20
Police, Indian, test for, 226
Metropolitan, 228
Polyopia, monocular, in catar*
act, 139
Positive angle alpha and
gamma, 187
convergence, 172
Pray's letters, 133, 134
Presbyopia, 142
accommodation in, 142
anisometropia and, 157
definition of, 142, 148
eyestrain in, 149
symptoms of, 148
tables for accommodation
in, 146, 147
treatment of, 149
Presbyopic point, 148
Prescription forms for glasses,
138, 205 ^ _
Primary deviation in strabis-
mus, 192
Principal axis, 9
focus, 10, 20
meridians in astigmatism,
116
axis of, 119
plane, 22
points, 20
Prism dioptre, 6
rotary, 7
test for binocular vision, 153
test for malingering, 183
Prisms, 5
action of light on, 2, 5
angle of, 5
deviation of, 5
measurement of, by cen-
trads, 6
numbering of, :;
rotating, 7
uses of, 7, 176, 178, 182, 183
Prison service, vision required
for, 228
Progressive myopia, 108, in
Projection, visual, 77
Prominence of eyeball in myo-
pia, 105
Public v^orks, Indian Service,
vision tests for, 225
INDEX
241
Punctum proximum of accom-
modation, 31, 34
in emmetropia, 35
in hyperopia, 35, 88
in myopia, 35, loi
of convergence, 51
Punctum remotum of accommo-
dation, 34
in hyperopia, 35, 85
in myopia, 35, 99
of convergence, 50
Pupil, contraction of, hiding
aberration, 18, 34
in accommodation, 34, 42
in hyperopia, 94
in myopia, 105
Pupillary centre, 42
Qualities of a good refraction
ophthalmoscope, 59
Railways, vision tests for ser-
vice on English, 228
Indian, 225, 227
Range of accommodation and
convergence. See Amplitude
Rectus externus, 39, 40, 41, 176,
182
abducting power of, 178
insufficiency of, 176, 182
tenotomy of, in strabismus,
197
Rectus internus, 39, 41, 175
adducting power of, 175
insuflSciency of, 175, 181
tenotomy of, in strabismus,
196
Reduced eye, 21
Reflection, angle of, 57
by mirrors, 57
Refraction of light, i
angle of, in prisms, 5, 7
by lenses, 8
by prisms, 3, 5
by spherical surfaces, 19
by the eye, 20
explanation of, 2
index of, 3
Region of accommodation, 37
Regular astigmatism, 116
of lenSj i2ij 123
Regulations for use in testing
for Services, 222
Relation between accommodation
and convergence, 38, 52, 90,
lOI
Relative accommodation, 38
frequency of different forms
of ametropia, 25
of small errors of astig-
matism, 135
hyperopia, 91
range of accommodation
and convergence, 52
Remotum, punctum. See Punc-
tum
Retina, 19, 72
exudations or tumours of,
causing hyperopia, 92
in myopia, 107
Retinal detachment in myopia,
108
image, size of, 23
Retinoscopy, 74
Reversal, point of, in retinos-
copy, 78
Reversible spectacles, 158
Risley's rotary prism, 7
Rod test, Maddox, 44, 45
Room, dark, 203
Rotating prisms, 7
Rotation, axis of, of eye mus-
cles, 39, 40
Royal Air Service, vision tests
in, 222
Scale, Maddox, for distant
vision, 44, 45
for near vision, 45, 53
Scholar's frame, 210
Schools, ophthalmic hygiene
in, no
Scissors movement in retinos-
copy, 80
Sclerotic, 23
in myopia, 104
Secondary axes, 9
deviation in strabismus,
192
Shadow test, 74
Shape of eyeball in hyperopia,
95
in myopia, 103
16
242
THE REFRACTION OF THE EYE
Shell shock, 170
Short-sight. See Myopia
Simple hyperopia astigmatism,
119
myopic astigmatism, 119,
Skiascope, Marple's, 83
Skiascopy, 74
Snellen's coloured glass test, 43,
153. 183
type, 26, 202
Spasm of accommodation, 94,
loi, 105, 201, 218
Spectacles, 208
bi-focal, 16
bridge of, 209
for aphakia, 161
for regular astigmatism,
for irregular astigmatism,
141
for myopia, iii
for presbyopia, 150
frames, 209
Franklin, 17
reversible, 158
scholar's frame, 210
stenopaic, 141
Spherical aberration, 18
lenses, 9
Squint. See Strabismus
Standard eye, 21, 25
Staphyloma, 100, 107
Static lenticular astigmatism,
121, 123
Stenopaic disc, 139, 204
slit, 115, 134, 204
spectacles, 141
Stevens on heterophoria, 174
Strabismometer, 192
Strabismus, 184
alternating, 185
angle of, 192
apparent, 95, 185, 187
concomitant, 184
constant, 185
convergent, 91, 95, 184, 193,
220
divergent, 184
heredity in, 190
intermittent, 186
monolateral, 185
Strabismus, paralytic, 184, 192
periodic, 186
treatment of, 96, 193
vertical, 185
Sturm, focal interval of, 118
Supplement, fusion, 54, 102, 171
Supra-orbital neuralgia in eye-
strain, 164
Symmetric astigmatism, 119
Symptoms of astigmatism, 121
of eyestrain, . 92, 122, 149,
162, 173
of hyperopia, 92
of myopia, 105
of presbyopia, 148
Table of accommodation power
at different ages, 143, 144,
145. 147
of inches and dioptres, 14
Tabloids, ophthalmic, 200
Tenon's capsule, 39
Tenotomy in heterophoria, 182
in strabismus, 196
Test for aphakia, 159
for astigmatism, 125
for hyperopia, 95
for malingering, 183
for muscles, 44
for myopia, 106
letters. Fray's, 133, 134
types for distance, 26, 202
Dixey's, 202
for near vision, 28, 203
Testing optical centre of lenses,
209
Tinted glasses, 113, 204
Toric lenses, 16, 160
Trial case, 203
frame, 204
Tscherning's theory of accom-
modation, 23
Unequal contraction of ciliary
muscle, 121
sight in the eyes. See
Anisometropia
Velocity of light, i
Vertical strabismus,
Vertigo, 167
185
INDEX
243
Virtual focus^ 11
image, 11
Vision, acuity of, 27
in absolute hyperopia, 92
in astigmatism, 118
in myopia, 105
record of, 26
Vision, binocular, 42
tests for, 43, 153
monocular, 43, 157
obscurations of, 94, 149
testing, apparatus for, 202
tests for the Services, 221
Visual angle, 25
axis, 187
projection, 77
Visual standards for recruits in
chief European armies, 229
Vitreous opacities in myopia,
105
shrinking of, in myopia,
108
Voigtlaender's lens, 63
Worth's amblyoscope, 195
theory of squint, 191
Yellow spot, 72
changes in, in myopia, loS
Zone of Zinn, ^3
Bailliire, Tindall &' Cox, 8 Henrietta Street, Covent Garden, LondoH
OPINIONS OF THE PRESS
"... a trustworthy guide for the student or the practitioner, and will enable
him thoroughly to understand (he principles on which errors of refraction and
defects of the ocular muscles should be treated . . . the symptoms and diagnosis
of myopia are particularly well written."— Lanccf.
"The preface of this excellent handbook contains the keynote of the whole
book : ' I have tried to make the following pages . . . essentially practical.'
Whether the average medical student can afford time to peruse this book or not, he,
at all events, may know that by possessing it he has a plain practical guide . . .
very practical monograph, which we can thoroughly recommend to students,
specialists, and general practitioners."— M^d/caZ Press and Circular.
" The preliminary chapters dealing with elementary and physiological optics are
particularly well written, the chapters on accommodation and convergence
deserving special praise ... its existence is justified by the freshness with which
the subject is treated, by its all-round excellence, and by its conciseness and
lucidity." — Ediltburgh Medical Journal.
" The book is singularly free from clerical blunders, is written in a clear and
attractive way, and is illustrated by many diagrams and several plates. We
consider that Mr. Clarke has produced a useful and trustworthy book, and one
that we can cordially recommend to students and to practitioners alike." —
Ophthalmoscope.
" It is throughout a thoroughly practical book, and can be recommended to
students requiring a reliable guide to this most important branch of ophthalmic
work. " — Practitioner.
" It is simple, concise, and lucid." — Dublin Medical Journal.
" It is clear in expression, easily readable, and not much given to mathematics. , . .
Asthenopia receives an adequate share of attention, such as it deserves but does not
always get"— Medical Chronicle.
" It is thoroughly sound and up to date. It is not too long and not too recondite.
The style exhibits those merits of simplicity and lucidity which are essential to
exposition and explanation, and the illustrations and diagrams are in keeping with
the style. The aspect of eye-work which we have endeavoured to indicate we
cordially recommend to the notice of our readers ; and we can say with confidence
that in the study of these important matters it would be impossible to find a guide
more reliable, or an exponent more lucid and instructive, than Mr. Ernest Clarke
has shown himself in these pages to be." — Journal of Balneology.
"General principles as well as practical rules are carefully explained, and the
recognized difficulties of the subject receive adequate treatment ... in all respects
his book commands confidence, and we heartily wish it an abundant success." —
Polyclinic Journal.
"The book is well written, clear, and definite, the paper and printing are good,
and it is a most useful addition to a student's library. It is well illustrated, and
very lucid in description." — Guy's Hospital Gazette.
"Those who are familiar with the work of Mr. Ernest Clarke know how care-
fully and thoroughly he does his work. This volume is the very best of the class
that goes into everything with care and thoroughness. We congratulate the
author in being able to present the profession with such a complete work on
refraction in such comparatively small bulk. The type, paper, and illustrations
are excellent. The coloured plates are specially good. The book merits the
highest commendation."— Canarfa Lancet.
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