THE LIBRARY
OF
THE UNIVERSITY
OF CALIFORNIA
LOS ANGELES
REFRACTION
HOW TO REFRACT
THORINGTON
BY THE SAME AUTHOR.
Rctinoscopy (The Shadow Test) in the Determination
of Refraction at One Meter Distance with the Plane
Mirror. 54 Illustrations, a number of which are in
Colors. Fifth Edition. Cloth, net, $1.00
From The Medical Record, New York.
" It presents a clear, terse, and thorough exposition of an
objective method of determining refraction errors which is de-
servedly increasing in popularity. In our opinion the author is
amply justified in declaring that its great value in nystagmus, young
children, amblyopia, aphakia, and in examining illiterates and the
feeble minded, cannot be overestimated, and we agree with him in
reminding those who attempt retinoscopy, fail, and ridicule it, that
the fault is behind and not in front of the mirror. The book is well
printed and usefully illustrated."
The Ophthalmoscope and How To Use It. With De-
scriptions and Treatment of the Principal Diseases of
the Fundus. With 12 Colored Plates and 73 other
Illustrations. Cloth, net, $2.50
HORIZONTAL SECTION OF THB RIGHT ~E\K.(Landois.)
Cornea, b. Conjunctiva, c. Sclerotic, d. Anterior chamber containing the
aqueous humor. <?. Iris. //', Pupil, g. Posterior chamber. /. Petit's canal.
j. Ciliary muscle, k. Corneoscleral limit, i. Canal of Schlemm. m. Choroid.
. Retina, o. Vitreous humor. No. Optic nerve, q. Nerve-sheaths, p. Nerve-
fibers. Ic. Lam ilia cribrosa. h. Crystalline lens. or. Ora serrata. pc. Ciliary
processes. The line, O A, indicates the optic axis ; S r, the axis of vision ; -, the
position of the fovea centralis. A'n. Nodal point, x. Equator of lens. /. Ex-
ternal rectus muscle, s. Internal rectus muscle. Z. Optic nerve-sheath. //.
Sclerotic.
Ml'SCLES OF THE EYE. TENDON OR LlGAMKXT OF ZlNN.
I. Tendon of Zinn. 2. External rectus divided. 3. Internal rectus. 4. Inferior
rectus. 5. Superior rectus. 6. Superior oblique. 7. Pulley for superior oblique.
I. Inferior oblique. 9. Levator palpebrae superioris. 10, 10. Its anterior expan-
sion, it. Optic nerve.
REFRACTION
AND
HOW TO REFRACT
INCLUDING SECTIONS ON OPTICS, RETINOSCOPY, THE
FITTING OF SPECTACLES AND EYE-GLASSES, ETC.
BY-
JAMES THORINGTON, A.M., M.D.,
PROFESSOR OH DISEASES OF THE EYK IN THE PHILADELPHIA POLYCLINIC AND COLLBGB
FOR GRADUATES IN MEDICINE; MEMBER OF THE AMERICAN OPHTHAL-
MOLOGICAL SOCIETY; FELLOW OP THE COLLBGB OF
PHYSICIANS OF PHILADELPHIA, ETC.
UbirO
TWO HUNDRED AND FIFTEEN ILLUSTRATIONS
THIRTEEN OF WHICH ARE COLORED
PHILADELPHIA
P. BLAKISTON'S SON & CO,
IOI2 WALNUT STREET
1907
COPYRIGHT, 1899, BY P. BLAKISTON'S SON & Co.
COPYRIGHT, 1900, BY P. BLAKISTON'S SON & Co.
COPYRIGHT, 1904, BY P. BLAKISTON'S SON & Co.
WM. F. FELL COMPANY
iUECTROTYPERS. PHINTCF
PREFACE TO THE THIRD EDITION.
The second edition of this work was published in 1900
and was exhausted two years later, at which time a few
changes were made in the text and the edition was again
republished under date of 1902. This virtually made a
third edition, but it was not so called. In preparing this,
the third, edition and to enhance its value the writer has
gone over the text very carefully and added fifteen new
illustrations together with a description of several new
instruments which have lately been brought forward as
material aids in estimating refractive errors.
120 S. EIGHTEENTH ST., PHILADELPHIA, PA.
August,
PREFACE
This book has been written at the request of the many
students who have attended the author's lectures on
"Refraction" at the Philadelphia Polyclinic ; and while it
is intended for all beginners in the study of Ophthalmology,
yet it is especially for those practitioners and students who
may have a limited knowledge of mathematics and who can
not readily appreciate the classic treatise of Bonders.
In the preparation of the manuscript and in arranging these
pages the writer has planned to be systematic and practi-
cal, so that the student, starting with the consideration of
rays of light, is gradually brought to a full understanding
of optics ; and following this, he is taught what is the stan-
dard eye, and then is given a description of ametropic eyes,
with a differential diagnosis of each, until finally he is told
how to place lenses in front of ametropic eyes to make
them equal to the standard condition.
By being dogmatic rather than ambiguous, with occa-
sional repetitions to avoid frequent references, and by simple
explanations and a definite statement of facts, the writer has
aimed to make the text more concise and comprehensive
than if encumbered with lengthy mathematic formulas or
with any discussion of disputed points.
The chapter on Retinoscopy embraces descriptions of that
method of refracting, both with the plane and with the
concave mirror ; but no matter how carefully expressed, the
Xll PREFACE.
student will frequently confuse the two, and he is therefore
referred to the author's manual on " Retinoscopy with the
Plane Mirror."
Of the two hundred illustrations used to elucidate this
work, nearly all are newly made, and were drawn or photo-
graphed by the author. Those in colors, on page 145, and
the diagrams of astigmatic eyes, as also several others, are
original.
The author desires to tender his thanks to Dr. Helen
Murphy, of Philadelphia, and to Dr. J. Ellis Jennings, of
St. Louis, Mo., for many valuable suggestions.
120 S. i8xH ST., PHILADELPHIA, PA.
November, 1899.
PREFACE TO SECOND EDITION.
The first edition of this work was published in November,
1899, and it is indeed gratifying to the author that the work
has found such favor as to call for a second in so short a
time.
In preparing this edition and to make it more lucid than
the first, the writer has carefully reviewed the original text
and made some changes in the phraseology.
The writer takes this opportunity to thank his many
friends and correspondents, at home and abroad, for their
complimentary letters and reviews, which are gratefully
appreciated.
November, igoo.
CONTENTS.
CHAPTER I.
PACK
OPTICS, .......................... 9
CHAPTER II.
THE EYE. THE STANDARD EYE. THE CARDINAL POINTS. VIS-
UAL ANGLE. MINIMUM VISUAL ANGLE. STANDARD ACUTE-
NESS OF VISION. SIZE OF RETINAL IMAGE. ACCOMMODATION.
MECHANISM OF ACCOMMODATION. FAR AND NEAR POINTS.
DETERMINATION OF DISTANT VISION AND NEAR POINT.
AMPLITUDE OF ACCOMMODATION. CONVERGENCE. ANGLE
GAMMA. ANGLE ALPHA, ............... 59
CHAPTER III.
OPHTHALMOSCOPE. DIRECT AND INDIRECT METHODS, ..... gg
CHAPTER IV.
EMMETROPIA. HYPEROPIA. MYOPIA, ............ I0 -,
CHAPTER V.
ASTIGMATISM, OR CURVATURE AMETROPIA. TESTS FOR ASTIGMA-
TISM, ......................... 122
CHAPTER VI.
RETINOSCOPY, ....................... j.g
CHAPTER VII.
MUSCLES,
CHAPTER VIII.
CYCLOPLEGICS. CYCLOPLEGIA. ASTHENOPIA. EXAMINATION OF
THE EYES ....................... 20 g
XIV CONTENTS.
CHAPTER IX.
PAGE
How TO REFRACT, 228
CHAPTER X.
APPLIED REFRACTION, 244
CHAPTER XI.
PRESBYOPIA. APHAKIA. ANISOMETROPIA. SPECTACLES, .... 271
CHAPTER XII.
LENSES, SPECTACLES, AND EYE-GLASS FRAMES. How TO TAKE
MEASUREMENTS FOR THEM AND How THEY SHOULD BE
FITTED, 297
INDEX, 3 o 7
LIST OF ILLUSTRATIONS.
FIG. PAGE
1. Illustrating Intensity of Light, 10
2. Convergent Pencil, 1 1
3. Convergent Pencil, II
4. Divergent Pencil, 12
5. Reflection, 13
6. Reflection from Plane Mirror, 13
7. Lateral Inversion, 14
8. Reflection from Concave Mirror, 15
9. Erect Image Formed by Concave Mirror, 16
10. Inverted Image Formed by Concave Mirror, 17
11. Image Formed by Convex Mirror, 18
12. Perpendicular to Plane Surfaces, 19
13. Refraction, 19
14. Critical Angle, 20
15 and 16. Angle of Refraction, 21
17. Density, . 21
18. Index of Refraction, 22
19. Maximum Deviation, 23
20. Minimum Deviation, 23
21. Angle of Deviation, 24
22. Displacement, 25
23. Centrad, 25
24. Prism Diopter, 25
25. Neutralization of Prisms, 26
26. Correction of Diplopia, 29
27. 28, and 29. Convex Lenses, 30
30, 31, and 32. Concave Lenses, 31
33. Prism Formation of a Convex Lens, 31
34. Prism Formation of a Concave Lens, 31
35. Parallel Rays Passing Through a Convex Lens, 32
36. Parallel Rays Passing Through a Concave Lens, 33
37. Conjugate Foci, 34
38. Ordinary Foci, 35
39. Negative Focus, 36
40. Nodal Points, 36
41. Optic Center, 37
42. Inverted Image Formed by a Convex Lens 38
43. Erect Magnified Image Formed by a Convex Lens, 40
44. Image Formed by a Concave Lens, 40
45 and 46. Cylindric Lenses, 44
47. Cylinder Axis, 44
48. Parallel Rays Passing Through a Convex Cylinder, 44
XVI LIST OF ILLUSTRATIONS.
FIG. PACK
49. Parallel Rays Passing Through a Concave Cylinder, 45
50. Trial Case, 46
51 and 52. Trial-frames, 47 and 48
53. Combining Sphere and Cylinder, 50
54, 55, and 56. Finding Optic Center of a Lens, 55
57 and 58. Finding Cylinder Axis, 56
58, 59, and 60. Action of a Cylinder, 56 and 57
61. Standard Eye, bo
62. Angle of View, 6l
63 and 64. Size of Retinal Image, 62
65. Minimum Visual Angle, 63
66 and 67. Five-minute Angle, 64
68. Retinal Image in the Standard and Ametropic Eyes, 65
69. Crystalline Lens at Rest and Accommodating, 68
70. Accommodation 69
71. Hyperopic Eye at Rest, 71
72. Myopic Eye at Rest 72
73. Randall's Test-letters 74
74. Wallace Test-letters 75
75. Illiterate Card, 75
76. Kindergarten Card, 76
77. Gould's Test-letters, 77
78. Gothic Type for Testing the Near Point, 8l
79. Block Letters for Testing the Near Point, 82
80. Meter Angle of Convergence, 84
81. Angle Gamma, 85
82. Positive Angle Gamma, 86
83. Negative Angle Gamma, 87
84. Loring Ophthalmoscope, 89
85. Direct Ophtha'.moscopy, 90
86. Emmetropia with the Ophthalmoscope, 95
87. Hyperopia with the Ophthalmoscope, 96
88. Myopia with the Ophthalmoscope, 97
89. Indirect Ophthalmoscopy, 99
90. Condensing Lens, 99
91 and 92. Luminous Ophthalmoscope, 102
93. Emmetropia, . 103
94. Emmetropic and Ametropic Eyes 23 mm. Long, 104
95. Hyperopic Eye at Rest, 107
96. Hyperopic Eye Refracted, 107
97. Parallel Rays Entering a Myopic Eye, 113
98. Myopic Eye at Rest, 114
99. Myopic Eye Refracted, 114
100. Astigmatic Lens, 123
101. Simple Hyperopic Astigmatism, 126
102. Simple Myopic Astigmatism 127
103. Compound Hyperopic Astigmatism, 127
104. Compound Myopic Astigmatism, 128
105 and 106. Mixed Astigmatism, 129
107. Symmetric Astigmatism 130
1 08. Asymmetric Astigmatism, 130
109. Astigmatism with the Rule, 131
LIST OF ILLUSTRATIONS. XV11
FIG. PACK
no. Astigmatism against the Rule, 131
111. Placido's Disc, 135
112. Stenopeic Slit, 135
113. Green's Astigmatic Charts, 137
114. Astigmatic Clock-dial, 138
115. Astigmatic Clock-dial in Black, 139
116. Author's Pointed Line Test, 141
117. Perforated Disc 142
118. Pray's Letters 142
119. Scheiner's Disc, 143
1 20. Scheiner's Disc in Hyperopia, 143
121. Scheiner's Disc in Myopia, 144
122 and 123. Cobalt-blue Glass, 145
124. Refrangibility of Cobalt-blue Glass, 146
125 to 136, inclusive. The Diagnosis of the Different Forms of Ame-
tropia with Cobalt-blue Glass, .' 147
137. Thomson's Ametrometer, 149
138 and 139. Ophthalmometer, 150 and 151
140 and 141. Mires or Targets, 152
142. Indirect Ophthalmoscopy, 155
143. Author's Schematic Eye, 156
144. Point of Reversal, 157
145 and 146. Author's Mirror with Folding Handle, 158
147. Author's Iris Diaphragm Chimney, 159
148. Position of Light and Mirror, 160
149. High Myopia as Seen with the Concave Mirror, l6l
150. Hyperopia as Seen with the Concave Mirror, 161
151 and 152. Rate of Movement of Retinal Illumination in Hyperopia
and Myopia, 164 and 165
153. Retinal Illumination in Emmetropia, 166
154. Band of Light, , 169
155- Axonometer, 170
156. Scissor Movement, 172
157. Positive Aberration, 173
158. Negative Aberration 173
159 and 1 60. Reisner Retinoscope, 174
161 and 162. Luminous Retinoscope, 175
163. Homonymous Diplopia, 178
164. Heteronymous Diplopia, 179
165. Scale for Testing Lateral Insufficiency, 187
166 and 167. Maddox Rods, 188
168. Rotary Prism of Risley, 189
169. Phorometer 190
170. Strabismometer, .... 200
171. Angle of Deviation in Strabismus, 201
172. Monocular Blinder, 203
173. Worth's Amblyoscope, 204
174. Aphakia, 278
175 and 176. Franklin Bifocals, 284
177. Mork's Bifocals, 284
178 to 184, inclusive. Cement Bifocals, 285 and 286
185 and 1 86. Acromatic Bifocals, 286
XV111 LIST OF ILLUSTRATIONS.
FIG. PACK
187 and 188. Solid Bifocals, 287
189 to 193, inclusive. Half Lenses 288
194. Toric Lenses, 290
195 and 196. Trifocals, 295
197 to 206, inclusive. Different Sizes and Shaped Lenses, . 298 and 299
207. Measuring Interpupillary Distance, 301
208 and 209 Fitting of Spectacle Bridge, 302
210. Measurement of Bridge, 303
211. Measurement for Spectacles, 304
212. Measurement for Eye-glasses, 304
213. Distance Frames, . . 305
214. Near Frames, . 305
215. Measurements for Guards, 305
REFRACTION
HOW TO REFRACT.
CHAPTER I.
OPTICS.
Optics (from the Greek OXTO/JLUI, meaning "to see") is
that branch of physical science which treats of the nature
and properties of light. "
Catoptrics (from the Greek xdToxrpoy, meaning " a mir-
f ror") and dioptrics (from the Greek diimrpov, meaning
" to see through ") are subdivisions of optics ; the former
treating of (incident and reflected rays) and the latter of the
^refraction of lightjpassing through different media, such as
air, water, glass, etc., but especially through lenses.
Light. Light may be defined as that form of energy
which, acting upon the organs of sight, renders visible the
objects from which it proceeds. This form of energy is
/ ^ **
propagated in waves in all directions from a luminous body,
.and with a velocity in a vacuum of about 186,000 miles a
second. In the study of a luminous body, such as a candle-,
lamp-, or gas-flame, the substance itself must not be con-
sidered as a single source of radiation, but as a collection
^
of minute points, from every one of which waves proceed
^HMMMfMMpMW*
in all directions and cross one another as they diverge from
their respective points. The intensity of light deer
10 REFRACTION AND HOW TO REFRACT.
as the square of the distance from the light increases : for
example, if an object is twice as far from a luminous body
as another of the same size, it will receive one-fourth as
much light as the latter. Figure i shows t\vo cards, one
is twice as far from the light as the other and receives only
one-fourth as much light as the card nearest to the light.
Ray. Ray (from " radius ") is used in optics in prefer-
ence to wave, and /means the smallest subdivision of li^ht
traveling in a straight line) Rays of light are considered
as incident, emergent, reflected, refracted, divergent, par-
allel, and convergent.
FIG. I. Illustrating Intensity of Light.
Incident Rays. Rays of light are said to be incident
when they strike the surface of an object. (See Fig. 8.)
/ Emergent Rays. Rays of light are emergent when they
.' have passed through a transparent substance. (See Fig. 1 3 .)
Reflected Rays. Rays of light are reflected when they
rebound from a polished surface. (See Fig. 8.)
Refracted Rays. A ray of light undergoes refraction
when it is deviated from its course in passing through any
transparent substance.
Divergent Rays. Rays of light proceed divergently
from any luminous substance, but, in the study of refrac-
tion, only those which proceed from a point closer than six
/meters are spoken of as divergent. (Fig. I.)
Parallel Rays. The greater the distance of any lumin-
ous point, the more nearly do its rays approach to paral-
OPTICS.
II
lelism ; this is evident in a study of rays coming from such
distant sources as the sun, moon, and stars. For all prac-
tical purposes in the
study of refraction,
rays of light which
proceed from a dis-
tance of six meters or
more are spoken of
as parallel, although
this is not an absolute
fact, as rays of light
at this distance still
maintain a slight
amount of divergence.
If the pupil of the
emmetropic eye is represented by a circular opening four
millimeters in diameter, then rays of light from a luminous
point at six meters (6000 mm.) will have a divergence of
when they enter such a pupil.
Convergent Rays. Convergent rays are the result of re-
FIG. 2. Parallel Rays Reflected by a Concave
Mirror Forming a Convergent Pencil.
FIG. 3. Parallel Rays Refracted by a Convex Lens Forming a Convergent
Pencil.
flection from a concave mirror or refraction through a con-
vex lens. (See Figs. 2 and 3.) y-
' A Beam. This is a collection or series of parallel rays.
(See Fig. 3.)
A Pencil. A pencil of light is/a collection of convergent
12 REFRACTION AND HOW TO REFRACT.
or divergent raysj ' Convergent rays are those which tend
to a common point (see Fig. 3), whereas divergent rays are
those which proceed from a point and continually separate
as they proceed.) (See Fig. 4.) This point is called the
radiant point.
A Focus. This is the point of a convergent or divergent
pencil ; the center of a circle ; the point to which converg-
ing rays are directed.
A Positive or Real Focus. This is the point to -which
rays are directed after passing through a convex lens or
after reflection from a concave mirror. (See Figs. 3 and 2.)
A Negative or Virtual Focus. This is the point from
which rays appear to
diverge after passing
through a concave lens J
(see Fig. 36), or Rafter
reflection from a convex
mirror, or after refraction
through a convex lens
when the light or object
FIG. 4. Illustrating a Divergent Pencil. j s c l oser to the lens than
its principal focus (see
Fig. 43), or after j reflection from a concave mirror when
the light or object is closer to the mirror than its principal
focus,/ (See Fig. 9.)
The principal phenomena of light are absorption, reflec-
tion, and refraction.
Absorption. Rays of light from the sun falling upon
the green grass are partly absorbed and partly reflected.
The grass absorbs some of the rays and sends back or
reflects only those rays which together produce the effect
of green. A piece of red glass owes its color to the fact
that it transmits only that portion of the light's rays whose
combined effect upon the retina is that of red. (The relative
OKJU^/~>
Of ('.'; i/l V '' I
OPTICS. 1 3
proportion of absorption and reflection of rays of light
depends greatly upon the quality of the suiface whether
light colored or polished, or dark colored or rough. ^)
Reflection. From the Latin rcflectcrc, "to rebound."
This is the sending back of rays of light by the surface on
which they fall into the medium through which they came.
\Yhile most of the rays falling upon the surface of a trans-
parent substance pass through it, with or without change
'in their direction, yet some of the rays are reflected, and it
is by these reflected rays that surfaces are made visible.
D
FK;. 5.
FIG. 6.
!A substance that could transmit or absorb all the rays of
light coming to it (if such a substance existed) would be
invisible. ( Reflection, therefore, always accompanies refrac-
tion, and, if one of these disappear, the other will disappear
also. )
Laws of Reflection. (i) The angle of reflection is
equal to the angle of incidence. (2) The reflected and in-
cident rays are in the same plane with the perpendicular to
the surface. (See Fig. 5.)
If A B represent a polished surface and I the incident
ray, then P D I is the angle of incidence ; R being the re-
REFRACTION AND HOW TO REFRACT.
R
E F
LEG
TION
1 ~
fl
T 3
03J
HOIT
fleeted ray, then P D R, equal to it, is the angle of reflection.
I D, P D, and R D lie in the same plane.
A reflecting surface is usually a polished surface (a
mirror), and may be plane, concave, or convex.
Reflection from a Plane Mirror. {Rays of light are
reflected from a plane mirror in the same direction in which
they fall upon it : if parallel, convergent, or divergent be-
fore reflection, then they are parallel, convergent, or diver-
gent after reflection. An object placed in front of a plane
mirror appears just as far back in the mirror as the object
is in front of it. (See
Fig. 6.]}
A B represents a plane
mirror with E F, rays
from the extremes of the
object I, reflected from
the mirror A B, and meet-
ing at the observer's eye
as if they came from the
object I in the mirror.
(See Visual Angle, p.
62.) The apparent dis-
-7 tance of the object I from the observer is equal Jo the
combined length of the incident and reflected rays.
The appearance of an image in a plane mirror is not
exactly the same as that of the object facing the mirror ;
it undergoes what is known as lateraljm^sion. This is
best understood by holding a printed page in front of a
plane mirror, when the words or letters will read from right
to left (See Fig. 7.) An observer facing a plane mirror
and raising his right hand, his image apparently raises the
left hand.
Tilting a plane mirror gives an object the appearance of
FIG. 7. Lateral Inversion.
moving in the opposite direction to that in which the mirror
is tilted.
Spheric Mirrors. A spheric mirror is a portion of a
reflecting spheric surface ; its center of curvature is therefore
the center of the sphere of which it is a part. (' Spheric mir-
rors are of two kinds concave and convex.)
Reflection from a Concave Mirror (Fig. 8). Parallel
rays are reflected from a concave mirror, and are brought to
a focus in front of it. This point is called the principal focus
(P.P.). The^principal axis of a concave mirror is a straight
line drawn from the mirror through the principal focus and
v;
/'
~ <
~^~-
^.P'F':-^..J ' ^- -- .
FIG. 8.
the center of curvatureYi i), and(a secondary axis (2', 2',
2', 2') is any other straight line passing from the mirror to
the center of curvature (C.C.).J Rays which diverge from
'any point beyond the principal focus are reflected con-
vergently (G J). Rays which diverge from any point closer
than the principal focus are reflected divergently (V V).
Images Formed by a Concave Mirror.-j^To find the
position of an image as formed by a concave mirror, two
rays may be used/ one drawn from a given point on the
object to the mirror, and parallel to its principal axis, and
reflected through the principal focus (P.P., Figs. 9 and 10);
the other, the secondary axis, from the same point, passing
/S ^ Ft^
/y c* +*
1 6 REFRACTION AND HOW TO REFRACT.
through the center of curvature. The place where the
secondary axis and the reflected ray or their projections in-
tersect gives the position of the image. Unlike the plane
mirror, which produces images at all times and at all dis-
jtances, the concave mirror produces either an erect, virtual,
'and enlarged image, if an object is placed closer than its
principal focus, or an enlarged inverted image if the object
is between the principal focus and the center of curvature.
By withdrawing the mirror in the former instance the
erect image increases slightly in size, and in the latter the
inverted image diminishes in size. /'At the principal focus
there is no image formed.)
FIG. 9.
Figure 9 shows an erect, virtual, and enlarged image of
A R which is closer to the mirror than the principal focus.
Parallel rays from A and R are reflected to the principal
focus, P.P. Lines drawn from the center of curvature
through A and R to the mirror are secondary axes ; these
lines and those reflected to the principal focus do not inter-
sect in front of the mirror, but if projected, will meet at a
and r behind the mirror, forming a magnified image of
A R. If the mirror is withdrawn from the object, the
erect magnified image will increase in size, but at the prin-
cipal focus no image will be formed, as the rays are reflected
parallel.
Figure 10 shows a real inverted image of A R at a r ;
A R situated beyond the principal focus. Lines drawn
from A and R through C.C. are secondary axes. Parallel
rays from A and R converge and cross at the principal
focus (P.F.).
Where D P and F E intersect the secondary axes, the in-
verted image a r of A R is situated. ^Vhen the object, as
in this instance, is situated beyond the center of curvature,
the image is smaller than the object. , As the image and
object are conjugate to each other, they are interchange-
able, and in such a case the image would be larger than
the object and inverted. /This is always true when the
FIG. 10.
object is situated between the center of curvature and the
principal focus. j(vVhen an object is situated at the center
of curvature, its image is equally distant and of the same
size, but inverted.
Tilting a concave mirror gives an object placed inside of /
r i
its principal focus the appearance of moving as the mirror
is tilted ; but if the object is situated beyond the principal
focus, the object appears to move in the opposite direction.
Reflection from a Convex Mirror. All rays are re-
flected divergently from a convex mirror, and parallel rays
diverge as if they came from the principal focus situated
behind the mirror at a distance equal to one-half its radius
18
REFRACTION AND HOW TO REFRACT.
of curvature. The principal focus of a convex mirror is
therefore negative. The foci of convex mirrors are virtual.
are
Images Formed by a Convex Mirror. These
^always virtual, erect, and smaller than the object^) The
closer the object, the larger the image ; and the more distant
the object, the smaller the image. (Tilting a convex mirror,
,the image does not appear to change position..
In figure 1 1 parallel rays from the object A R are reflected
from the mirror as if they came from the principal focus situ-
ated at one-half the distance of the center of curvature, C.C.
Lines drawn from the extremes of the object to C.C. are
C.C.
P.F.
FIG. ii.
secondary axes, and f' the image is situated at the point of
intersection of the secondary axes and the rays from the
principal focus^ and as these meet behind the mirror, the
jimage is virtual and erect.
Refraction. From the Latin refrangere, meaning " to
bend back " /. e., to deviate from a straight course. /Refrac-
tion may be defined as the deviation which takes place in
the direction of rays of light as they pass from one medium
into another of different density)*
(* As ordinarily understood in ophthalmology, refraction has come to mean
the optic condition of an eye in a state of repose or under the physiologic effect
of a cycloplegic.
OPTICS. 19
Two laws govern the refraction of rays of light :
1. A ray of light passing from a rare into a denser
medium is deviated or refracted toward the perpendicular.
2. A ray of light passing from a dense into a rarer
medium is deviated or refracted away from the perpen-
dicular.
Aside from these laws, there are other facts in regard
to rays of light that should have consideration. A ray
of light will continue its straight course through any
/ number of different transparent media, no matter what their
I densities, so long as it forms right angles with the surface
ICE
\
/ FLINT GLASS
> CROWN "
3 PLATE "
$
>
FIG. 12
or surfaces. (Such a ray is spoken of as the normal or per-
pendicular} {such surfaces are plane, the surfaces and per-
pendicular forming right angles.) (See Fig. 12.) In any
lease of refraction the incident and refracted rays may be
[supposed to change places.
Figure 1 3 shows the perpendicular (P P) to a piece of
plate glass with plane surfaces. The ray in air incident at
O on the surface S F is bent in the glass toward the per-
pendicular, P P. The dotted line shows the direction the
ray would have taken had it not been refracted. As the
ray in the glass comes to the second surface at R, and
2O REFRACTION AND HOW TO REFRACT.
passes into a rarer medium, it is deviated from the perpen-
dicular, P P. The ray now continues its original direction,
but has been deviated from its course ;(it has undergone
lateral displacement)
Critical Angle or Limiting Angle of Refraction. This
is /the angle of incidence which just permits a ray of light
in a dense medium to pass out into a rare medium) The
^ J
(size of the critical angle depends upon the index of refrac-
tion of different substances. \ Figure 14 shows an electric
light suspended in water. The ray from this light which
forms an angle of 48 35' with the surface of the water
FIG. 14. Critical Angle.
will be refracted and pass out of the water, grazing its sur-
face ; but those rays which form an angle greater than
48 35' will not pass out of the water, but will be reflected
back into it. /The surface separating the two media be-
comes a reflecting surface and acts as a plane mirror.^
The critical angle for crown glass is 40 49'.
Index of Refraction. By this is (meant the relative
density of a substance or the comparative length of time
required for light to travel a definite distance in different
substances. The ('absolute index <>f n-fraction is the
density or refractive power of any substance as compared
OPTICS.
21
with a vacuum.) According to the first law of refraction, a
ray of light passing from a rare into a dense medium is
refracted toward the perpendicular ; (in other words, the
angle of refraction is smaller, under these circumstances,
than the angle of incidence. J) In the study of the compara-
tive density of any substance it
fwill be seen that the angle of
refraction is usually smaller the
more dense the substance^ this
is well illustrated in figures 15
and 1 6.
I The greater the density, the
/slower the velocity or the more
effort apparently for the wave or ray to pass through
the substance. This is illustrated in figure 17, where a
ray or wave of light is seen passing at right angles through
different media. A ray passes through a vacuum without
apparent resistance, but in its course through air it is
slightly impeded, so that/air has an index of refraction of
FIG. 15.
FIG. 16.
/ / / / /
/
Ice
'X_/'X_/-\_x
Glass
^\^^^
Diamond
Vacuum
Air
FIG. 17.
1.00029+ when compared with a vacuum); but as this is so
slight, air and a vacuum are considered as one for all pur-
poses in refraction. To find the index of refraction of any
substance as compared witrravacuun^orjur^tis neces- I
sary to divide the sine of the angle of incidence by the sine I
of the angle of refraction.
22
REFRACTION AND HOW TO REFRACT.
In figure 18 the angle of incidence P C I is the angle
formed by the incident ray I with the perpendicular, P P.
The angle of refraction P C R is the angle formed by the
refracted ray with the perpendicular, P P. Drawing the
circle P H P O around the point of incidence C, and therx p*
drawing the sines^TD
X. and B F, perpen-
diculars to the per-
pendicular P P.raivide
the sine D X of the
angle of incidence by
the sine F B of the
angle of refraction to
obtain the index of
refraction) in this in-
stance, water as com-
pared with air. D X
equaling 4 and F B
equaling 3, then 4 di-
vided by 3 will equal , or
1.33 -J-, the index of refraction of water as compared with air.
(To find the index of refraction of a rare as compared
with a dense substance, divide the sine of the angle of
refraction by the sine of the angle of incidence /. c., air
as compared with water would be ^, or 0.75. \
INDEXES OF REFRACTION.
Water . . . . .
.177
Cornea . . . .
.7777
, Crown plass.
.C
Flint elass
.58
Crystalline lens, nucleus,
.4.7
" " intermediate layer,
.41
" " cortical layer. ,
3Q
OPTICS. 23
A prism is a wedge-shaped portion of a refracting
medium contained between two plane surfaces. The sides
of a prism are the inclined surfaces. The apex is where
the two plane surfaces meet. The base of the prism is the
thickest part of the prism. The refracting angle is the
angle at which the sides come together.
Position of a Prism. When a prism is placed in front
of an eye, its position is indicated or described by the direc-
tion in which its base is situated : base down means that the
thick part of the prism is toward the cheek ; base up means
that the thick part of the prism is toward the brow ; base
in means that the thick part of the prism is toward the
IMG. 19. FIG. 20.
nose ; and base out means that the thick part of the prism
is toward the temple.
Prismatic Action. Rays of light passing through a
f \
prism are^a.hvays'i-efracted toward the basejbf the prism.
If an incident ray is perpendicular to the surface of a prism,
there will be only one refraction) and that takes place at
the point of emergence. ^The angle of incidence in this
instance will equal the angle of the prism, and the maximum
deviation takes place, as all the refraction is done at one
surface./
In figure 19 the incident ray (I) is perpendicular to the
surface A B, and is not refracted until it comes to the sur-
24 REFRACTION AND HOW TO REFRACT.
face A C at E, when it is bent toward the base B C, all the
refraction taking place at the surface A C.
\ If an incident ray forms an angle other than a right
1 ingle with the first surface of the prism, then it will be re-
iiracted twice as it enters and as it leaves the prism.
^In figure 20 X N is the perpendicular to the surface A B
The ray (I) incident at N is refracted toward this perpen-
dicular and follows the course N E inside of the prism.
On emergence it is refracted from the perpendicular E P of
J;he surface A C, and in the direction of the base of the prism. )
/ If the incident ray (I) so falls upon the surface A B that
the refracted ray (N E) is parallel to the base (B C), and
the emergent ray is such that the angle of emergence
equals the angle of incidence (I N X), as in this instance, then
the angles of incidence and of emer-
gence are equal, and the deviation is
at a minimum, or the least possible. J
Angle of Deviation (Fig. 21).
This is the angle formed between the
directions of the incident and emergent
rays, and measures the total devia-
FlG 2I tion. In all prisms of ten degrees
or less the angle of deviation is equal
,7\
to half the angle of the prism, but in prisms of more than
ten degrees the angle of deviation increases.
Summary. Prisms do not cause rays of light to con- ^
verge or to diverge ; rays that are parallel before refraction
are parallel after refraction. (Therefore, prisms do not form
images ; prisms have no foci.y
Effect of a Prism. An object viewed through a prism
has the appearance of being displaced, and in a direction
opposite to the base /. e., toward the apex.
Rays from the object (X, Fig. 22) strike the prism at C,
OPTICS.
undergo double refraction, and, falling upon the retina of
the eye, are projected back in the direction in which they
were received, and the apparent po-
sition of X is changed to X', away
from the base of the prism and
toward the apex.
Numbering of Prisms. Form-
ytrly, prisms were numbered by their
refracting angles ; now, however,
two other methods are in use :
jDennet's method, known as the centrad
'/method, known as the prism- diopter.
Dennett's Method (Fig. 23). The unit, or centrad (ab-
breviated V ), is a prism that will deviate a -ray of light the
Y^-jj- part of the arc of the radian. This is calculated as
follows : As much of the circumference of a circle is taken
as will equal the length of its radius of curvature ; this is
^called the arc of the radian, and equals 57.295 degrees.;
The arc of the radian is then divided into 100 parts. /A
FIG. 22.
and Prentice's
Radius
FIG. 23.
I Meter
FIG. 24.
prism, base clown, at the center of curvature that will devi- /[>
ate a ray of light downward just T ^-$ part of the arc of the
radian is a one centrad, and equals T ^g- of 57.295 degrees,
or 0.57295 of a degree. )
26
REFRACTION AND HOW TO REFRACT.
9876543210
Ten centrads will deviate a ray of light ten times as
much as one centrad, or 10 X 0.57295 = 5.7295 degrees,
etc.
Prentice's Method (Fig. 24). The unit, or prism-diopter
(abbreviated P.D.,or/\),is a prism
that will deviate a ray of light
> just i cm. for each meter of dis-
tance that is, the -^ part of
the radius measured on the tan-
gent. The deviation always be-
ing i cm. for each meter of dis-
tance, i P. D. will deviate a ray of
light 2 cm. for 2 meters of dis-
tance ; 3 cm. for 3 meters, etc.
The comparative values of cen-
trads and prism-diopters is quite
uniform up to 20, but above 20
the centrad is the stronger.
Neutralization of Prisms.
Knowing that rays of light are de-
viated by centrads and prism-
diopters up to 20, in the ratio of
i cm. for each meter of distance,
then tc^ find the numeric strength
of any prism all that is necessary
is to hold the prism over a series
of numbered parallel lines, sepa-
rated by an interval of i cm. or
fraction thereof, and note the
amount of displacement) For example, figure 25 shows a
series of vertical lines */j of a cm. apart, and numbered from
o to 9; an X is placed at the foot of the o line. Holding a
prism, base to the right, at a distance of ^ of a meter (as
98765
I 3
2
:
:
D
X
FIG. 25.
OPTICS.
the lines are ^ of a cm. apart) and looking through the prism
at the X on the o line, it will be seen that the X has been
displaced to the line to the left corresponding to the number
of centrads or prism-diopters in the prism ; in this instance
three.
TABLE SHOWING THE EQUIVALENCE OF CENTRADS IN PRISM-DIOPTERS
AND IN DEGREES OF THE REFRACTING ANGLE (INDEX OF
REFRACTION 1.54).
'TO
CENTRADS.
PRISM-DIOPTERS.
REFRACTING ANGLE.
I.
I.
I.oo
2.
2.0001
2. 12
3-
3.0013
3 .i8
4-
4.OO28
4-23
5-
5-0045
5. 28
6.
6.0063
6. 3 2
7-
7.0II5
7-35
8.
8.0172
8. 3 8
9-
9.0244
9-39
10.
10.033
10. 39
ii.
II.O44
ii. 3 7
12.
12.057
i2-34
13-
I3-074
I3.29
14.
14.092
14. 23
15-
15.114
I5.i6
16.
16.138
i6.o8
17-
17.164
i6.98
1 8.
18.196
i 7 .8 5
19.
19.230
i8.68
20.
20.270
i9 -45
25-
25-55
23-43
30-
30.934
26.8i
35-
36.50
29. 72
40.
42.28
32.i8
45-
48.30
34. 20
50.
54-514
35-94
60.
68.43
38-3i
70.
84.22
39-73
80.
102.96
40. 29
90.
I26.OI
40-49
100.
155-75
39. H
>rism may be ncutralix.cd by placing another prism
in apposition to it, with their bases oppositejso that in look-
/V""^ ->
L-^- 4yt**4&**.
V*y /4<t-w-~^~
28 __ REFRACTION AND HOW TO REFRACT.
ing through the two prisms at a straight line, no matter at
what distance, the straight line will continue to make one
straight line through the prisms ; the strength of the neu-
tralizing prism will equal the strength of the prism being
neutralized.
Uses of Prisms. I. To detect malingerers who profess
.-"/ *"'
monocular blindness so as to obtain damages for supposed
injuries, or who wish to escape war service, or those cases
of hysteric blindness wishing to create sympathy. This
"~test or use of a prism is(_known as the diplopia test^) and is
practised as follows : A seven P. D., base up or down, with
a blank are placed in the trial-frame corresponding to the
" blind " eye ; nothing is placed in front of the seeing- eye.;
the trial -frame, thus armed (without fhe patient seeing what
is being done), is placed on the patient's face and he is in-
structed to read the card of test-letters on the wall across
the room. While he is thus busy reading, and purposely
contradicted by the surgeon, so as to get his mind from his
condition, the surgeon suddenly removes the blank from
the " blind " eye. The patient exclaiming that he sees two
cards and two of all the letters proves the deception.
2. ^Occasionally, to counteract the effects of strabismus,
or diplopia due to a paralysis of one or more of the extra-
ocular musclesj For example : A patient looking at a
point of light focused on the macula (M) of the left eye
(L), the right eye being turned in toward the nose, receives
the rays upon the retina to the nasal side of the macula,
and hence projects the rays outward to the right, giving a
false image to the right side ; a prism of sufficient strength
is then placed with its base toward the temple (base out)
over the right eye, so that the rays from the light may fall
upon the macula (M), and the diplopia will be corrected
(See Fig. 26.)
OPTICS. 29
3. To test the strength of the extra-ocular muscles : A
patient looking with both eyes at a distant point of light
is made to see one light just above another by placing a
3 P. D., base down or up, before either eye, and if a 2^
P. D. did not produce diplopia when similarly placed, the
strength of his vertical recti is then represented by 2^
P. D. The strength of the prism placed base in which,
FIG. 26.
if increased, would produce diplopia is the strength of the
externi ; and the strength of the prism or prisms placed
base outward which, if increased, would produce diplopia
is the strength of the interni.
4. For exercise of weak muscles. (See p. 191.)
Lenses. A lens is a portion of transparent substance
(usually of glass) having one or both surfaces curved.
There arc t\v<> kinds of lenses spheric and cylindric.
3O REFRACTION AND HOW TO REFRACT.
Spheric Lenses. Abbreviated S. or sph. Spheric
lenses are so named because their curved surfaces are sec-
tions of spheres. A spheric lens is one which refracts
-rays of light equally in all meridians or planes. Spheric
lenses are of two kinds convex and concave.
A convex spheric lens is thick at the center and thin
at the edge. (Figs. 27, 28, 29.) The following are synony-
mt>us terms for a convex lens : (i) Plus ; (2) positive ; (3)
collective ; (4) magnifying. A convex lens is denoted by
the sign of plus ( + ).
Varieties or Kinds of Convex Lenses.
I. Planoconvex, meaning one surface flat and the other
convex. It is a section of a
\ A V sphere. (See Fig. 27.)
\ / \ \\ 2. Biconvex, also called con-
vexoconvex or bispheric, for the
reason that it is equal to two
planoconvex lenses with their
/ \l I/ plane surfaces together. (Fig.
FIG. 27. FIG. 28. FIG. 29. 28.)
3. Concavoconvex. This lens
has one surface concave and the other convex, the convex
surface having the shortest radius of curvature. (Fig. 29.)
^ The following are synonymous terms for a concavoconvex
lens : (i) Periscopic ; (2) convex meniscus ; (3) converging
meniscus (meniscus meaning a small moon). (See Fig. 29.)
. / A periscopic lens enlarges the field of vision, and is of
/ especial service in presbyopia.
A Concave Spheric Lens. Such a lens is thick at
the edge and thin at the center. (Figs. 30, 31, 32.) The
following are synonymous terms for a concave lens: (i)
Minus ; (2) negative ; (3) dispersive ; (4) minifying. A
concave lens is denoted by the sign of minus ( ).
. ^^ '
OI'TICS.
Varieties or Kinds of Concave Lenses.
1. J^anoconcai'C, meaning one surface flat and the other
concave. (Fig. 30.)
2. Biconcave, also called concavoconcave or biconcave
spheric, for the reason that it is
equal to t\vo planoconcave lenses
with their plane surfaces to-
gether. (Fig. 31.)
3. Conyexoconcavf. This lens
has one surface convex and the
other concave, the concave sur-
face having the shortest radius
of curvature. (Fig. 32.) The
following are synonymous terms for a concavoconvex lens :
(i) Concave meniscus ; (2) diverging meniscus ; (3) peri-
scopic.
17 V7
A
FIG. 30. FIG. 31. FIG. 32.
FIG. 33.
FIG. 34.
(A. spheric lens may be considered as made up of a series
of prisms which gradually increase in strength from the
center to the periphery, no matter whether the lens be con-
cave or convex. /
32 REFRACTION AND HOW TO REFRACT.
In the convex sphere the bases of the prisms are toward
the center of the lens, whereas in the concave the bases of
the prisms are toward the edge. (See Figs. 33, 34.)
Knowing that a prism refracts rays of light toward its
base, it may be stated as a rule that every lens bends rays
of light more sharply as the periphery is approached i. c.,
at the periphery the strongest prismatic effect takes place.
Lens Action. --As a ray of light will travel in a straight
line so long as it continues to form right angles with sur-
faces) then the ray A in figure 35 passes through the bicon-
vex lens unrefracted, or without any deviation from its
course whatsoever, for at its points of entrance and emer-
FIG. 35.
gence the surfaces of the lens are plane to each other. This
ray is called the axial ray, and /the line joining the centers
of curvature of the two surfaces i
The axis of a planoconvex or planoconcave lens is the liiu
drawn through the center of curvature perpendicular to the
plane surface.
The ray B in figure 35, though parallel to the ray A,
forms a small angle of incidence, and must, therefore, be
refracted toward the perpendicular to the surfaces of the
lens, and, passing through the lens, will meet the axial ray
at P.P. The rays C, D, and K, also parallel to A and B,
form progressively larger angles with the surface of the lens,
OPTICS.
33
and finally meet the axial ray at P.P. < It will be seen at
once that the rays all meet at P.P., showing the progres-
sively stronger prismatic action that takes place as the per-
iphery of the lens is approached..'
In figure 36 we have similar rays, A, B, C, D, and E,
passing through a con-
cave lens. The axial
ray A passes through
the centers of curvature
unrefracted, but the rays
B, C, D, and E are pro-
gressively refracted more
and more as the periph- FIG. 36.
ery is approached. The
ray E in each instance is refracted the most.
The action of a convex lens is similar to that of a concave
mirror, and the action of a concave lens is similar to that of
a convex mirror.
Principal Focus. The principal focus of a lens may be
defined (i) as the point where parallel rays, after refrac-
tion, come together on the axial ray ; or (2) as the shortest
focus ; or (3) as the focal point for parallel rays.
Focal Length. This is (the distance measured from the
optic center to the principal focus. The principal focus
of an equally biconvex or biconcave lens of crown glass is
situated at about the center of curvature for either surface
of the lens.)^A. lens has two principal foci, an anterior and
a posterior, according to the direction from which the par-
allel rays come, or as to which radius of curvature is re-
ferred to.) (Seep. 60.) Figure 35 shows parallel rays, B,
C, 1), and E, passing through a convex lens and coming
to a focus on the axial ray (A) at P. F. ; and as the path of
a ray passing from one point to another is the same, no
34 REFRACTION AND HOW TO REFRACT.
matter what its direction, then if a point of light be placed
at the principal focus of a lens, its rays will be parallel after
passing back through the lens. This is equivalent to what
' takes place in the standard or emmetropic eye. An eye, in
other words, which has its fovea situated just at the princi-
pal focus of its dioptric media, such an eye in a state of rest
receives parallel rays exactly at a focus upon its fovea, and
therefore is in a condition to project parallel rays outward.
Conjugate Foci. Conjugate meaning "yoked to-
gether." ^The point from which rays of light diverge
(called the radiant) and the point to which they converge
(called the focus) are conjugate foci or points. ) For in-
stance, in figure 37 the rays diverging from A and passing
FIG. 37.
through the lens converge to the point B ; then the points
A and B are conjugate foci. They are interchangeable, for
if rays diverged from B, they would follow the same path
back again and meet at A. The path of the ray C C' is
the same whether it passes from A to B or from B to A :
there is no difference. It is by the affinity of these points
for each other, with respect to their positions, that they are
called conjugate.
The conjugate foci are equal when the point of diver-
Igence is at twice the distance of the principal focus. The
v equivalent to conjugate foci is found in the long or myopic
eye ; an eye, in other words, which has its fovea situated
further back than the principal focus of its dioptric media,
the result being that rays of light from the fovea of such an
OPTICS. 35
eye would be projected convergently after passing out of the
eye, and would meet at some point inside of infinity. In
other words, only those rays which have diverged from some
/ point insidq of six meters will focus upon the fovea of this
/ long eye. ( The fovea of the myopic eye represents a con-
I jugate focus./ A myopic eye is in a condition to receive
divergent rays of light at a focus on its retina and to emit
convergent rays.
Ordinary Foci. When rays of light diverge from some
point inside of infinity (six meters) they will be brought to
a focus at some point on the other side of a convex lens,
beyond its principal focus ; this point is called an ordinary
focus. (A lens may have many foci, but only two principal
FIG. 38.
fociJ The further away from a lens the divergent rays pro-
ceed, the nearer to the principal focus on the other side of
the lens will they converge. As the divergent rays are
brought closer to the lens they reach a point where they
will not focus, but will pass parallel after refraction. c_This
point is the principal focusj^ (See Fig. 38.) (^A lens, there-
fore, has as many foci as there are imaginary points on the
axial ray between the principal focus and infinity/)
(When rays of light diverge from some point closer to a
lens than its principal focus, they do not converge, but,
after refraction, continue divergently ; their focus now is
negative or virtual, and is found by projecting these diver-
gent rays back upon themselves) to a point on the same
30 REFRACTION AND HOW TO REFRACT.
side of the lens from which they appeared to come. (See
Fig- 39-)
This is the equivalent of what takes place in a short or
hyperopic eye, an eye which has its macula closer to its
dioptric media than its principal focus. In a state of rest
FIG. 39.
the fovea of such an eye would project outward divergent
rays, and would be in a position to receive only convergent
rays of light at a focus upon its fovea.
Secondary Axes. In the study of the direction of a ray
of light passing through a dense medium with plane sur-
FIG. 40.
faces, it was found that it underwent lateral displacement
(see Fig. 1 3), and(so in lenses there is a place where rays
undergo lateral displacement) Figure 40 shows a convex
lens of considerable thickness, and on each side is drawn a
radius of curvature (C C). The ray indicated by the arrow
7\
OPTICS. 37
passed through the two surfaces, has undergone lateral
displacement, but continues in its original direction ; such
rays are called secondary rays or axes. The incident ray
is projected toward N 1 in the lens on the axial ray, and
the emergent ray, if projected backward, would meet the
axial ray at N 2 . (These points on the axial ray are such
that a ray directed to one before refraction, is directed to
the other after refraction. The points N 1 and N 2 are ^
spoken of as nodafpoints. /Every lens, therefore, has two
nodal points, but in thin lenses the deviation of the second-
ary rays is so slight that, for all practical purposes, only
FIG. 41. ~>^tt^^ Jr>*-W^ / c
^ "^
^one nodal point is recognized. \ It is spoken of as the optic /V^.
' center. When writing prescriptions for glasses, this point of
having the lenses or glasses -made as thin as possible, must
be borne in mind.
/ Optic Center. This term is used synonymously with
nodal point, and/ is the point where the secondary rays (sia.\\\
Fig. 4 1 ) cross the axial ray. It is^not always the geometric
center) /Rays of light crossing the optic center in thin lenses
are not considered as undergoing refraction^) (See Fig. 41.)
Action of Concave Lenses. Rays of light passing
through a concave lens, no matter from what distance, are
^always refracted divergently, and its focus is, therefore,
always negative" or virtuah)and is found by projecting these
35 REFRACTION AND HOW TO REFRACT.
divergent rays backward in the direction from which they
appear to come until they meet at a point on the axial ray.
I The principal focus and conjugate foci of concave lenses
I are found in the same way as in convex lenses. (See Figs.
' 36, 44-)
Images Formed by Lenses. ^An image formed by a
lens is composed of foci, each one of which corresponds
to a point in the object. Images are of two kinds real and
virtual.
A Real Image. This is^an image formed by the actual
meeting of rays) such images can always be projected on
to a screen.
A Virtual Image. This is one that is formed by the
prolongation backward of rays of light to a point.
FIG. 42.
/To find the position and size of an image it is necegsary
to obtain the conjugate foci of the extremes of the object,
as the image of an object is equal to the sum of its inter-
mediate points. iOnly two rays are required for this pUr-
pose, one parallel to the axial ray, and one secondary ray
passing through the optic center ; the image of the extreme
point of the object will be located at the point of inter-
section of these rays. In figure 42 A B is an object in
front of a convex lens, o is the optic center and P. F.
the principal focus. A ray drawn from A parallel to the
axial ray o, and a secondary ray from the same point drawn
OPTICS.
through the optic center, will give at their point of inter-
section the conjugate focus of the luminous point A, which
will be at A'. In the same way the conjugate focus of B
and points intermediate in the object may be obtained. A'
B' is a real inverted image of A B ; /the size of the image
of A B depends upon the distance of the object from
the lens. The relative sizes of image and object are as ,
their respective distances from the optic center of the lens.)
For example, if an object ten millimeters high is three
meters (3000 mm.) from the optic center of a lens, and its
image is sixty millimeters from the lens, the image will be
ToMhr or To" f tne s ' ze f tne object ; that is, the image will
be -^ of ten millimeters (the height of the object) namely,
I of a millimeter high.
As conjugate foci are interchangeable, then in figure 42
if A! B' was the object, the image A B would be the image
of A' B', and, therefore, larger than the object.
Three facts should be borne in mind in the study of real
images formed by a convex Inis : ^~\^JjuC \
1. The object and image are interchangeable.
2. The'object and the real image are on opposite sides of "T)
the lens, and,
0> ^ /
3. As the rays which pass through the optic center
cross each other at this point, the real image must be in-
verted. A&*ruS\>W^
^MMH^^M* ' *f L3
Rays of light from an object situated at the distance of Q^ J /
the principal focus would proceed parallel after refraction, r~ r > 1 - t '
P-
and no image of the object would be obtained.
/ If an object is situated just beyond the principal focus,
/ then the image would be larger than the object, real and
/ inverted. (See Fig. 42, reversing image for object.)
If an object is situated at twice__the distance of the prin-
cipal focus, then its image would be of the same size, real,
4O REFRACTION AND HOW TO REFRACT.
inverted, and at a corresponding distance, as these conju-
gate foci are equal.y
If an object is situated at a greater distance than twice
the principal focus, and nearer than infinity, its image will
be real, inverted, and smaller than the object.
FIG. 43.
Rays of light from an object situated closer to a lens
than its principal focus would be divergent after refraction,
and could only meet by being projected backward ; the
image would, therefore, be larger than the object, erect, and
FIG. 44.
virtual. Such an image is only seen by looking through
the lens ; the lens in this instance being a magnifying
glass. (Fig. 43.)
Images Formed by Concave Lenses. These images
are always erect, virtual, and smaller than the object. (See
Fig. 44.) A concave lens is, therefore, a minifying lens.
&AC&
OPTICS. 41
Parallel rays from the extremes of the object A ix form the
divergent ray A' and R' after refraction. Secondary rays
pass through the optic center o unrefracted, A" and R".
At the points of intersection where these rays meet after
being projected backward, the image of A R is found,
erect, virtual, and diminished in size. This image is only
;een by looking through the lens.
Numeration of Lenses. -/-Formerly, lenses were num-
bered according to their radii of curvature in Paris inches
(27.07 mm.). The unit was a lens that focused parallel
rays of light at the distance of one English inch (25.4 mm.)
from its optic center,/
As lenses for purposes of refraction were never as strong
as the unit, they were numbered by fractions, thus showing
their relative strength as compared to this unit ; for instance,
a lens that was one-fourth the strength of the unit was
expressed by the fraction ^, or a lens that was one-
sixteenth the strength of the unit was expressed as ^g-, etc.,
[the denominator of the fraction indicating the focal length
'of the lens in Paris inches.
There are three objections to this nomenclature : (i)
The difference in length of the inch in different countries ;
(2) the inconvenience of adding two or more lenses num-
bered in fractions with different denominators ylj -f -%
-f- ^; (3) the want of uniform intervals between num-
bers.
In the new nomenclature, and the one that is now quite
universal, known as the metric or dioptric system (diopter,
abbreviated D.),/^f lens has been taken as the unit which
has its principal focus at one meter distance (39.37 English
inches), commonly recognized as 40 inches. c."> / rv
^Lenses in the dioptric system are numbered according
to their refractive power and not according to their radii of
42 REFRACTION AND HOW TO REFRACT.
curvature.") (The strength or refractive power of a dioptric
lens is, tKerefore, the inverse of its focal distance.) To find
jthe focal distance of any dioptric lens in inches or centi-
Imeters, the number of diopters expressed must be divided
linto the unit of 40 inches or 100 cm. For example, a
2 D. lens has a focal distance of 40 -4- 2 equals 20 inches ;
or 100 cm. -4- 2 equals 50 cm. A -(-4 D. has a focal dis-
tance of 40 -4- 4, equaling 10 inches, or 100 -4- 4, equaling 25
cm. /Lenses that have a refractive power less than the unit
are not expressed in the form of fractions, but in the form of
decimals ; for example, a lens which is only one-fourth, one-
half, or three-fourths the strength of the unit is written 0.25,
0.50, 0.75, respectively, and their focal distances are found
in the same way as in dealing with unitsj) 0.25 D. has a
focal distance of 40 H- 0.25 or 100 -4- 0.25, equaling 160
inches or 400 cm. ; 0.50 D. has a focal length of 40 -4-
0.50 or 100 -4- 0.5-0, equaling 80 inches or 200 cm. ; 0.75
D. has a focal length of 40 -4- 0.75 or 100 -4- 0.75
equaling 53 inches or 133 cm. Unfortunately, 0.25 D.,
0.50 D., and 0.75 D. are frequently spoken of as twenty-
five, fifty, and seventy-five, which occasionally leads to
confusion in the consideration of the strength and focal dis-
tance. The student should learn as soon as possible to
change the old nomenclature into the new, as he will have
to make these changes in reading other text-books.
To change the old " focal length " or inch system of
numbering lenses into diopters, divide the unit (40 in.) by
the denominator of the fraction, and the result will be an
approximation in diopters ;/ for example, y^- equals y$
or 4 D. ; -^V equals |^ or 2" D. The following table,
from Landolt, gives the equivalents in the old and new
systems :
OPTIQS.
43
OLD SYSTEM.
NEW SYSTEM.
I.
II.
III.
IV.
V.
VI.
VII.
VIII.
No.
No.
No.
of the
Focal
Focal
of the
Focal
Focal
Cories-
Lens,
Distance
Distance
Equiva-
Lens,
Distance
Distance
poiKlinir
Old
in English
in Milli-
lent in
New
in Milli-
in English
of the Old
System.
Inches.
meters.
Diopters.
System.
meters.
Indies.
System.
72
67.9
1724
0.58
0.25
4000
IS7 48
166.94
60
56.6
'437
0.695
' 0.5
2OOO
7874
8346
48
45-3
1150
0.87
o.75
1333
52-5
5563
42
39-6
1005
0.99
i
1000
3937
41-73
36
34
863
I 16
'25
800
3i 5
3339
3
28.3
718
i 39
i-5
666
26.22
2779
24
22.6
574
1-74
i-75
571
22.48
23-83
20
18.8
477
2.09
2
500
19.69
20.87
18
I?
431
2-31
2.25
444
1748
1853
16
15
38i
2.6
2-5
400
15-75
16.69
15
I4.I
358
2.79
3
333
"3 i?
"3-9
14
13-2
335
2.98
3-5
286
ii 26
n-94
13
12.2
312
3-20
4
250
9.84
10.43
12
II. 2
287
3-48
45
222
8-74
9 26
II
I3
261
3.82
5
2OO
7.87
8-35
10
9-4
239
4.18
5-5
182
7 16
76
9
8-5
216
4.63
6
166
654
693
8
7-5
190
5-2.S
7
'43
563
597
7
6.6
167
5-96
8
125
4.92
5-22
(>K
6.13
155
642
9
III
4-37
4-63
6
5-6
142
70
10
IOO
394
4 '7
5%
52
132
7-57
ii
91
3-58
38
5 f
4-7
119
8.4
12
83
3-27
3-46
4%
42
106
9-4
13
77
3-3
3-21
4 f
3-8
96
10.4
14
7i
2.8
2.96
3%
3-3
84
11.9
15
67
2.64
2.8
3Y4
3-1
79
12.7
16
62
244
2.59
3
2.8
71
14.0
17
59
2.32
2 46
2K
2.6
66
15-1
18
55
2 17
2.29
*S
2.36
60
17.7
20
5
1.97
2.09
2*
2.1
53
18.7
2
1.88
48
20.94
Cylindric Lenses. Abbreviated cyl., c, or C. A cylin-
dric lens, usually called a " cylinder," receives its name from
being a segment of a cylinder parallel to its axis. (See
Fig. 45. T Occasionally cylinders are made with both sur-
faces curved, and are then equivalent to two planocylinders
with their plane surfaces together. A cylinder may be
defined as a lens u'hich refracts rays of light opposite to its
axis. This definition should be carefully borne in mind in
contradistinction to a spheric lens, which refracts rays of
light equally in all meridians. /A cylindric lens has no one
44
REFRACTION AND HOW TO REFRACT.
common focus or focal point, but a line of foci, which is
parallel to its axis.)
Axis of a Cylinder. That dimension of a cylindric lens
which is parallel to the axis of the original cylinder of'
FIG. 45.
FIG. 46.
FIG. 47-
which it is a part is spoken of as the axis, and is indicated
on the lens of the trial-case by a short diamond scratch on
the lens at its periphery, or by having a small portion of
FIG. 48.
its surface corresponding to the axis ground at the edges,
or it may be marked in both ways. (See Fig. 47.) Cylin-
ders are of two kinds convex and concave. (Figs. 45, 46.)
Cylinder Action. /A convex cylinder converges parallel
OPTICS.
45
FIG. 49.
rays of light so that after refraction they are brought into
a straight line which corresponds to the axis of the cylin-
der j/for instance, a +5 cyl. will converge parallel rays so
that they come together in a straight line at the dis-
tance of eight inches,
or twenty centimeters,
and this straight line
will be parallel to the
axis of the cylinder.
(Fig. 48.)
A concave cylin-
der diverges rays of
light opposite to its
axis, as if they had diverged from a straight line on the
opposite side of the lens. (Fig. 49.)
Spherocylinders. A spherocylinder is a combination
of a sphere and a cylinder, and is/Therefore a lens which has
one surface ground with a spheric curve and the other sur-
face cylindric/ A spherocylindric lens is/also spoken of as
an astigmatic lensj 1 (See Fig. 100.) A spnerocylindric lens
is{pne which has two focal planes.j Spherocylinders have
different curves : the spheric curve may be convex, with the !
cylindric surface convex ; or the spheric surface may be
concave, with the cylindric surface concave ; or the spheric
surface may be convex, with the cylindric surface concave ;
or the spheric surface may be concave, with the cylindric
surface convex.
The Trial-case (see Fig. 50). This case contains pairs
of plus and minus spheres and pairs of plus and minus
cylinders ; also prisms numbered from ^ or ^ to 20 A.
The spheres are numbered in intervals of o. 12 up to 2 S.;
and from 2 S. up to 5 S. the interval is 0.25 S.; and from
5 S. to 8 S. the interval is 0.50 S.; and from 8 S. to 22 S.
46 REFRACTION AND HOW TO REFRACT.
the interval is i S. The /cylinders have similar intervals,
but seldom go higher than 6 or 8 cyy
The trial-case also contains a trial -frame, which is used
to place lenses in front of the patient's eyes. (See Fig. 51.)
The eye -pieces of such a frame are numbered on the
periphery in degrees of half a circle, so that the axis of a
cylinder can be seen during refraction. The left of the
FIG. 50.
horizontal line in each eye-piece is recognized as the start-
ing-place, or zero (o), and the degrees are marked from
left to right on the lower half, counting around to the
horizontal meridian, which at the right hand is numbered
1 80; this horizontal meridian is, therefore, spoken of as
horizontal, zero (O), or 1 80 degrees. The meridian midway
between zero and 1 80 is spoken of as vertical, or 90 degrees.
In some countries the meridians are differently num-
OPTICS.
47
bered (see Fig. 52); for example, the vertical meridian is
called zero, and the degrees are marked on each side of
zero up to 90 degrees. Only the upper half of the eye-piece
is thus numbered, so that when a cylinder has the upper end
of its axis inclined toward the nose, the record would be so
many degrees of inclination to the nasal side ; or if the upper
end of the cylinder was inclined toward the temple, the
record would be so many degrees to the temporal side.
For example, in the right eye 1 5 degrees nasal would
mean axis 75 on the ordinary trial-frame, and 15 degrees
temporal would mean 105 degrees.
The trial-case also contains other accessories, such as
blanks or blinders, a stenopeic slit, pin-hole disc, etc., all of
which are referred to in the text.
Combination of Lenses. The sign of combination
is ^.
Combining Spheres. Any number of spheric lenses
placed with their optic centers over each other, and sur-
4 8
REFRACTION AND HOW TO REFRACT.
faces together, will equal one lens the value of their sum :
for example, -f-2 S. Q -f I S. Q -(-3 S. will equal -j-6 S.;
or a 2 S. ^ I S. (^ 3 S. will equal a 6 S.
If a plus and minus sphere, each of the same strength, be
placed with their optic centers together, the refraction will
be nothing, for the one will neutralize the effect of the other ;
for instance, +48. and 4 S. will be equivalent to a piece
of plane glass, as the 4 S. will diverge rays of light as
much as the -(-4 S. will converge them, and the result
FIG. 52.
is, rays of light parallel before refraction are parallel after
passing through such a combination. (If, however, a plus
and a minus sphere of different strengths are placed together,
the value of the resulting lens will equal their difference, in
favor of the higher numberj for instance, -{-48. and 2
S. will equal a -\-2 S., the 2 S. neutralizing 2 S. of the
+48., leaving +2 S.
Combining Cylindric Lenses. -(-Any number of cylin-
dric lenses placed together, with their axes in the same
OPTICS. 49
meridian, are equal to a cylinder the value of their sum ;
for example : -j-2 cyl. axis 90 degrees and +3 cyl. axis 90
degrees will equal a -f 5 cyl. axis 90 degrees ; or 2 cyl.
axis 1 80 degrees and 3 cyl. axis 180 degrees will equal
a 5 cyl. axis 180 degrees ; or 2 cyl. axis 180 degrees
and +i cyl. axis 180 degrees will equal a I cyl. axis
1 80 degrees.
As a cylinder refracts rays of light only in the meridian
opposite to its axis, this opposite meridian can always be
found by the following simple rule :
" Add 90 u'hcn the given axis is yo or less than po, and
subtract go when the given axis is more than 90." j
For example : -f- 3 cyl. axis 90 refracts rays of light in
the 1 80 degree meridian (90 + 90= 180); or +3 cyl.
axis 75 refracts rays of light in the 165 meridian (75+90
165). A 3 cyl. axis 135 refracts rays of light in the
45 meridian (135 less 90 =45). A 2 cyl. axis 180 re-
fracts rays of light in the 90 meridian (180 less 90 = 90).
Combining two cylinders of the same strength and
same denomination, with their axes at right angles to
each other, will equal a sphere of the same strength
and same denomination. For instance, +3 cyl. axis 90
and +3 cyl. axis 180, placed together, will equal a +3 S.
/. e. y the +3 cyl. at axis 90 will converge parallel rays in
the 1 80 meridian, while the -f- 3 cyl. axis 1 80 will converge
parallel rays in the 90 meridian, producing a principal
focus /therefore any sphere is also equal to two cylinders
of its same strength and same denomination with their
axes at right angles to each otherj
(Combining cylinders of different strength, but of the
same denomination, with their axes at right angles to
each other, such a combination will equal a sphere and
a cylinder of the same denomination. ) For example :
REFRACTION AND HOW TO REFRACT.
-\-2 cyl. axis 75 O +3 cyl. axis 165 will equal -f-2 S. O
~\-i cyl. axis 165. The -\-2 cyl. axis 75 takes -f 2 of the
-j~3 cyl. axis 165 and makes a + 2 S., leaving -f- 1 cyl. axis
at 165 ; the result is then -f-2 S. O + I cyl. axis 165.
Or 3-5O cyl. axis 15 O - 4.50 cyl. axis 105, will
equal 3.50 S. O I cyl. axis 105. The 3.50 axis 15
takes 3.50 of the 4.50 and makes a 3.50 S.,
leaving I cyl. axis 105; this I cyl. axis 105 is now
joined to the 3-5O sphere, making 3-5O S. O I cyl.
axis 105.
Combining a sphere and a cylinder of the same
strength, but of differ-
ent denomination, will
'equal a cylinder of the
opposite sign and opposite
axis from the cylinder
given. For example : -f- 1
sphere O I cyl. axis 1 80
will equal -f- I cyl. axis 90.
The -f- i S. equals two -f- I
cylinders, one at axis 90
and one at axis 180, and
the I cyl. at axis 180 is
neutralized by the -f- 1 cyl.
at the same axis, leaving
-f- I cyl. axis 90. This may be better understood by the
diagram (Fig. -49).
Or 3 S. O +3 cyl. axis 90 equals 3 cyl. axis 180.
The 3 S. is equal to two 3 cylinders, one at axis 90
and one at axis 180 ; the one at axis 90 is neutralized by
the +3 cyl. at the same axis, leaving 3 cyl. axis 180.
Combining a Sphere with a Weaker Cylinder of Dif-
ferent Denomination. Such a combination should be
1.00
FIG. 53.
OPTICS. 5 1
changed to its simplest form of expression, and will equal
a sphere of the same denomination, of the value of their
difference, combined with a cylinder of the same strength
as the cylinder given, but of opposite sign and axis. For
example: -j~4 S. O i cyl. axis 180. The minus one
cylinder is refracting in the 90 degree meridian, therefore it
reduces the strength of the +4 S. in this axis, making it a
plus 3. The horizontal or 1 80 degree meridian of the plus
4 S. has not been altered, but still remains -(-4, and the re-
sult is, plus 3 in the vertical meridian and plus 4 in the
horizontal meridian, equaling, therefore, -j~3 S. ^ -f I
cyl. axis go.
The following rule will be of service in making this
change, and, in fact, this rule will apply in any instance
where the sphere and cylinder are of different denomina-
/s^
tion, no matter what their respective strengths may be : |(/^
Rule. Subtract the less from the greater, and to the res2tlh. {
prefix the sign of the greater ; continue isith this the same\ ^',(Y^
strength cylinder, using the opposite sign and opposite axis.\ ^T^* \
Example : +2.25 S. O 0.75 cyl. axis 75 degrees ; sub- /**'
tracting the less from the greater ( 0.75 from +2.25), and\
prefixing the sign of the greater (+), will leave -}- 1.50 S. ;
and combining with this the same strength cylinder (0.75),
with opposite sign and axis (+ and 165), will be +0.75
cyl. axis 165. Result, +I-5O S. O +0.75 cyl. axis 165.
Combining a Sphere and Cylinder of the Same De-
nomination. This is recognized as the minimum or
^ simplest form of expression, and is seldom changed. For
example : 2 S. O 6 cyl. axis 180 is considered as the
thinnest lens and the one with the least weight that can be
made by such a combination. It may be changed, how-
ever, by the reverse of the rule above given, and will equal
8 S. O -f 6 cyl. axis 90.
REFRACTION AND HOW TO REFRACT.
Combining Two Cylinders of Different Denominations
with Opposite Axes. Commonly called crossed cylinders.
Such combinations can be written in three ways :
1. -)-Cyl. ^ cyl. axes opposite.
2. -f Sphere O cyl. (cylinder stronger than sphere).
3. Sphere +cyl. (cylinder stronger than sphere).
y For example : i.oo cyl. axis 180^ +2.50 cyl. axis 90
may be changed to one of the following :
I.oo S. O +3-5 cyl. axis 90 ; or
^+2.50 S. C 3-50 cyl. axis 180.'
\J| ' \ \ i7, L^ t*T.
/K/Y^ n~V^^ J t^"
-j The first formula shows that the vertical meridian must
always be I and the horizontal or 1 80 meridian must
always be +2.50, and with this clearly in mind, the second
/^?4rfkf '&ird formulas will be understood. In the second
&** ' formula ( i.oo S. O +3.50 cyl. axis 90) the +3.50 cyl.
is only equal to +2.50, as it has I D. neutralized by I
of the I sphere. In the third formula (+2.50 S. O
" ^^/3/5 cyl. ax i s !8o) the 3-5 cylinder is only equal to
HT.OO cylinder, as it has 2.50 neutralized by +2.50 of
the sphere.
In any spherocylindric combination the meridian in which
the axis of the cylinder lies has the strength of one lens, and
the meridian opposite to the axis of the cylinder has the com-
bined values of sphere and cylinder /. c., i.oo S. O
-(-3.50 cyl. axis 90 means i.oo on the axis (90) of the
cylinder, and opposite to the axis therefore at 1 80, it equals
+ 2.50 (1 and 4-3-50).
Crossed cylinders in themselves are seldom ordered in a
prescription, preference being given to a spherocylindric
combination, ('when to order a plus sphere with a minus
cylinder, and when to order a minus sphere with a plus
cylinder, depends upon the individual lenses.) For example :
-(-0.50 cyl. axis 90 O 5.00 cyl. axis 180 equals +0.50
S. O 5.50 cyl. axis iSoor 5 S. O +5. 50 cyl. axis 90.'
Preference would be given to the plus sphere combina-
tion, on account of thinness and lesser weight of the lens.
The following formula, i cyl. axis 180 degrees ^ 3
cyl. axis 90 degrees, equals i.oo S. ^ +4 cyl. axis 90,
or +3 S. ^ 4 cyl. axis 180, and for similar reasons
preference would be given to the minus sphere combination.
Whichever combination makes the thinnest and lightest
weight of glass is the one to be ordered, as a rule.
The student should practise these combinations at the
trial-case, and be able at a glance to change one formula
into another without diagram or rule.
Prescription Writing. In writing prescriptions for
lenses the right eye is indicated by one of three signs R,
Rt, or O. D., the latter from the Latin for right eye,
OCH/HS Dexter. The left eye is also indicated in one of
three ways L, Lt, or O. S., the latter from the Latin
for left eye, Oculus Sinister.
A prescription may call for any one of the following :
-(-Sphere, written 4 4 D. or -(-4.00 D. S. or -f 4 S. or -} 4 sph.
Sphere, written 2 D. or 2.00 D. S. or 2 S. or 2 sph.
-(-Cylinder, written -f 4.00 D. C. or -f 4 C. or -(-4 cyl. (axis as indicated).
Cylinder, written 2.00 D. C. or 2 C. or 2 cyl. (axis as indicated),
-f Sphere and -f- cylinder, written +2.00 S. ^ -\-2.oo cyl. axis 90 degrees.
Sphere and cylinder, written 2.00 S. ^ 2.00 cyl. axis 180 degrees.
-f Sphere and cylinder (cylinder stronger than sphere), 4 2 - S. O 3-OO
cyl. axis 180 degrees. C
Sphere and -f cylinder (cylinder stronger than sphere), 2.00 S. ^ 43- )
cyl. axis 90 degrees.
A plus cylinder and minus cylinder may be prescribed,
and, if so, their axes must be at right angles to each other.
An occasional exception to this may be found in irregular
astigmatism. Or a prism with its base indicated may be
^
REFRACTION AND HOW TO REFRACT.
&!
\j "Nadded in any one of the foregoing formulas ; for example :
* 2 S. O 2.OO cyl. axis 1 80 O 2 A base in ; or the direc-
>tion of the base may be abbreviated as follows : B. I.,
^ meaning base in ; B. O., meaning base out ; B. U., meaning
base up ; and B. D., meaning base down.
Prescriptions are never written for two spheres.
Prescriptions are never written for two cylinders at the
ame axis.
Prescriptions are never written for two cylinders at axes
Bother than those at right angles to each other, except, as
just noted, in irregular astigmatism.
For obvious reasons prescriptions are never written for a
sphere and two cylinders^ except in irregular astigmatism.
Recognition of Lenses?
A convex sphere is thick at the center and thin at the
r\<r 1 edge. It has the power of converging rays of light ; hence,
L \ V^f strong, it is a burning glass. Objects viewed through a
i Q convex lens as it is moved before the eye, from left to
r right and right to left or up and down, appear to move
^ in an opposite direction to that in which the lens is
5V' v moved. The weaker the lens, the slower the object
~~% (appears to move ; and the stronger the lens, the faster the
V apparent movement of the object. (A convex lens being a
V magnifier, has the effect of making objects appear larger
) and closer when it is moved away from the observer's eye ;
v,or if brought toward the eye, objects already enlarged V
viappear smaller and more distant^
A To Find the Optic Center of a Convex Lens. Look-
^ing at a vertical straight line and passing a convex
lens before the eye from left to right has the effect of
displacing 'to ward thff^right edge of the lens that portion of'
the line seeir\ through the lens (see , Fig. 54), and as the
lens is slowlV moyed^$till further to th& right, the displacec
1
OPTICS.
55
portion of the line will finally coincide with the original
straight line, making one continuous line through the lens.
(See Fig. 55.) Marking this straight
line on the surface of the lens, and
then turning the lens to the opposite
meridian and repeating the examina-
tion, and marking the lens as before,
the optic center will be in the lens
beneath the point of intersection of the
t\vo lines. (See Fig. 56.)
A concave sphere is thick at the
edge and thin at the center, and has
the power of causing rays of light
to diverge. When moved before the
eye from left to right and right to left
or up and down, objects appear to move in the same direc-
tion as that in which the lensjs moved.
A concave lens being a minifier, makes objects appear
FIG. 55.
FIG. 56.
smaller and more distant as the glass is moved away from
the eye, and if brought closer to the eye, makes objects
apparently small appear somewhat larger and nearer.
REFRACTION AND HOW TO REFRACT.
Looking at a straight edge or line through a concave
sphere, and passing the lens from left to right, the portion
of the(line seen through the lens appears displaced toward
the center of the lens (see Fig. 57), and as the lens is still
further moved to the right, the displaced portion of the
line finally coincides with the original straight edge, as in
figure 55.)
The optic center of a concave lens is found in the same
way as the center of a convex lens.
A Convex Cylinder. When a convex cylinder is moved
FIG. 57.
FIG. 58.
in front of the eye in the direction of its axis, objects looked
at do not change their positions ; but when the lens is
moved in the direction opposite to its axjs ; the movement
of the object is the same as that of a convex sphere. Look-
ing at a straight edge through a convex cylinder, and
rotating it, has the effect of displacing away from its axis
that portion of the straight edge seen through the lens.
(See Fig. 58.)
A Concave Cylinder. When a concave cylinder is
moved in front of the eye in the direction of its axis, ob-
OPTICS. 57
jects looked at do not change their positions ; but when the
lens is moved in the direction opposite to its axis, the
movement of the object is the same as that of a concave
sphere. Looking at a straight line through a concave
cylinder, and rotating it, has the effect of displacing toward
its axis that portion of the straight line seen through the
lens. (See Fig. 59.) / A circle viewed through a strong con-
cave cylinder appears as an oval with its long diameter cor-
responding to its axis.^) (See Fig. 60.) (A circle viewed
through a strong convex cylinder appears as an oval with
its long diameter opposite to its axislN In place of using a
FIG. 59. FIG. 60.
straight line or straight edge to find the optic center of a
sphere or axis of a cylinder, two lines at right angles may
be substituted (see Fig. 56) or a protractor may be used.
A Prism. Objects viewed through a prism are dis-
placed toward its apex, and that portion of a straight line
seen through the prism never coincides with the straight
line.
Neutralization of Lenses. Having determined from
the foregoing description what the character of an indi-
vidual lens may be, then to neutralize its effect or find out
its strength a lens of opposite character is taken from the
$8 REFRACTION AND HOW TO REFRACT.
trial-case and held in apposition to it, and the two lenses
.are moved in front of the eye as a distant object is
/ observed. That lens or combination of lenses which stops
all apparent movement of the object is the correct neu-
tralizing lens. Spherocylindric lenses are neutralized by
finding out what sphere will correct one meridian and what
sphere will correct or neutralize the opposite meridian ; for
example, if a minus 2 S. stops all movement in one
meridian and minus 3 S. stops all movement in the other
meridian, then the lens being neutralized will be plus 2 S.
combined with a plus I cylinder. Or after a sphere neu-
tralizes one meridian, a cylinder may be combined until the
other meridian is neutralized.
CHAPTER II.
THE EYE. THE STANDARD EYE. THE CARDINAL
POINTS. VISUAL ANGLE. MINIMUM VISUAL AN-
GLE. STANDARD ACUTENESS OF VISION. SIZE OF
RETINAL IMAGE. ACCOMMODATION. MECHANISM
OF ACCOMMODATION. FAR AND NEAR POINTS.
DETERMINATION OF DISTANT VISION AND NEAR
POINT. AMPLITUDE OF ACCOMMODATION. CON-
VERGENCE. ANGLE GAMMA. ANGLE ALPHA.
The Eye. While the eye is considered as the organ of
vision, vet its function is to form upon its retina an inverted
image of any object looked at ; and if the retinal image is
distinct, the object will appear distinct ; if the retinal image
is blurred, the object will appear blurred. By means of
the optic nerve and tract the retinal impression or image
is placed in communication with the brain, which interprets
the image and completes the visual act.
The Standard Eye. For purposes of exact calcula-
tions it has been found necessary to project a standard or
schematic eye, whose nodal point (optic center) shall be
^seven millimeters back of the anterior surface of the cor-
- 11 ea and fifteen millimeters from the fovea (Helmholtz).
Allowing one millimeter for the thickness of the choroid
and sclera, such an eye would have an anteroposterior
measurement of about twenty-three millimeters. Parallel
rays of light passing into such an eye in a state of rest
would focus on the macula.
Cardinal Points (Fig. 61). Images formed upon the
retina are the result of refraction by three refracting sur-
faces and three refracting media. The refracting surfaces
59
6O REFRACTION AND HOW TO REFRACT.
are the anterior surface of the cornea and the anterior and
posterior surfaces of the crystalline lens. The refracting
media are the cornea (and aqueous humor forming a convex
lens), the crystalline lens, and the vitreous humor. These
refracting surfaces and media represent a compound dioptric
system, centered upon /the optic or principal axis i. e., a
line drawn from the pole of the cornea to a point between
the nerve and fovea.J
^On the principal axis are situated the anterior and poste-
rior principal foci (see p. 33), the anterior and posterior nodal
FIG. 61.
points, and the anterior and posterior principal points. The
anterior principal focus is situated upon the optic axis
N / 13.745-)- mm. in front of the corneal apex. The pos-
terior principal focus is situated 15.61+ mm. back of the
posterior surface of the lens\ (The nodal points are situ-
ated about 7 mm. back of the cornea, and correspond
approximately to the optic center of this compound re-
fracting system ; and as they are so close together, they are
considered as one for all purposej) in the study of the for-
mation of images. The first or anterior principal point is
MINIMUM VISUAL ANGLE. 6 1
situated 1.75 mm. back of the anterior corneal surface, and
the second or posterior principal point is situated 2.10 mm.
behind the anterior surface of the cornea. The principal
points are so closely situated that they are considered as
one. The anterior focal distance equals 15.49+ mm. and
the posterior focal distance equals 20.71 + mm.
The Visual Angle, or Angle of View. The visual angle
is the angle formed by rays of light from the extremes of an
object passing to the nodal point of the eye ; (or the visual
angle may be defined as the angle which the object subtends
at the nodal point) of the compound refracting system of
the eye. Rays of light from the extremes of an object
FIG. 62.
directed to the nodal point of the eye pass through unre-
fracted, and continuing their straight course, fall upon the
retina, forming an inverted image of the object. (See Fig.
62.)
The size of the retinal image depends upon the size and
the distance of the object from the nodal point of the eye.
/ Objects, therefore, which are seen under the same visual
angle must have the same sized retinal image. (See Fig.
63.)
If the arrows i, 2, 3, and 4 represented a child, a man, a
tree, and a church, respectively (some distance apart), they
would form the same sized retinal images, and if the eye
\\cre guided alone by the size of the retinal image, it would
62
REFRACTION AND HOW TO REFRACT.
judge erroneously; but, by experience, distance and com-
parison of size are brought into judgment.
If, however, arrows 2, 3, and 4 are placed at the side of
arrow I, then their resulting images would increase in size
according to the size of their respective visual angles. (See
Fig. 64.)
The nearer an object to the eye, the larger the visual
FIG. 63.
angle and retinal image ; the further away an object from the
eye, the smaller the visual angle and retinal image. An ob-
ject, to retain the same sized visual angle, must, therefore, be
made larger the further it is removed from the eye ; this is
demonstrated in figure 63, where arrow I, to be seen
FIG. 64.
under the same visual angle which it has at present, would
have to be as large as arrow 4, at the distance of arrow 4.
Minimum Visual Angle. This is the smallest visual
angle in which a standard eye can still recognize an object
and give it a name ; this angle is also spoken of as the
SIZE OF RETINAL IMAGES. 63
limiting angle of vision. In figure 65, for example, the
letter D at a distance of six meters is recognized as the
letter D : it is plainly seen ; but if placed beyond six meters,
it would form a . smaller visual angle, and could not with
certainty be called D.
To be seen at a distance of twelve meters and still
occupy this same visual angle, D would have to be made
twice as large /. e., the size of F ; and to be seen at
twenty-four meters, it would have to be four times its pres-
ent size, or the size of P. Thus, while the letter D, seen
clearly at six meters, would have to be made proportion-
ately larger as it is 'removed from the eye, then to occupy
FIG. 65.
the same visual angle it would have to be made smaller
if brought closer to the eye and kept within this limiting
angle. In figure 65 D, F, and P can be seen closer to
the eye than their respective distances call for ; but the pur-
pose is to find the greatest distance from the eye at which
they can be seen, as this represents the maximum acuteness
of vision, or maximum sharpness of sight.
Standard Acuteness of Vision. As it was necessary for
purposes of calculation to have a standard or emmetropic
eye, so it is essential to have a standard acuteness of vision
which will be consistent with the standard or emmetropic
eye, and thus have some method of recording numerically
any departure from this standard visual condition.
64 REFRACTION 'AND HOW TO REFRACT.
The standard acuteness of vision is the power of the eye to
x distinguish letters and characters occupying an angle of five
minutes. (Every letter is, therefore, so proportioned that it
will measure just five minutes in the vertical and hori-
zontal meridians, and be reducible to twenty-five parts or
squares, each measuring
one minute vertically and
horizontally.* (See Fig.
66.))
*
FIG. 66. FIG. 67.
letter F drawn in a five-
minute square, and each stroke of the letter, and space
between the strokes, measuring just one minute in width.
As the tangent of the angle of five minutes is expressed
by the decimal .001454, then to calculate the size of
any letter or character which should be seen clearly and dis-
tinctly by the standard eye at a certain definite distance, it
is necessary to multiply the distance in millimeters by this
tangent of the angle of five minutes. Letters or characters
made on this scale are called standard letters. For ex-
ample, letters to be seen under an angle of five minutes at
a distance of one meter (1000 mm.) would have to be
1.454 mm. square (1000 X .001454). At six meters
(6000 X .001454) = 8.7 mm^ etc.
Size of Retinal Images.-^-The size of the retinal image
depends upon two factors the size of the object itself and
its distance from the nodal point) In the standard eye it
has been stated that the nodal point was 7 mm. back of
the cornea and 1 5 mm. in front of the retina ; then an
object 8.7mm. square situated 6000 mm. in front of the
* There are two letters in the alphabet which are exceptions to this rule,
L and O. L can be seen under an angle of two minutes and O can be seen
under an angle of three minutes.
SIZE OF RETINAL IMAGES.
/ye would have a retinal image -g-^f^ of 8.7, or 0.02 -f- mm.,
and this is the size of the retinal image in a standard eye,
^looking at a standard letter at six meters' distance. A good A.^
rule for finding the size of the retinal image is to multiply
the height of the object by the nodal distance and divide
by the distance. /In other words, the size of the retinal
image is to the size of the object as their respective dis-
tances from the nodal point.
Refraction in ophthalmology has most to do with eyes
whose measurements are not according to the standard or
emmetropic condition, and which have their retinas closer
to or further from the nodal point than 15 mm. (spoken of
as ametropic). The
retinal images in /^~ ~~~~N M
such eyes will be
smaller in the former
and larger in the
latter. (See Fig. 68.)
Accommodation. FIG. 6s.
/ This may be de-
>j scribed as the power of the eye to focus rays of light upon its
retina from different distances at different times. In other
words, the eye can not focus rays of light upon its retina from
different points at one and the same time. For example, the
point of a pencil held six inches in front of the eye is not seen
clearly (is hazy) as the eye looks at a printed page thirteen
inches beyond ; and, vice versa, the printed page is not seen
distinctly if the point of the pencil is looked at. In the
study of convex lenses it was noticed that when an object
was brought closer than infinity, the focus of the lens was
correspondingly lengthened ; and so, in the photographer's
camera, to keep the focus on the ground-glass or sensitive
plate as the object is brought toward the camera, it is
6
66 REFRACTION AND HOW TO REFRACT.
necessary to push the lens forward by means of the accor-
dion plaits ; but the human eye does not lengthen or
shorten in this way. Normally, the eyeball is inextensible,
and to accomplish this same purpose the ciliary muscle
must contract, causing the crystalline lens to become more
convex, and thus keep the rays of light entering the eye at
a focus upon the fovea.
The Mechanism of Accommodation. To appreciate
this, it is necessary to understand something of the anatomy
of the ciliary body, of which the ciliary muscle is a part.
The ciliary body is circular in form and occupies a small
. (3 mm.) area in the eye, just beneath the sclera, at its cor-
neal junction. (See Fig. 61.) In section the ciliary body is
triangular in shape, the base of the triangle measuring about
0.8 mm. and facing toward the anterior chamber, the apex
of the triangle extending backward beneath the sclerotic.
The ciliary body lies in apposition to the sclera, but has
only a very minute attachment to it, at the sclerocorneal
junction, called the ligamentum annulare, or pectinatum.
That portion of the ciliary body lying next to the hyaloid
membrane of the vitreous humor is composed of folds,
known as the ciliary processes, seventy or more in number.
A portion of the ciliary body is composed of muscular
fibers disposed in flat bundles, which interlace with each
other, forming a sort of plexus, and called the ciliary mus-
cle. This muscle, by the character of its fibers, has been
subdivided into three parts : (i) Meridional (2) radiating ;
and (3) circular or sphincter fibers. The (meridional are
the longest, lie next to the sclerotic in lamellae, parallel
with it, and pass back to join the choroid coat of the eye,)
(forming what is known as the tensor choroidea?, or muscle
|of Brucke or Bowman. The radiating fibers are fan-shaped,
few in number, and scattered through the ciliary body.
THE MECHANISM OF ACCOMMODATION. 6/
The circular or sphincter fibers also called annular are
sometimes referred to as the muscle of Miiller, or com-
pressor lentis, and are the most important fibers in the
consideration of accommodation ; they form a sphincter
ring concentric with the equator of the lens. Attached to
the ciliary body, well forward on its inner side, near the
' base of the triangle, is the ligament of the lens (zonule of
Zinn), and it in turn sends fibers to the anterior and poste-
rior capsule of the lens. This ligament of the lens occu-
pies an interval of about 0.5 mm. between the ciliary body
and the periphery of the lens, and is a constant factor in
all conditions of the healthy eye.
During the act of accommodation the following changes
take place in the eye :
1. The ciliary^ muscle contracts.
2. The ciliary muscle (sphincter), by contracting, makes
a smaller circle.
3. The tensor choroideae draws slightly upon the choroid
(compressing somewhat the vitreous body), and these two
sets of fibers, sphincter and meridional, acting together,
i relax the ligament of the lens, with the result that
4. The lens fibers, no longer held in check, become re-
laxed, and by their own inherent quality (elasticity) allow
the lens to become more convex, especially on its anterior
surface.
5. The anterior surface of the lens being made more
convex, approaches the cornea.
| 6. The posterior surface of the lens becomes slightly
/more convex, but retains its position at the pole.
7. The lens axis is lengthened, but the equatorial diame-
ter diminishes, thus keeping up the uniform interval between
the equator of the lens and the ciliary body, as previously
referred to. The lens docs not increase in volume.
68
REFRACTION AND HOW TO REFRACT.
8. The anterior chamber becomes slightly shallower at
the center and deeper in the periphery.
9. That portion of the iris resting upon the anterior cap-
sule of the lens is pushed forward, espe-
cially at its pupillary edge.
10. The iris contracts, producing a
smaller pupil ; but it must be remembered
that contraction of the iris is not an essen-
tial condition in accommodation. The shape
of the cornea is not changed during con-
traction of the ciliary muscle.
The following table shows the compara-
tive measurements of a lens at rest and
during the height of accommodation in a
healthy emmetropic eye of ten years. The dotted lines
in figure 69 indicate the changes in the shape of the lens
at the height of accommodation.
FIG. 69.
AT REST.
Radius of curvature of anterior surface of lens, . 10 mm.
" " posterior " " . 6 "
Distance from anterior surface of cornea to ante-
rior surface of lens, 3.6 "
Anteroposterior diameter, on axis, 3.6 "
Distance from anterior surface of cornea to poste-
rior surface of lens, 7- 2 "
Equatorial diameter, 8.7 "
HEIGHT OF
ACCOMMODATION.
6 mm.
5-5 "
3-2
4
7.2
8.2
Far Point. Latin, punctum rcmotum ; abbreviated p. r.
^or r. The far point may be (defined as the greatest distance
at which an eye has maximum sharpness of sight, or the
most remote point at which the eye, in a state of rest, has
maximum acuity of vision^) Infinity (sign of infinity, oo )^
is the far point of an emmetropic eye.
The standard or emmetropic eye, when looking at distant'
objects, receives parallel rays of light at a focus upon its
AMPLITUDE OF ACCOMMODATION. 69
fovea (Fig. 70), and also emits parallel rays ; under these
conditions the ciliary muscle is not acting, the eye is in a
condition of complete repose, of rest, of minimum refrac-
tion, and is adapted for its far point.
Near Point. Latin, punctual proximum ; abbreviated
p. p. or p. This may be .defined as the nearest point at
which an eye has maximum sharpness of sight^ or the
nearest point to the eye at which it has distinct vision, the
lens is in the condition of greatest convexity, of maximum
refraction.
Amplitude of Accommodation. This is also called the
range * or power f of accommodation, and may be^e-
fined as the difference between the refraction of the eye in
a state of rest (or adapted for its far point) and in a condi-
tion of maximum refraction, or adapted for its near pointy
For example, an emmetropic eye has infinity for its far
point, and if 10 cm. distance is its near point, then the dif-
ference between the lens adapted for infinity and locm. will
be 10 D., as 10 cm. represents the focal length of 10 D. In
other words, there is no accom-
modation used for infinity, but
there is an accommodation of
10 D. for the near point, which
is the amplitude or power of
accommodation. The emme-
tropic eye in a state of accom- FIG. 70.
modation adds on to the ante-
rior surface of its lens what is equivalent to a convex
meniscus. Figure 70 shows an emmetropic eye at rest
receiving parallel rays of light at a focus upon its retina,
* Range applies to the space between the far and near points,
f Power applies to the force or strength or diopters necessary to change
the refraction from the far to the near point
7O REFRACTION AM) HOW
and it also shows the same eye in its maximum state of
accommodation for a point 10 cm. distant; the broken
line representing what is equivalent to a convex meniscus,
added to the anterior surface of its lens.
When the distance of the near point is known in inches
or centimeters, the equivalent in diopters is found by divid-
ing 40 by the near point in inches, or by dividing roo by
the near point in centimeters. The near point being 10 cm.,
! or 4 inches (10 into 100 or 4 into 40) the amount of accom-
modation will be 10 D.
In the study of healthy emmetropic eyes it has been
found that the power of accommodation gradually dimin-
ishes as the eye passes from youth to old age. This is
the result of one or more changes : the lens fibers lose
their elasticity, becoming sclerosed, or the ciliary muscle
grows weak, or both of these changes may exist together.
Rarely the cornea may flatten. A knowledge of the power
of accommodation~~isabsolutely essential, so that any vari-
ations from the standard condition may be noted. The fol-
lowing table gives the ages from ten to seventy-five years,
respectively, with five-year intervals, and the near point
consistent with each, as also the amplitude of accommoda-
tion for each period.
I AMPLITUDE AMPLITUDE
YEAR. /NEAR POINT. IN DIOPTERS. YEAR. NEAR POINT. IN DIOPTERS.
10 ></7 cm. 14 45 28cm. /| 3.5
15 2^8.5 " 12 > 50 40 " / ^ 2.5
20 4io " 10 55 55 ">/ 3<? 1.75
25 -f%i2 ." 8.5 60 loo "U^/i *
30 1457*^ 7 ''5 U3 "$'>/> 0.75
35 18 Jf> S-S 7 400 " 0.25
40 My'TV 4 ' 5 75
I > ' / This table of near points applies only to emmetropic eyes
/or those eyes which are made emmetropic by the adjust-
-C. ^
/ ' ^ ,
FAR POINT. 7 1
ment of suitable correcting lenses. The table of amplitudes,
however, is the same, with a few exceptions, for all eyes of
whatever degree or amount of ametropia.
For a better appreciation of the amplitude of accom-
modation it is necessary to understand the two forms of
eyes already referred to in figure 68.
First, the eye which has its retina closer to its refractive
media than the principal focus ; such an eye is spoken of
/as a short or hyperopic eye. (H in Fig. 68.) (Hyperopia :
^/ \ Greek, o-^p, over; and &<!', eye.)
This eye in a state of rest (under the influence of atro-
pin) will emit divergent rays of light, and is, therefore, in a
FIG. 71.
condition to receive only convergent rays of light at focus
upon its retina. (See Fig. 71.) Parallel rays would not
focus upon the retina of such an eye, but, if possible, would
focus back of the retina.
Second, the eye that has its retina beyond the principal
focus of its dioptric media (M in Fig. 68) ; such an eye is
spoken of as a long or myopic eye (Greek, /wstv, to close ; V',
eye). This eye always emits convergent rays, and is, there-
fore, in a state to receive divergent rays of light at a focus
upon its retina. (See Fig. 72.) Parallel rays would not
focus upon the retina of a myopic eye, but in the vitreous
in front of the retina.
The Far Point of a Hyperopic Eye. This must neces-
72 REFRACTION AND HOW TO REFRACT.
sarily be negative (see Fig. 71), and is found by projecting
the divergent emergent rays backward to the imaginary
point behind the retina from which they appear to have
diverged. A hyperopic eye, to receive parallel rays of
light at a focus upon its retina, must, therefore, accommo-
date, and the amount of accommodation thus exerted will
remove the near point just that much from the eye as com-
pared with an emmetropic eye. For example, according to
the table of amplitudes just given, an eye at twenty years has
I IO D. of accommodation, but if it uses 2 D. of this to make
' rays of light parallel, then it only has 8 D. left to accom-
modate inside of infinity, with the result that the near point
comes to only (8 into 100) 12.5 cm. ; or an eye which is
FIG. 72.
twenty-five years old has an amplitude of accommodation of
8.$ D., and if it has to use 4.5 D. for infinity, it would have
(4 into 100) a near point of 25 cm. (10 inches).
The Near Point of a Hyperopic Eye. From the de-
scription just given it will be seen at once that the near
point in hyperopic eyes is always further removed than in
the emmetropic eye for a corresponding age, and that the
..jar point depends upon the amount of accommodation t//'
that is left after the eye has accommodated for infinity .[ ^ y
The Far Point of a Myopic Eye. This is always posi-
tive and situated some place inside of infinity. It is found
by uniting the convergent emergent rays. (See Fig. 72.)
FAR POINT.
73
The far point of a myopic eye is the result of its strong
refracting power or the distance of its retina beyond the
principal focus of its dioptric media. The retina and far
> point of a myopic eye are conjugate foci. (See Fig. 72.)
The myopic far point is equivalent to just that much refrac-
tion in excess of the emmetropic eye. An emmetropic eye
under the influence of atropin would require a -f- 2 S.
placed in front of it to make rays of light focus upon its
retina from a distance of 50 cm., and rays of light from the
retina of this eye with a -j-2 S. in front of it would focus
at 50 cm. This eye, then, equals a myopic eye of 2 D.
This myopic eye would have a far point of 50 cm. Where
^ /the rays of light meet as they come from a myopic eye in a
I state of rest is its far point.
The Near Point of a Myopic Eye. This is always
^>| closer than in the emmetropic eye for a corresponding age,
and depends upon the distance of its far point. For exam-
ple, an eye at twenty-five years has 8.5 D. amplitude of
accommodation, but if it has a far point of 70 cm., then its
near point will be represented by 8.5 D. and 70 cm., /. c.,
1.5 D., which would equal 10 D., or a near point of 10 cm.
. ^C-^W^
The following table gives the comparative near points jn an
emmetropic eye, a hyperopic eye of 2 D., and a myopic eye
of 2 D. :
AGE.
10
15
20
25
30
35
40
45
5
55
60
65
70
75
F.tnnu-tropia, p.p.
7
8.3
10
12
14
18
22
28
40
5S
100
UT
400
at
2 1 ). 11 vperopia, "
8.3
10
125
16
20
28.5
40
66
200
00
2 D. Myopia, "
6
7
-3
10
11
13
15-3
"
22
25
33
36
44
50
Determining the Vision. This may be considered as
the method of finding out what an eye can see without any
lenses placed in front of it ; in other words, determining the
vision may be defined as ascertaining the seeing quality of the
7
74
REFRACTION AND HOW TO REFRACT.
_
L 6 A
T G"Y D
FAVHU
DLOECN
CPAR'CEOU
unrefracted eye. The refraction of an eye should never be
confounded with the visual quality, as refraction applies to
the refractive media ; for example, an emmetropic eye with
a hemorrhage at the fovea would be practically without
visual quality, and yet its refraction or refractive condition
would be standard. The most acute vision is at the fovea
and the region immediately surrounding it, but this sensi-
bility diminishes as the fovea is departed from and the per-
ipheral portion of the retina approached ; this is due to the
fact that the cones are as
close as 0.002 mm. at the
macula, and not so close or
C_ f>^ numerous in the forepart of
^^ *^ the eye-ground.
Test-type or Test-let-
terS f r Distant vision --
To determine the vision we
employ cards on which are
engraved test-type or letters
of various sizes, constructed
so that each letter subtends
an angle of five minutes, as
suggested by Snellen, and
described on page 64.
Figure 73 shows such cards of test-letters, reduced in size.
The Roman characters just over the top of the letters indi-
cate the distance in meters that the letters should be seen by
the standard eye, and the little figures at the left of the letters
indicate the equivalent distance in English feet. The top
letter should be seen at 60 meters, and the bottom letters
at 3 meters ; the intervening letters are to be seen at the
respective distances indicated. As it is not unusual to find
eyes that have a seeing quality better than that obtained
O A L
G Y D D T
n
VFHUA
LE C~H D O
GEORLCPA
FIG. 73. Randall's Test-letters. Block
letters in black on cream-colored
cards.
TEST-TYPE OR TEST-LETTERS FOR DISTANT VISION. 75
with Snellen's type constructed on the angle of five min-
utes, Dr. James Wallace has constructed letters which sub-
tend an angle of only four minutes. Such a card is shown
in figure 74 and has a large field of usefulness. While
test-cards are usually white or cream-colored, with black
letters, Gould has white letters constructed on black
cards. (See Fig. 77.) As white stimu-
. Cj lates the retina and black does not, it
* will be recognized at once that in one
-Q ^3 instance the card, and in the other the
letters, produce the retinal stimulation.
The white letters seem to stand out from
the black card al-
most as if they were
embossed, giving a
clear-cut edge and
most soothing effect
to the eye under ex-
amination, and can
be recognized when
subtending a much
smaller angle than
the black letters. To
this card should be
-o o o
-X X Y V
a a 8 H M
-1 1 O O A R
V V T 3 S X
*AOO8JT
FIG. 74. Four Min-
ute Letters of Dr.
J. Wallace. This
card is constructed
principally for re-
flection purposes.
(See Fig. 7.)
->javoid reflection,
hung at an angle.
For aliens who do not know the
English letters, and for illiterates, a
special card has been made, known as
the illiterate or "dummy" card, with
characters consisting of lines shaped
111
3 LU
E 111 3
E 3 ui m
ui 3 E
FIG. 75.
like the capital letter " E," and made to conform to the five-
minute angle. As these letters are variously placed, the
patient is asked to tell, or indicate with his finger or fingers,
76
REFRACTION AND HOW TO REFRACT.
the direction in which the prongs of the "E" point: up,
down, to the right or left. This illiterate card (see Fig.
75) is much to be preferred to the German, Hebrew, and
" figure " cards occasionally displayed in clinics.
Kindergarten Card. To keep the attention of children
who do not know the alphabet, or who attend a Kinder-
garten school, the author has
prepared a card of test objects
as shown in figure 76. These
objects have been drawn up
to the angle of five minutes,
j . though some of their com-
ponent parts do not measure
Um up to one minute.
^^ Selection of Test-cards.
^J f iMt ^ e surgeon should have
several of these in duplicate
with the order of the letters
changed (Figs. 73, 77), as pa-
tients not infrequently and
unintentionally commit them
to memory. Care should be
exercised in the selection of
test-cards, to see that each
letter on the card measures
O
t
o a
*
A
a a T o &
FIG. 76. Author's Test Objects or
Kindergarten Card for Children
and Illiterates.
up to the standard square of
five minutes, as many of the
A's and R's and N's, etc., on
the old cards as seen in the
shops measure six and seven minutes horizontally. It is
a matter of choice with the surgeon whether to use test-
cards with the block or Gothic letters. It is well to have
both.
THE RECORD OF THE VISUAL ACUITY.
77
Method of Procedure. The test-card should be hung
on the wall with its ^ line five or six inches below the
level of the patient's eyes,, and illuminated by means of
reflected artificial light. This is always a certain quantity,
whereas daylight is too variable and not to be depended
upon. The patient should be placed with his back toward
any bright light, and at a distance of six meters from the card.
FIG. 77. Gould's Test-letters. Gothic letters in white on black cards.
Sometimes the surgeon's office is not six meters long, and
this distance must be obtained by using diagonal corners
of the room or by using a plane plate-glass mirror and a
specially prepared test-card with reversed letters (see Fig.
74), the card being hung as many meters in front of the
mirror as will make six meters when added to the length
78 REFRACTION AND HOW TO REFRACT.
of the office. While a distance of six meters is always to
jbe preferred, yet if this can not be obtained, the surgeon
I may use a distance of four meters, but never less than this.
Each eye should be tested separately, the fellow-eye being
shielded or covered by a card or opaque disc held in front
of it or placed in the trial-frame. The eye should never be
d shut, and any pressure upon the eyeball must be
avoided.
The record of the visual acuity is usually made in the
form of fractions, using Arabic or Roman notation ;
figures usually indicate feet, and Roman letters usually sig-
meters^ though there is no fixed rule for this. How-
ever expressed, the denominator indicates the size of the
I frjt^typjs which the eye reads, at (The distance indicated by the
y <fr numerator^ For example, if at VI meters the eye reads
I ^ jiR 6 l me f letters marked VI, then the record would be
J^ y|. This would be |-[J- if the numerator and denomina-
tor were expressed in feet. If the eye, at a distance of
VI meters, reads only the letters on the XII line, then
the record would be ^j, or |g- (feet). If the top letter
was the only one recognized at the distance of six meters,
then the record would be ^ (meters), or -f^ (feet). If
the eye reads the VI line, miscalling two letters, then the
record could be made in one of three ways, each indicating
the same thing. ~ ? ? (one question mark for each
miscalled letter), or "y| partly," would indicate that
the eye saw yj, but not each letter correctly. This way of
(making the record is not so explicit as that with question
marks. Or, ^yj^r+ would mean that the eye saw all of
^j^ and some of the letters of -^j- ; but this, too, is not so
definite as the first record and the one recommended.
DETERMINING THE NEAR POINT. 79
If the eye can not recognize any letter on the card at the
distance of VI meters, then the card should be brought
toward the patient, or the patient told to approach the card,
until the eye cznjnst make out the top letter and no more.
If this is seen at IV meters, then the record will be ~ ; if at
one meter, the record would be - r, etc. While it has
l*A
been stated that the visual record is usually made in the
form of common fractions, as just- described, yet there are
some who prefer to make the record in the form of deci-
mals ; namely, a vision of- would be.i.o, a vision of -
7 VI XII
would be 0.50, or a vision of j^j^ would be 0.25, or a
vision of -- would be o. I. "TVIost authorities prefer to make
their records in the form of common fractions.
In some instances the eye may not be able to distinguish
ain- letter on the card, no matter how close it may be
brought to the eye, and in such a case the vision is tested
holding the outstretched fingers between the patient's
eye and a bright light (an open window), and a note is
e of the greatest distance at which the eye can count
fingers ^if at ten inches, the record would be " fingers
counted at ten inches," or whatever the distance may be.
/This ability to recognize form is spoken of as " qualitative
light perception." Eyes that are not able to recognize
/form may still be able to distinguish light from darkness,
C^and this ability is tested by alternately covering and uncov-
ering the eye as it faces a light, or as light is reflected into
it from a mirror. If " qualitative light perception " is pres-
ent, the vision is recorded/t^P. , which means " light per-
jception," or the record may be made L. & S., which means
x^lpractically the same thing, "light and shade."
Determining the Near Point. Having obtained and
recorded the distant vision of an unrefracted eye, it is well
8O REFRACTION AND HOW TO REFRACT.
to also find out and note what is the nearest point to the
eye at which small type can be made out ; this is spoken
of as determining the near point.
Test-type or Test-letters for Near Vision. To deter-
mine the near point, we employ cards on which are printed
or engraved words or sentences, or a series of letters, so
that each letter in each word or sentence shall subtend an
angle of five minutes, at a given distance from the standard
eye ; for instance, letters that are to be seen at one meter
and occupy the angle of five minutes, must be 1.425 mm.
square ; letters that are to be seen at half a meter distance
must be 0.712 mm. square, etc. (Most of the "near" cards
in the market are very defective in this respect, and the near
types of Jaeger are becoming obsolete, as they are not
standard letters, but merely represent the various fonts of
printers' type.) The writer's card is one of Gothic type, as
shown in figure 78. Another card in block letters is shown
in figure 79. Above each series of letters is marked the
greatest distance (D) at which the respective letters can be
3een; these distances vary from 0.25 to 2 meters (25 to
200 cm.), which are ample for all purposes in estimating
the near point.
Method of Procedure to Find the Near Point. The
patient is seated so that the light entering the room will
come over his shoulder and fall upon the card of test-type
held in front of him. The surgeon, to one side of the
patient, holds the card in one hand and a meter stick in the
other, the eye which is not being tested is covered with a
card, and the patient is told to select the smallest type on
the card which he can read or spell, and as he continues to
do so (aloud), the surgeon gradually approaches the card
to the eye until the patient says that the letters commence
to grow "hazy" and he can scarcely decipher them; or
TEST-TYPE FOR NEAR VISION.
81
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82
REFRACTION AND HOW TO REFRACT.
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CONVERGENCE. 83
.'another way is to hold the card close to the patient's eye
'and gradually withdraw it until he can just recognize the
: letters ; when this point is reached, the distance from the eye
to tlie card is measured with the meter stick, and this dis-
tance, as also the size of the type which was read, is care-
fully recorded. For example, the patient selecting the type
marked 0.50 D. and is able to read it as close as 8 cm. and
no closer, the record will be " near point equals type 0.50
D. at 8 cm."; or abbreviated, would be "type 0.50 D.
= 8 cm."
In some instances the patient may not be able to read
any of the near type without the aid of a glass, and if so,
it will be necessary to place a plus sphere in front of the
eye to assist in finding the near point ; for example, if a
+ 28. was employed, then the record might be "near /' .- - -
point equals type 0.50 D. at 12 cm. with -}-2 S.," or " -{-2 /V^
S. = type 0.50 D. at 12 cm."
Convergence. Con, " together," and vergere, " to
turn " ; literally, turning together. This is the power of the
internal recti muscles (especially) to turn the eyes toward
the median line ; to " fix" an object closer than infinity.
Standard eyes, when looking at an object at a distance of
six meters or more, are not supposed to converge ; the
visual lines are spoken of as parallel and the power of con-
vergence is in a state of repose. The angle which the
visual line makes in turning from infinity ( oc) to a near
point is called -th~e"angle of convergence, and the angle
which is formed at one meter distance by the visual axis
with the median line is called tlTerneter angle, or the unit
of the angle of convergence. (See I, in Fig. 80.)
If the visual line meets the median plane at ^ of a
meter, it has then two-meter angles of convergence ; at
1^ of a meter, four-meter angles of convergence, etc. Or
8 4
REFRACTION AND HOW TO REFRACT.
five-meter angles means that the eye is converging to a
point ^ of a meter distant.
The size of the meter angle varies ; it is not the same in
all individuals ; in fact, the meter angle is smaller in children
than in adults, as a rule, on account of the shorter inter-
ocular distance. In children thisliistance is about 50 mm.,
whereas in adults it is, on the average, 60 or 64 mm.
While standard eyes, to see a point one meter distant
would converge just one meter angle, they would also
accommodate just one diopter ; to see a point at ^ of a
FIG. 80.
meter they would converge just three meter angles, and at
the same time would accommodate three diopters, etc.,
thus showing how intimately the powers of convergence and
accommodation are linked together, though it is possible
to converge without accommodation (see Presbyopia) or to
accommodate without convergence (paralysis of the in-
terni).
Far and Near Points of Convergence. Just as we have
a far and a near point of accommodation, we also have a far
and a near point of convergence. The far point of conver-
gence is the point to which the visual lines are directed
when convergence is at rest, or at a minimum. The near
ANGLE GAMMA. 85
point of convergence is the point to which the visual lines
are directed when the eyes are turned inward to their
utmost degree.
Infinity, or parallelism, is the position of the visual lines
in the standard eyes in^a state of rest (E o>, in Fig. 80).
Visual lines that diverge in a state of rest can only meet by
being projected backward, and, therefore, meet at an imag-
inary point behind the eyes (N, in Fig. 80) ; convergence is
then spoken of as negative, or minus ( ).
If the visual lines meet in a state of rest, then conver-
gence is spoken of as positive (-f ).
The amplitude of convergence is the distance measured
from the far point to the near point of convergence, and is
FIG. 81.
represented by the greatest number of meter angles of con-
vergence which the eyes can exert.
Angle Gamma. An understanding of what is known as
the angle gamma is important, that the observer may
understand and appreciate the real or apparent position of
the eyes when looking at a near or distant point. Figure 8 1
shows the line O A (optic axis) and the optic center, or
nodal point (N), is situated on this line in the posterior part
of the crystalline lens. The line V M is really a secondary
axis to this dioptric system of the eye, and unites the object
(V) with the fovea centralis at M ; this line is known as the
tst
fc^X. f m
tt*^tte/ ?^&9
S3 -a. ->^^*^
86 REFRACTION
/v
V- tf
AND HOW TO REFRACT.
"^H&* 4-^
#7w<
' ^ '^^ S~ '
visual line. ' The angle formed by the visual line with 'the
axis at the nodal point may be considered as the angle
gamma.* .
If the fovea centralis at M was situated on the optic axis
~*r~& A, then the visual line and optic axis would coincide,
and there would not be any angle gamma.
In hyperopic and emmetropic eyes the outer extremity
of the visual line lies $, 7,
'or, in some instances, as
much as 10 degrees to the
nasal side of the optic axis
(averaging about $ de-
grees), and is spoken of
as positive, and given the
plus sign. In some long
myopic eyes, however, the
outer extremity of the
visual line may lie to the
outer side of the optic
axis, when it is spoken of
as negative, and given the
minus sign.
To demonstrate the an-
gle gamma, the patient is
told to look at the point of a pencil or pen held in the
hand of the surgeon, about 1 3 inches distant (A in Figs.
82 and 83). If the angle gamma is positive, the eyes will
appear divergent to the observer, who looks at the position
* This is not a perfectly correct statement, as the real angle gamma is the
angle formed by the line of fixation V R with the optic axis, R being the
center of rotation. The angle V N O and the angle V R O being so nearly
equal, are, for all intents and purposes, considered as the same.
FIG. 82.
ANGLE ALPHA.
of the poles of the corneas or centers of the pupils. (See
Fig. 82.)
If the angle gamma is negative the eyes will appear
convergent that is, they appear to converge to a point in
front of the pencil. (See
Fig. 83.)
The amount of the
angle gamma can be
measured by using the
arc of the perimeter held
horizontally, the patient
being placed in the same
position as when having 1
his field taken. To do
this, while the eye fixes
the central point, the
surgeon passes a candle-
flame along the arc until
the catoptric image of
the flame is seen at the
center of the pupil ; this F g
position of the candle-
flame on the arc is noted in degrees, which is the size of the
angle gamma.
Angle Alpha. This is the angle formed by the long
axis of the corneal ellipse with the visual axis. In the
consideration of this angle it must be remembered that the
cornea resembles, in its central area, at least, an ellipsoid
of revolution, with the shortest radius usually in the verti-
cal meridian. The angle alpha is spoken of as positive
when the outer extremity of the long axis of the cornea is
to the outer side of the visual line, and negative when it
is to the nasal side.
<
CHAPTER III.
OPHTHALMOSCOPE.
DIRECT AND INDIRECT METHOD.
Ophthalmoscope. From oyffa^x;, "eye," and axo-siv,
" to observe " or " view "; literally, " to view an eye." An
instrument used for studying the media and interior of the
eye. The pupil of an eye in health appears to an observer as
black ; this is due to the fact that the observer's eye does
not ordinarilyintercept any of the rays of light which return
from the eye. Rays of light entering an eye are returned
toward their immediate source, and, therefore, if an observer
wishes to see into or study the interior of an eye, he must
have his own eye in the path of the returning rays. To
accomplish this, the observer places a mirror in front of his
eye, so that the reflected rays entering the eye are returned
toward the mirror. There is an infinite variety of these in-
struments in the market, but for the general student the
modified instrument of Loring appears to meet with most
favor. (See Fig. 84.)
This has a concave mirror with a radius of curvature of
40 cm., giving a principal focus, therefore, at 20 cm. The
sight-hole is round and about 3 ^ mm. in diameter, cut
through the glass ; this mirror can be tilted to an angle of
25 degrees. As an improvement over such a mirror, and to
take its place, the writer would recommend the mirror used
on his own ophthalmoscope, which has a radius of curvature
of 15 cm.; and the sight-hole, 2^ mm. in diameter, is not
cut through the glass, but is made by removing the quick-
88
OPHTHALMOSCOPE.
8 9
silver. The glass at the sight-hole gives additional reflect-
ing surface, and at the same time does away with much
annoying aberration which results when the glass is per-
forated.
FRONT
BACK
FIG. 84.
The small sight-hole is an advantage, also, in looking
into small pupils. The mirror, oblong in shape, 18 by 33
mm., is secured at the center of its ends, by two elevated
screws, to a hollow disc 4^ cm. in diameter, in which is a
revolving milled wheel, containing small spheres, each
about 6 mm. in diameter. The series of spheres ranges
8
9O REFRACTION AND HOW TO REFRACT.
from I D. to 8 D., and from -f I D. to +7 D. The
central aperture does not contain a lens, but is left open.
When it is desirable to use any lens stronger than 8
D. or + 7 D., there is an additional quadrant, which can be
superimposed and turned into place at the sight-hole ; it
contains four lenses, 0.50 D. and 16 D., also +0.50
D. and -{-16 D. With this quadrant and the spheres in
the milled wheel, any spheric combination can be made
from zero to 24 D. or to + 23 D. An index below the
sight-hole of the instrument records the strength of lens
FIG. 85.
that may be in use ; minus lens.es are usually marked in red
and plus lenses in white. pKwU^i V.\
How to Use the Ophthalmoscope. There are two ways
or methods by which the ophthalmoscope may be used
the direct and the indirect.
The Direct Method (see Fig. 85). Proficiency with
the ophthalmoscope does not come except from long and
constant practice, and several important matters should
receive very careful attention before the student attempts to
study the interior of an eye.
OPHTHALMOSCOPE. 9!
The Room. This should be darkened by drawing the
shades or closing the blinds ; the darker the room, the better.
The Light. This should be steady, clear, and bright ;
a good lamp is suitable, but an Argand burner gives more
intense light, and is to be preferred, especially if it is placed
on an extension bracket that can be raised or lowered
and is capable of lateral movement.
Position of Light and Patient. The light should be
several inches to one side and back of the patient, and on a
level with the patient's ear, so as to illuminate the outer half
of the eyelashes of the eye to be examined ; it may even
be well to have the tip of the patient's nose illuminated.
The patient should be seated in a comfortable chair
(without arms), and is instructed to look straight ahead into
vacancy, or at a fixed object if necessary, and is only to
change the direction of his vision when told to do so.
Under no circumstances should the patient be allowed to
look at a light, as this will contract the pupil.
For the beginner, it may be well to dilate the patient's
pupil with a solution of cocain or homatropin. The
student, however, should learn as soon as possible to see
into an eye without the aid of a mydriatic, as many patients
seriously object to the slight inconvenience that results
from the drugs mentioned.
The Observer. If the observer has any decided refrac-
tive error, he should wear his correcting glasses ; the reason
for this will be explained later. The observer should be
seated at the side of the patient corresponding to the eye
he is to examine. Examining the right eye, the observer
should be on the patient's right ; if the left eye, then on
the patient's left.
"When examining the right eye, the ophthalmoscope is
held in the right hand, before the right eye ; and in the left
92 REFRACTION AND HOW TO REFRACT.
hand, and before the left eye, when examining the left eye.
The silrgeon's eye should be a little higher than the patient's.
and observer should keep both eyes open. The
exception to this is when the patient has a squint,
when it will be necessary for him to cover the eye not being
examined, and in this way the eye under observation will look
straight ahead.
The surgeon holds the ophthalmoscope perpendicularly,
so that the sight-hole in the mirror is directly opposite to
his pupil and close to his eye. The side of the instrument
rests on the side of his nose or the upper margin is in the
hollow of the brow. The mirror is tilted toward the light,
surgeon's elbow should be at his side, and not form an
angle with his body.
With these several details carefully executed, the surgeon"
^begins his examination at a distance of about 25 or 30 cm.,
never closer ; and at this distance he reflects the light from
the mirror into the eye, and observes a " red glare," which
occupies the previously black pupil. This is called the
" reflex," and is due to the reflection from the choroidal
coat of the eye. The color of the reflex varies with the size
of the pupil, transparency of the media, the refraction, and
the amount of pigment in the eye-ground.
Having obtained the " reflex," it will be well for the be-
ginner to practise keeping the reflected light upon the pupil
by changing his distance, approaching the eye as close as
an inch or two ; this must be done slowly, and not with a
rush.
What the Observer Sees. Having learned to keep the
light on the pupil, the next thing is to study the transparency
of the media i. e., to find out if there is any interference
with the free entrance and exit of the reflected rays, such
as would be caused by opacities in the cornea, lens, lens
OPHTHALMOSCOPE. 93
capsule, or vitreous ; and, if present, to note their character
and exact location, whether on the visual axis or to one
side, etc. The next objective points will be mentioned
individually, and with the idea of systematizing the study.
The Optic Nerve. Also called the disc or nerve head
or papilla.
Color of the Optic Disc. This has been described as
resembling in color the marrow of a healthy bone, or the
pink of a shell, etc. ; yet this is not by any means a true
statement or description, as the apparent color of the nerve
controlled in great part by the surrounding eye -ground
whether this is heavily pigmented or but slightly so, or
whether there is an absence of pigment, as in the albino.
The student should be ready to make allowances for these
contrasts.
The shape of the disc varies : it may appear round, oval,
or even irregular in outline. Usually it is a vertical oval.-
The vessels on the disc which carry the blood to and
from the retina are not of the same caliber, nor do they
have the same curves and branches in all eyes or in the
same pair of eyes. The central artery may be single or
double (if it has branched in the nerve before entering the
eye), and enters the eye at the nasal side of the center of
the disc.
Approximating the central artery on its temporal side is
the retinal vein, which may also be double. The relative
normal proportion in size between arteries and veins is
nerally recognized as about two to three. The veins are
usually recognized by their larger size and darker color. *-
At or near the center of the disc is often seen a depression,
I known as the physiologic cup ; this may be shallow or
1 deep ; it may have shelving or abrupt edges ; it may even
ibe funnel-shaped.
94 REFRACTION AND HOW TO REFRACT.
At the bottom of the cupping is frequently seen a gray
stippling, the membrana cribrosa ; openings in the sclera for
the passage of the transparent optic nerve-fibers which go to
form the retina. Surrounding the disc proper is often seen
a narrow white ring ; this is sclera, and is known as the
^scleral ring. Just outside of this ring is frequently seen a
ring of pigment ; this is called the choroidal ring. In many
cases the choroidal ring is not complete, the pigment being
quite irregular, or possibly there may be just one large
>mass of pigment .to one side of the disc ; this is not patho-
logic.
The retinal arteries and veins, while possessing many
anomalies, and while occasionally an artery and vein are
seen to twine around each other, usually pursue a uniform
course up and down from the disc, and are named accord-
ingly i. c., upper nasal vein and artery ; upper temporal
' vein and artery ; lower, nasal artery and vein ; lower tem-
poral artery and vein.
T> The retina itself, in health being transparent, is not seen.
The fovea centralis, occupying the center of the macular
region, is about two discs' diameter to the temporal side
of the disc and slightly below the horizontal meridian.
The fovea is recognized because it is a depression, and its
edges give a reflex ; it is very small, and appears as a bright
J spot one or two mm. in diameter. The " macular region "
is the part of the eye-ground immediately surrounding the
fovea ; it contains minute capillaries, but it is impossible, in
Wealthy eyes, to recognize them with the ophthalmoscope.
The Choroid. This is distinguished by the character of
its circulation, the vessels being large, numerous, and flat-
^>tened, and without the light streak which characterizes the
retinal vessels. (iPigment areas between the vessels are also
diagnostic of this tunic) The choroidal circulation is best
OPHTH A LMOSCOPE.
95
studied in the blond or albino, and may be seen in many
eyes toward the periphery of the eye-ground.
In the foregoing description of the use of the ophthal-
moscope, etc., it is presumed that the instrument has been
used without any lens in position, and that the observer's
eye and the eye under examination are healthy emmetropic
eyes with the accommodation at rest. Figure 86 shows the
position of the light, L, the ophthalmoscope, the examiner's
and the examined eye under these conditions.
The divergent rays from the light (L) are reflected con-
FIG. 86.
vergently from the concave mirror, and focusing in the vitre-
ous, they cross' and form an area of illumination on the
retina at I I'. The retina, situated at the principal focus of
the dioptric media, naturally projects out from its indi-
vidual points rays of light which are parallel as they leave
the eye ; some of these pass through the sight-hole of the
mirror and meet upon the retina of the observer's emme-
tropic eye.
There are twt> very important points which must be
9 6
REFRACTION AND HOW TO REFRACT.
considered when using the ophthalmoscope in the direct
method : one is the direction which the rays of light take
> as they leave the eye under examination, and the other is
for the observer to keep his own eye emmetropic ; in other
words, the observer wearing his correcting glasses should
ji ot accommodate.
Figure 87 shows that rays of light passing out of an eye
divergently must be made parallel, so as to focus upon the
surgeon's own retina (emmetropic), and to do this it is
FIG. 87. T B indicate points at the edge of the disc from which rays pass
out of the eye divergently in the direction T x B', 1"' B', T' B', and being
received by the observer's eye, are projected backward, forming an erect
magnified image at T" W. This image is not so large as that seen when
looking into a myopic eye. (Fig. 88.)
necessary to turn a plus lens in front of the sight-hole of
the ophthalmoscope ; the strength of the convex lens thus
employed, other things being normal, is the amount of the
refractive error of the eye being examined.
Figure 88 shows rays of light passing out of an eye
convergently, and to have them parallel, so as to focus
upon his own retina (emmetropic), it is necessary to turn a
concave lens in front of the sight-hole of the ophthalmo-
OPHTHALMOSCOPE.
97
scope ; the strength of the concave lens thus employed,
other things being normal, is the amount of the refractive
error of the eye under examination.
The Observer's Accommodation. It has already been
stated that, when using the ophthalmoscope, the observer
should wear any necessary correcting lenses. If the ob-
server has a refractive error and does not wear his glasses,
he must deduct this amount from the lens used in the
ophthalmoscope. If he has two diopters of hyperopia
--B"
FlG. 88. T B indicate points at the edge of the disc from which rays pass
out of the eye convergently in the direction T' W , and, being received by
the observer's eye, are projected backward, forming an erect magnified
image at T" B" '. This image is much larger than that seen when look-
ing into the hyperopic eye. (Fig. 87.)
himself, and the lens used in the ophthalmoscope is plus
four diopters, then the eye under examination has only two
| diopters. It is not unusual for beginners to see the eye-
Aground (disc) in hyperopic eyes with a strong concave
lens ; this is due to the fact that they accommodate. Prac-
tice will overcome this habit, and it should be mastered as
- soon as possible. There are several ways of doing this :
one is to begin the examination at a distance of 30 or 40
9
98 REFRACTION AND HOW TO REFRACT.
cm. from the eye, with both eyes open, and to gradually
I approach the eye as close as 3 cm., imagining all the time
/ that one is looking for some remote point ; otherwise, if one
begins the examination close to the eye, and imagines he is
going to see an object about an inch away, he will most
invariably accommodate several diopters, with the result
^> that he turns a strong concave lens in front of the sight-
hole of the ophthalmoscope to neutralize his accommodation.
/ This explains how so. many beginners diagnose all cases
/of hyperopia as myopia. An excellent way to learn to
relax the accommodation is to practise reading fine print
at a distance of about thirteen inches through a pair of
plus three lenses, placed before the surgeon's emmetropic
eyes. Another good way to learn to relax the accommo-
dation is to practise on one of the many schematic eyes
found in the shops. (Fig. 143.)
Size of the Image of the Eye-ground (Figs. 87 and
88). In concluding the subject of the direct method of
examination it may be interesting to note the apparent size
of the image of the eye-ground, which, it must be remem-
^bered, is virtual, erect, and enlarged ; in fact, it seems to
be at some distance behind the eye, and if the student has
paid close attention to the study of images as formed by
convex lenses, detailed in chapter i, he need not have any
difficulty in appreciating these facts.
The optic disc of an emmetropic eye, as seen through
ophthalmoscope, appears to be about 25 mm. in diam-
iter, and about 250 mm. away. The retina of the emme-
tropic eye is about 1 5 mm. from its nodal point ; then the
actual size of the emmetropic disc is -/-$ of 25, or -f , or
1.5 mm. ; then 15 is to 250 as 1.5 is to 25, or 16.6 the
magnification, in other words, when the emmetropic disc is
observed, it appears about 16.6 times larger than it actually is.
]u~
I
OPHTHALMOSCOPE.
99
The Indirect Method (see Fig. 89). Practising this
method, the observer sees a larger part of the eye-ground
FIG. 89.
at one time, but it is not so perfect in detail nor is it mag-
nified to the same extent as in the direct method. The
observer does not have to get so close to his patient, which
is a decided advantage in some clinical cases. Unfortun-
ately, as a preliminary
step, it is often neces-
sary to dilate the pu-
pil. In addition to
the ophthalmoscope,
there is also required
a convex lens of
known strength and
large aperture ; the
one which comes in
the case with the
scope is usually too small and too strong for general use.
The writer prefers his plus 13 D. with metal rim and conve-
nient handle, shown in figure 90 (reduced one-third in size).
FIG. 90.
IOO REFRACTION AND HOW /to KKFKACT.
_r* X
This is held at about three inches in front of the eye
under examination, the observer resting his little and ring
fingers on the temple of the patient. The light may be
7 over the patient's head, or to the side corresponding to the
eye under examination, the patient being instructed to look
with both eyes open toward the surgeon's right ear when
ithe right eye is being examined, and toward the surgeon's
left ear when the left eye is examined.
With a +4 D. in the ophthalmoscope held close to his
eye, the surgeon seats himself in front of the patient at
.*> about sixteen inches distant, and reflects the light through
the condensing lens into the patient's eye, and then
approaches or moves away from the eye until he recog-
nizes clearly a retinal vessel or the disc ; he must remem-
ber, however, that he is not looking into the eye, but is
(viewing an aerial image formed between the convex lens
land the ophthalmoscope ; this image is not only inverted,
but undergoes lateral inversion, so that the right side
of the disc becomes the left side of the image, and vice
versa ; the upper side of the disc becomes the lower side
of the image, and vice versa. As the direct method gives
Ian erect, virtual, and enlarged image, the indirect method
\produces an inverted, real, and small image. The principle
pf the direct method is similar to a simple microscope, and
the indirect to a compound microscope.
I The size of the image depends upon the refraction of
( the eye and the distance of the convex lens from the eye
under examination. Mn the standard eye this is always the
same, no matter how far away from the eye the convex lens
is held. )To estimate the size of the image in the standard
eye, all that is necessary to know is the principal focal dis-
tance of the lens employed ; if a +13 D., then the image
is formed at 75 mm. (three inches), and remembering that the
"
OPHTHALMOSCOPE. IOI
retina in the eye is 1 5 mm. back of the nodal point, the
size of the image will be to the size of the disc (if that is
what is looked at) as their respective distances, or as 15 is
to 75 which equals 5, the magnification.
The purpose of the +4 D. in the scope is to take the
place of the eve -piece in the microscope, and, therefore, to
magnify the image at the same time it relieves the observer's
(accommodation. In high myopia the -(-4 D. may be dis-
pensed with.
The Luminous Ophthalmoscope (Figs. 91 and 92).
DcZeng Patent. This instrument is a combination of the
Loring ophthalmoscope just described, with the addition of
an electric light attachment. The mirror is somewhat dif-
ferent from the mirror on the Loring instrument. It is
f
plane, circular in form, 14 millimeters in diameterfwith one
millimeter of its upper area cut off horizontally^- The flat
top edge of the mirror is on a level with the lower edge of
the sight-hole in the ophthalmoscope. The observer in
looking through the sight-hole always looks over the mirror
and never through it. J The mirror is placed at an angle of
43 degrees. The handle of the instrument carries the elec-
tric wires to a small lamp, and between the lamp and the
mirror is placed a very strong convex lens. The rays of
light from the filament falling upon the convex lens are
refracted very coiivergently, and after reflection from the
mirror converge to a point one inch distant. (See Fig. 92.)
This instrument is ideal for both the direct and indirect
method and has the following points of merit: The mirror
and light arc stationary, thus giving the observer any
liberty of movement necessary without any loss of reflec-
tion from the mirror; the mirror never requires any tilting;
the brilliancy or intensity of the illumination at the ftindus,
by virtue of the light being so close to the mirror, far ex-
IO2
REFRACTION AND HOW TO REFRACT.
ceeds that of the nonluminous instrument; for the same
reason the size of the retinal illumination is made about five
FIG. 91. FIG. 92.
FIGS. 91 and 92. DeZeng Luminous Ophthalmoscope. Two-thirds size.
times larger than that by the old style instrument. The heat
from the electric lamp (2^4 volts, ^ ampere) is infinitesimal.
CHAPTER IV.
EMMETROPIA. HYPEROPIA. MYOPIA.
KMMETROPIA.
Emmetropia ('>, "in"; plrpw, "measure"; fy 1 ,
" eye ") literally means an eye in measure, or an eye which
has reached that stage of development where parallel rays
of light will be focused on its retina without any effort of
accommodation. As the emmetropic eye is the ophthal-
mologist's ideal unit of measurement or goal in refraction,
the beginner should know this form of eye thoroughly, so
that he may recognize any departure from this standard
condition. The emmetropic eye may be described in vari-
ous ways, and while these descriptions may appear like
repetitions, they are given for purposes of illustration :
, The standard or schematic eye: Authorities differ some-
what in the exact measurements of a schematic eye, but
the one suggested by Helm-
holtz is certainly worthy of
careful consideration. (See
P- 59-)
An emmetropic eye is one
which, in a state of rest
(without an}- effort of accom- FIG. 93.
modation whatever), receives
parallel rays of light exactly at a focus upon its fovea.
(See Fig. 93.)
An emmetropic eye, therefore, is one which, in state of
rest, emits parallel rays of light. (See Fig. 93.)
103
104
REFRACTION AND HOW TO REFRACT.
An emmetropic eye is one
whose fovea is situated exactly
at the principal focus of its re-
fractive system. (See Fig. 93.)
*l An emmetropic eye is one
the vision of which, in a state
of rest, is adapted for infinity.
\ ' An emmetropic eye is one
[which has its near point consist-
ent with its age. (See p. 70.)
An emmetropic eye is one
which does riot develop pres-
7byopic symptoms until forty-
five or fifty years of age. (See
p. 272.)
\ An emmetropic eye, in con-
tradistinction to a myopic eye
(see p. 115), is spoken of as
ia healtKy eye, or one which
A shows the least amount of irri-
tation in its choroid and retina.
Because we refer to Hclm-
holtz's schematic eye as an em-
metropic eye, it will not do to
say that all eyes that measure
just 23 mm. in their antero-
posterior diameter are emme-
tropic (Fig. 94); for while an
FIG. 94. I. Emmetropia. 2. Myopia
due to a strong lens. 3. Hyperopia
due to a weak lens. 4. Myopia due
to a short radius of curvature of cornea.
5. Hyperopia due to a long radius of
curvature of cornea. The anteropos-
terior diameter of all these eyes is just
23 mm.
AMETROPIA. IO5
eye maybe just 23 mm. in length, it may have its refractive
system stronger or weaker than is consistent \vith its length,
making it,. if stronger, a myopic or long eye, and, if weaker,
a short or hyperopic eye. An eye, to be emmetropic,
therefore, no matter what its length, must have its refractive
apparatus of just such strength that, in a state of rest, the
principal focus will coincide exactly with the cones at the
fovea. (Fig. 93.)
cu AMETROPIA.
Ametropia (a priv. ; /^r/>..>, "a measure" ; o<?>s, "sight")
literally means " an eye out of measure." An ametropic
eye is one which, in a state of rest, docs not form a distinct
image of distant objects upon its retina. An ametropic
eye may be defined as one which, in a state of rest, does
not focus parallel rays of light upon its fovea. fAn eye
which is not emmetropic is ametropic.J) There are two
of ametropia axial and curvature ametropia.
Axial ametropia is the condition in which the dioptric
apparatus refracts equally in all meridians, but the retina
of the eye, when at rest, is either closer to, or further away
from, the nodal point than the principal focus. (See Figs.
95 and 97.) The refraction is measured on the length of
the anteropostcrior axis of the eye ; hence its name, axial
metropia.
Curvature ametropia, in contradistinction to axial ame-
tropia, is the condition in which the dioptric apparatus does
not refract equally in all meridians, and with the result that
there is no focusing of all the rays at any one point ; or
curvature ametropia may be considered as that condition
in which parallel rays of light entering an eye have two
focal planes for two principal meridians at right angles to
each other. Curvature ametropia is commonly spoken of
as astigmatism. (See Chap, v.)
| ____
IO6 REFRACTION AND HOW TO REFRACT.
Varieties of Axial Ametropia. Axial ametropia is of
two forms : one in which the eye has its fovea closer to the
Idioptric apparatus than its principal focus (see Fig. 95),
| known as the hyperopic, short, or flat eye ; and the other
form of the eye in which the fovea is further away than its
principal focus, known as the myopic or long eye. (See
Fig. 97.)
HYPEROPIA OR HYPERMETROPIA.
Hyperopia (t>-lp, "over"; &?, "eye") literally means
an eye which does not equal the standard condition, or an
eye which is less than the standard measurement. Hyper-
opia is often abbreviated H. About twenty per cent,
of all eyes have simple hyperopiaj) The hyperopic eye is
spoken of as far-sighted, and the condition as one of far-
sightedness. The hyperopic eye may be described in many
different ways :
1. The " natural eye," or "the eye of nature." tf> X^n^***
2. The "short eye." This term is used on account of
its fovea lying closer to the dioptric apparatus than the
principal focus.
3. Parallel rays of light passing into a hyperopic eye in
P a state of rest fall upon its retina or fovea before they focus.
I (See Fig. 7 1 .)
4. Rays of light from the fovea of a hyperopic eye in a
state of rest pass out divergently (see Fig. 95), and the
condition is equivalent to a convex lens refracting rays of 1
light which proceed from a point close'r to the lens than its
principal focus. (See Fig. 39.)
5. A hyperopic eye is one which, in a state of rest, can
receive only convergent rays of light at a focus upon its
fovea (Fig. 95) ; therefore, to repeat : the hyperopic eye, in
a state of rest, emits divergent rays and receives convergent
rays at a focus upon its fovea.
HYPEROPIA. IO7
6. As convergent rays are not found in nature, and are,
therefore, artificial, a hyperopic eye is one which, in a state
of rest, requires a convex lens to focus parallel rays of light
on its fovea. (See Fig. 96.)
7. A hyperopic eye is one which must^accommodate for
infinity, and, in fact, for all distances ; in other words, a
hyperopic eye in use is in a constant state of accom-
modation.
8. A hyperopic eye having to use some of its accommo-
dative power for infinity, must, in consequence, have its
near point removed beyond that of an emmetropic eye of
corresponding age. (See p. 72.)
9. From the description contained in 3, it follows that
FIG. 95. FIG. 96.
the far point of a hyperopic eye in a state of rest is negative
( ), and is found by projecting the divergent rays back-
ward to a point behind the retina. (See Fig. 95.)
10. From the description contained in 6, and the descrip-
tion of accommodation on page 67, it is natural to find the
ir.
retina and choroid of many hyperopic eyes in a state of
irritation.
11. From the description contained in 6 and 7, and on
page 273, it follows that symptoms of presbyopia manifest
themselves earlier in hyperopic than in any other form of
eyes.
12. From the description contained in 5, and this may
IO8 REFRACTION AND HOW TO REFRACT.
appear like repetition), it follows that a hyperopic eye will
'"* accept a plus glass for distant vision. (See Fig. 96.)
13. From 6 it is evident that the circular fibers of the
ciliary muscle must become highly developed ; much more
so than the longitudinal fibers. Microscopically, a section
of the ciliary muscle on this account will bear evidence of
>iS ,/^th(f character of the eye from which it came.
Causes of Hyperopia. It is a well-known fact that the
^eyes of the new-born are, with comparatively few excep-
tions, hyperopic ; such eyes are supposed to grow in their
anteroposterior diameter, and at adolescence to reach that
stage of development called emmetropia. It is also a well-
known fact that this ideal condition of emmetropia is very
rarely attained, the length of the eyeball not increasing in
proportion to the strength of its refractive system.
Eyes may approximate the emmetropic condition, but
very seldom remain so, passing into the condition in which
the fovea lies beyond the principal focus, becoming what is
known as long, or myopic.
A standard eye may be made hyperopic by removing
its lens ; the condition following cataract extraction. (See
Fig. 174.)
/ An eye may possibly become hyperopic in old age, from
flattening of the lens due to sclerosis of its fibers.
Any disease which will cause a flattening of the cornea
in a standard eye will produce hyperopia.
^>A. diminution in the index of refraction of the media
of the standard eye will produce hypcropial^^Aist/Tfy
Subdivisions of Hyperopia. For purposes of study
hyperopia has been divided into six classes or forms :
I. Facultative hyperopia (abbreviated Hf.) is a condi-
tion of the eye in which the patient can overcome the error
by using his accommodation. It is a condition of early
H\TEROPIA. IO9
t
life, and is voluntary. The patient can see clearly, with or
without a convex glass.
2. Absolute hyperopia (abbreviated Ha.). This is hy-
Iperopia that can not be overcome by the accommodative
I effort. It is generally a condition of old age, and is invol-
untary ; facultative hyperopia in youth becomes absolute in
old age. ^ Old age, in fact, may develop each variety except
latent hyperopia. Absolute hyperopia exists whenever the
defect is of so high a degree that it can not be overcome
by the accommodation or when the accommodative power
itself is gone.
3. Relative hyperopia (abbreviated Hr.) is where ac-
commodation is assisted in its efforts by the internal recti
muscles ; in other words, the eyes squint inward.
4. Manifest hyperopia (abbreviated Hm.) is repre-
sented by the strongest convex lens through which an eye
can maintain distinct distant vision. Manifest hyperopia,
therefore, includes facultative and absolute.
5. Latent hyperopia (abbreviated HI.) is the amount
\of hyperopia which an eye retains when a plus lens is
'' (placed in front of it. /Or latent hyperopia is the difference
between the manifest hyperopia and that lens which an eye
would select if its accommodation was put at rest with a
cycloplegic (atropin)J For example, an eye accepts a
-f- 1.25 S. as its manifest H., and, when atropin is instilled,
would accept +2.75 S. for the same distant vision ; then
the difference between the manifest +1-25 S. and +2.75
S. (the total) is +1.50 S., which is the latent hyper-
opia.
6. Total hyperopia (abbreviated Ht.) is the full amount
--> of the hyperopia ; or is represented by the strongest glass
which an eye will accept, and have clear, distinct vision
when in a state of rest. &/v
IIO REFRACTION AND HOW TO REFRACT.
Symptoms and Signs of Hyperopia.- These are many
and various ; the principal one, however, and the one that
^generally causes the patient to seek relief, is headache.
Headache caused by the eyes is usually frontal, and is
denominated " brow ache " ; it may be frontotemporal ;
the pain or discomfort starting in or back of the eyes may
extend to the occiput or all over the head, and be accom-
panied with all kinds of nervous manifestations. The most
characteristic distinguishing feature of ocular headache is
that it comes on while using the eyes, and gradually grows
worse as the use of the eyes is persisted in ; and, likewise,
the headache gradually ceases after a few minutes' or hours'
rest of the eyes. Vertex headache, or a feeling of weight
on the top of the head, has been preempted by the gyne-
cologist, and is not usually classed as ocular. The ciliary
muscle being the prime factor in causing the headach
the writer feels justified in calling it the " headache mus-
cle." " Sick headaches " are largely due to eye-strain.
Various functional disorders, such as dyspepsia, constipa-
tion, biliousness, lithemia, chorea, convulsions, epileptoid
diseases, hysteria, melancholia, etc., are, according to some
few authorities, attributable to this condition. See Asthen-
opia, page 219.
Blepharitis marginalis, styes, and conjunctivitis are
frequently present, and in truth the hyperopic eye on
^ this account can often be diagnosed in public outside
of the surgeon's office. A feeling as of sand in the eyes,
ocular pains or postocular discomfort, a dryness of the
iids, as if they would stick to the eyeballs, are common
complaints, and part of the conjunctivitis. Other patients
have their eyes filling with tears (epiphora) as soon as they
-jr begin reading, etc. A drowsiness or desire to sleep often
comes on after or during forced accommodation.
r
/
HYPEROPIA. I I I
Congestion of the choroid and retina, as evidenced by the
ophthalmoscope, often go together with the blepharitis and
conjunctivitis.
The patient complains that the print blurs or becomes dim
after reading, and this is especially apt to occur by artificial
light. When the "blur" comes on, he has to stop and
rub his eyes or bathe them ; and then, with additional light,
he is able to continue the reading for a short time longer,
when the blur again returns and the effort must be given up.
Strong light stimulates the accommodation. The " hyper-
> opic blur" is nothing more or less than a relaxation of the
accommodation.
In children hyperopia sometimes simulates myopia, from
the fact that the child in reading holds the print very close
to the eyes. He does this in order to get a larger retinal
image and to relieve his accommodation ; the retinal image
is not clear, and the child has to read slowly ; the retinal
image is composed mostly of diffusion circles. The child
holds the print close to his eyes to avoid using his total
accommodation, which he might have to do if he held the
print at a respectable distance.
He also calls into play the orbicularis palpebrarum, and
narrows the palpebral fissure, looking through a stenopeic
slit, as it were. These cases of simulated myopia can be
7
jquickly diagnosed by :
I. The narrow r palpebral fissure during the act of reading,
and reading very slowly, as each letter has to be studied.
7 2. The fact that very few children have myopia.
3. The comparatively good distant vision, as a rule,
which myopes never have, unless the myopia is of very
small amount.
4. The ophthalmoscope.
The beginner in ophthalmology should be on his guard
I 1 2 REFRACTION AND HOW TO REFRACT.
for these "pseudo-myopias," and not be guilty of putting
concave lenses on hyperopic eyes.
Diagnosis of Hyperopia. This form of ametropia may
be recognized in many ways :
1. Blepharitis marginalis, if present, is generally due to
hyperopia.
2. Hyperopic eyes are said to be small, and to have
. small pupils, which facts are generally confirmed ; but
myopic eyes sometimes appear small, and have small pupils
also.
3. A narrow face and short interpupillary distance are
quite indicative of hyperopia, but these indexes are not
infallible.
4. A child with one eye turned inward toward the nose
,
' (convergent squint) has hyperopic eyes, as a rule ; the
hyperopia generally not being of the same amount in the
two eyes, the squinting eye usually being the more
hyperopic.
5. It has been authoritatively stated that light-colored
irises are seen in hyperopic eyes and dark irises are to be
\found in myopic eyes, and yet this is not always correct.
German students, with their blue irises, will average from
50 per cent, to 60 per cent, of myopia.
I 6. Hyperopic eyes, with few exceptions, have excellent
/ distant vision : often ~, or even better. The student should
be on his guard for this, and not imagine, because a
patient has vision, that he is emmetropic ; on the con-
trary, hyperopic eyes accommodate for distance, and obtain
this acute vision by effort.
7. The patient gives a history of accommodative asthe-
nopia, with or without headaches coming on during or
after the use of the eyes.
8. The distant vision of a hyperopic eye may remain
MYOPIA. -II3
unchanged or may be improved with the addition of a
convex lens, which latter would be impossible in emme- /
i ">/ A^^Lti^i* ->t^ &^r
tropia and myopia. ' '^C~^'t^ r*4- -7^^ t-*c* *
9. The near point of a hyperopic eye without glasses lies
beyond that of an emmetropic eye for a corresponding age.
10. A hyperopic eye can see fine print clearly through a
convex lens at a greater distance than its principal focus,
which would not be the case in any other form of eye.
Other tests for determining hyperopia are with (i i) the
ophthalmoscope, (12) the retinoscope, (13) Scheiner's test,
(14) Thomson's ametromcter, and (15) the cobalt-blue
glass test, commonly spoken of as the chromo-aberration
test. These tests are described in the text'
MYOPIA. ..,,
Myopia (v^b, "to close"; a><}>, "eye") means, liter-
ally, "to close the eye," and this origin of the name has
arisen from the fact that many long eyes (myopic) squint
the eyelids together when they endeavor to see beyond
their far pointNBrachymetropia is another name occasionally
mentioned for the same kind of eye. Myopia is abbreviated
About 1.5 per cent, of all eyes have simple myopia.
The myopic eye is spoken
of as near-sighted, and the
condition as one of near-
sightedness. The myopic
eye may be described in
many different ways :
1. The long eve. The
- r IG. 97-
origin of this name is
purely anatomic, thoffovea lying beyond the principal focus
of the refracting system. (See Fig. 97.)
2. Parallel rays of light entering a myopic eye focus in
114
REFRACTION AND HOW TO REFRACT.
the vitreous humor before they can reacli the fovea. (See
Fig- 97-)
3. Rays of light from the fovea of a myopic eye pass out
of the eye convergently (see Figs. 72 and 98), focusing at
FIG. 98.
some point inside of infinity. The refractive condition of
ja myopic eye is similar or equivalent to a convex lens
-^ Irefracting rays of light which proceed from some point
(further away than its principal focus. (See Fig. 37.) The
/nearer the emergent rays of light focus to the eye (in a
state of repose), the longer the eye ; and the further away
the emergent rays focus from the eye, the nearer the eye
approaches to emmetropia, or normal length.
4. A myopic eye is one which receives rays of light
which diverge from some point closer than six meters at a
focus on its fovea and which
emits convergent rays. (See
Fig- 37> ar >d also description
of conjugate foci.)
5. As parallel rays can not
focus on the fovea of a
myopic eye, it is necessary to
give parallel rays entering the
eye a certain amount of divergence, so as to place the focus
at the fovea ; and to accomplish this, a concave lens must
be used. (See Fig. 99.) A myopic eye, therefore, is one
FIG. 99.
MYOPIA. 115
which requires a concave lens to improve distant vision.
(See Fig. 99.)
/6. A myopic eye is one whose distant vision is made
worse by the addition of a convex lens.
7. A myopic eye is one which does not accommodate for
,. t
distance.
8. A myopic eye having a refracting system stronger
than is consistent with its length, or vice versa, greater
length than is consistent with its dioptric system, naturally
does not use any accommodation except for points inside of
its punctum remotum, and with the result that its ampli-
tude of accommodation is used near by ^consequently, a
myopic eye is one which has a near point closer than an ,
emmetropic eye of corresponding age. ) (See p. 73.)
9. From the description contained in 3 it follows that the
far point of a myopic eye is positive (jj^). j^i vry(
10. From the description contained in 3 and 7, it also
follows that the myopic eye does not develop presbyopic
symptoms until late in life.
1 1. From 6 and 9 it follows that the circular fibers of
the ciliary muscle are not used to the same extent in a
myopic eye as in the emmetropic and especially in the
hyperopic eye. Microscopically, a section of a ciliary
muscle on this account will bear evidence of the character/
| of the eye from which it came, and have the longitudinal
' fibers more in evidence. In some very long myopic eyes
there may not be any circular fibers recognized.
i 2. I -AX-S in which the myopia is progressive are spoken
of as " sick eyes."/ "r Ln^*+*-^*-*-<~ >lE>2<*-*<^- P*J*-*-~*^'
" ' ~ ^
Causes of Myopia. Any disease or injury which will /
so alter the refracting system of an eye that parallel rays /
must focus in front of the fovea will produce the form of <^ r *-**^-^
eye known as long or myopic. This may be brought about >
in different ways : A shortening in the radius of curvature
of the cornea, such as comes with conic cornea and staphy-
loma of the cornea ; an increase in the refractive power
^ &*
of the lens from swelling, as often precedes cataract, and
is spoken of as "false second sight ; /cyclitis and irido-
cyclitis, which diseases cause- a relaxation of the lens
ligament, allowing the lens to assume a greater convexitvj
or ciliary spasm may produce temporarily the same con-
dition.
Technically, however, myopia is quite universally under-
stood to mean a permanent elongation of the visual axis
of the eye beyond the principal focus of its refracting
system.
Heredity is certainly a predisposing factor to myopia,
but this does not mean that the babe is necessarily born
with long eyes. On the contrary, the eye is very likely
hyperopic at birth, and/what the child may inherit is weak
(eye tunics,) Such eyes, when placed under strain or what
xfto them is overuse, soon become elongated. This may
also be brought about or assisted by poor hygienic surround-
ings, poor health, or develop after an attack of typhoid
or one of the~eruptive fevers.
.T^Three causes for the elongation of eyes have been brought
forward by able authorities and expounded as theories, any
one of which, or all three, may appear conspicuously in in-
dividual cases.
1. Anatomically, the size of the orbit and the broad
face give a long interpupillary distance and cause excessive
convergence (turning inward of the eyes) when the eyes fix
at the near point.
2. Mechanically, when the eyes are far apart and
attempt to converge, the external recti muscles press upon
the outer side of the globes, flattening the eyes laterally,
MYOPIA. 1 1 7
with the result that the point of least resistance for the
compressed contents of the globes is at the posterior pole
of the eye, and here it is that the pressure shows itself, by
an elongation of the eye backward in its anteroposterior
diameter. ^This combination of the anatomic and mechanic
theories may explain in great part the presence of myopia
in the average German student or any broad-faced indi-
vidual.^
3. The inflammatory theory is that a low grade of
inflammation attacks the tunics of the eye, especially at
the posterior pole,, and is spoken of as macular chor-
(oiditis ; this is brought about by faulty use of the eyes,
in the school or in the home, in a poor light or too
glaring a light improperly placed, or by using the eyes
with the head bent over the work so that the return circu-
lation from the retina and choroid is interfered with./ This
inflammation or congestion of the tunics of the eye may
be primary in itself or secondary to the anatomic and
mechanic causes. Be this as it may, the conditions exist,
and go to show more and more that myopia is actually
acquired and not per sc congenital. " The inherited con-
genital anomalies of refraction, particularly astigmatism, are
responsible for the myopic eye, by virtue of the pathologic
changes they occasion in hard-worked eyes rather than any
inherited predisposition to disease." (Risley, "School
1 Ivgiene.")
Symptoms and Signs of Myopia. While the myope
may complain of headache and symptoms of accommodative
/asthenopia, yet the principal visual complaint will be the
, inability to see objects distinctly which lie beyond the far
point. The myope's world of clear vision is limited to the
distance of the far point, where the rays of light leaving his
eye come to a focus. Every object situated beyond the far
Il8 REFRACTION AND HOW TO REFRACT.
point is blurred and indistinct, and the further the object
from the far point, the more indistinct it becomes. The
myopic child at school soon ranks high in the class, is fond
of study, of books, music, or needlework, according to the
sex. The myope, in other words, is usually literary in
taste. Myopes avoid out-of-door sports, such as foot-ball,
base-ball, golf, etc.
Diagnosis of Myopia. This form of ametropia may be
recognized in various ways :
1. The prominent eyeball. This is not a positive sign of
myopia, though this and other signs are mentioned for the
reason that they are often present in the myopic condition.
2. The broaa face and (3) long interpupillary distance
are quite significant of myopia, and yet the broadest face
with longest interpupillary distance the writer ever saw
was in a hyperopic subject.
4. Divergent squint usually indicates myopia, and this
condition is often brought about by an inability to converge,
' or one eye may be more myopic than its fellow, with the
result that the more myopic eye turns out and soon be-
comes amblyopic.
5. (It has been stated that myopic eyes usually have
dark-colored irises>( but this is often a fallacy, as is only too
evident in the German student with his blue iris.
The foregoing are but signs of myopia, and are recog-
nized by inspection ; they should be looked for and care-
fully estimated, and each given its due consideration.
'Subjective and objective symptoms are the true tests of
myopia, and are as follows :
6^>Poor distant vision ; inability to see numbers on the
houses across the street or on the same side of the street ;
history of passing friends without speaking to them. The
myope enjoys close work and takes little or no interest in
MYOPIA. I 19
sports. A history, in other words, that is in keeping with
a vision of short range.
7.-^Good near vision ; ability to see the finest print or to
thread the finest needle or do the finest embroidery.
8. The near point is closer than that of an emmetropic
eye of corresponding age. (See p. 73.)
I 9. Distant vision is made worse by the addition of a
convex lens. The writer prefers to teach the diagnosis of
myopia in this way, and not to say that a concave lens
will improve distant vision ; of course it will, but he does
not want the student to put concave lenses before the
eye of the young "pseudo-myope," referred to under
Hyperopia.
io.(ihe far point is brought nearer by the addition of a
convex lens./ Objective methods of determining myopia
are by means of the
11. Ophthalmoscope.
12. Retinoscope.
13. Schemer's method.
14. Thomson's ametrometer.
15. Chromo-aberration test.
Direct Ophthalmoscopy in Axial Ametropia. Pro-
ficiency in this method comes only by perseverance and
long practice. It should not be employed to the exclusion
of other and more exact methods. To estimate with the
ophthalmoscope which lens is. required to give an eye
emmetropic vision, three very important facts should receive
careful attention :
1. The distance between the surgeon's and patient's eye.
2. The surgeon's and patient's accommodation.
3. The surgeon's own refractive error.
First, the surgeon should have his eye as close to the
patient's eye as possible, usually at 13 mm. ; this is the
I2O REFRACTION AND HOW TO REFRACT.
/anterior principal focus of the eye, and is the distance at
'/which the patient will wear his glasses.
Second, as already explained, the observer's and patient's
accommodation should be in repose. The most difficult
part for the student to learn is to relax his accommoda-
tion. The ambitious student strains his accommodation
; (ciliary muscle) in his haste, and with the result that he
' thinks all eyes myopic and all eye-grounds as affected
with " retinitis."
Third, the surgeon, if not emmetropic, must wear any
necessary correcting lenses ; otherwise, the lens in the
ophthalmoscope will record his and the patient's error
together, and deductions must be made accordingly. For
instance, if the surgeon is hyperopic -f 2 S., and does not
wear his glasses, and the ophthalmoscope records the fundus
as seen clearly with -j- 5 S., this would mean that the patient
had +3 S. (2 of the 5 S. being the surgeon's error); or
if the fundus is seen without any lens in the ophthalmoscope,
then the patient's error would be 2 S. (the surgeon's
+ 2 S. from o leaving 2 S.) ; or if the ophthalmoscope
showed 2 S., then the patient's error would be 4 S. ;
or if the ophthalmoscope registered -\-2 S., then the
patient would be emmetropic, and this -j-2 S. is the sur-
geon's error.
Rules. i. When the surgeon and patient are both hy-
peropic or both myopic, the surgeon must subtract his cor-
rection from the lens which shows at the sight-hole in the
) J ophthalmoscope.
2. When the surgeon's eye is hyperopic or myopic, and
the eye of the patient is the opposite, he must add his cor-
! rection to the lens at the sight-hole in the ophthalmoscope.
With the foregoing details clearly in mind and carefully
executed, the surgeon selects small vessels near the macula
frt4A*X
MYOPIA. 121
for his observations. If it .is impossible to see these on
account of the small pupil, then he will have to observe the
larger vessels at the disc (nerve-head, or papilla).
\Yhenever the vessels in the macular region are seen
clearly with one and the same glass in the ophthalmo-
scope, the refractive error can be approximated as one of
axial ametropia, and/every three diopters, plus or minus, or
any multiple of three diopters, represent very closely one
millimeter of lengthening or shortening of the anteropos-
terior diameter of the eye.\ For example, any eye that
takes a plus 3 S. to make it emmetropic is just I mm. too
short ; any eye that takes a minus 3 S. to make it emme-
tropic is about i mm. too long. It will be observed, however,
under the head of curvature ametropia (astigmatism), that
every 6 D. cylinder represents about I mm. in length, as
measured on the radius of curvature of the cornea. The
following table, from Nettleship, gives the exact equiva-
lents in millimeters for axial ametropia :
H ..... i D. = o.3 mm. M ..... I D.=o.3 mm.
61). 2 " 6 D.= 1.75 "
9D.= 3 " 9D.= 2.6 "
12 D.=4 " I2D.= 3.5 "
i8D.= 5
Indirect Method. See page 99 for a full description of
this method. Slowly withdrawing the objective lens, and the
[disc remaining unchanged in size, signifies emmetropia ; if
the disc grows uniformly smaller, it means H., and if it
grows uniformly larger, it means M. (See Fig. 142.) This
is merely a method of diagnosis, and is never used for
definite measurements.
ii
*
CHAPTER V.
ASTIGMATISM, OR CURVATURE AMETRO-
PIA. TESTS FOR ASTIGMATISM.
Astigmatism (from the Greek, , priv. ; a-l-fiia, "a
point"). Optically, astigmatism may be defined as the re-
fractive condition in which rays of light from a point, passing
through a lens or series of lenses, do not focus at a point*
In ophthalmology astigmatism is recognized as that con-
dition of the refractive system of an eye in which rays of
light are not refracted equally in all meridians, and the
resulting image of a point becomes (an oval, a line, or a
circle.^ (See Fig. 100.)
Or astigmatism is that condition of an eye in which there
are two principal meridians, of greatest and least ametropia,
each having a different focus.
In the standard eye the cornea is represented as a section
of a sphere; anatomically, however, the cornea is generally
found to be an ellipsoid of revolution, with its shortest radius
of curvature (normally 7.8 mm.) in the vertical meridian.
In the study of astigmatism the meridians of minimum
and maximum refraction alone are considered ; they are
spoken of as the principal meridians, and are at right
angles to each other.
With very few exceptions most eyes have some degree
of astigmatism. The standard or emmetropic eye is an
*In an article published by Dr. Swan M. Burnett, in "The American
Journal of Ophthalmology," for December, 1903, entitled ' Astigmia or Astig-
matism," he draws attention to the fact that astigmatism is an erroneous word,
and gives the true origin of the word from oriyfir]-r)^, meaning a mathematic
point, whereas "anyfia" really means a "blemish" or "brand." He there-
fore urges the change from astigmatism to astigmia, with the word "astigmic''
as the adjective.
122
I2 3
extremely rare condition, and plain myopic eyes (long eyes)
i(.4t1iout any astigmatism are almost as rare as the emme-
tropic condition ; and while plain hyperopic eyes are seen,
--yet statistics show that fully eighty per cent, of hyperopic
eyes have astigmatism.
.. Astigmatism is located in the cornea or lens, or it may
be a condition of both structures in one and the same eye.
Astigmatism of the lens may increase, diminish, or neutral-
ize the corneal astigmatism. Astigmatism, however, is more
? often a condition of the cornea than of the lens. , 7 /
Figure 100 shows parallel rays of
light passing through an astigmatic lens
in which the vertical meridian has the
H'
H
-^ ^i i
V
V
FlG. IOO.
shortest radius of curvature, with the result that those rays
which pass through the vertical meridian V V come to a
focus before those in the horizontal meridian H H', which
has the longest radius.
Intercepting the refracted rays at I, 2, 3, 4, 5, and 6, the
image would be at I a horizontal oval, at 2 a horizontal
line, at 3 a circle, at 4 a vertical oval, at 5 a vertical line,
and at 6 a vertical oval. The space between the points of
foci of the two meridians (2 and 5) is known as Sturm's
interval. The importance of this space or interval is that
124 REFRACTION AND HOW TO REFRACT.
it represents astigmatism. Sturm's interval is the quantity
which must be found in correcting astigmatism.
Causes of Astigmatism. Most cases of astigmatism
are congenital, and some can be traced to heredity. Ac-
quired astigmatism may result from conic cornea, cicatrices
following ulcers or wounds of the cornea, or be a tempo-
rary condition from pressure of a chalazion or other
growth ; and, in fact, astigmatism may develop from any
disease or injury that will cause a lengthening or shortening
/ .. 'or inequality in one or more of the meridians of the cornea
or lens. Swelling of the different sectors of the lens will
cause astigmatism. The visual line not passing through
the center of the cornea is a cause of astigmatism, and
astigmatism is the usual result following extraction of the
\ xdens. Tenotomy of one or more of the extraocular mus-
cles will often change the corneal curvature.
Irregular Lenticular Astigmatism.-This is a normal
y^ condition of all clear lenses.) It is often infinitesimal in
amount, and on this account does not interfere with vision.
It is caused by the different sectors of the lens or by the
individual lens-fibers themselves not being uniform in their
refracting power. /In this form of astigmatism a light does
not appear to have a distinct edge, but, on the contrary,
the edge has radiations passing from it, giving the light a
stellate qppparanrp. ^ There is no known glass that vj -ilL 5
correct this variety of astigmatism, ^r******* **" <*!* "**j*
Physiologic Astigmatism. This is due to lid pressure,
or temporarily to extreme pulling or contraction of the extra-
ocular muscles. It is a voluntary astigmatism, and therefore
not constant. It is not a condition of all eyes. The writer
has demonstrated with the retinoscope and ophthalmometer
that the condition can be produced in eyes not otherwise
astigmatic. Drawing the lids together in the act of squint-
ASTIGMATISM. 125
ing or frowning, the patient can press the cornea from
above and below, and give the horizontal meridian of the
cornea a longer radius of curvature and the vertical meri-
dian a shorter radius ; or with the eye looking into the
telescope of the ophthalmometer, no overlapping of the
mires is noted, but in some instances when told to open the
eye widely and "stare" into the instrument, as much as
lj or ^ of a diopter of astigmatism may be recorded.
This "transient" astigmatism should never be corrected
with a glass.
Subdivisions of Astigmatism. In addition to the
astigmatisms just described, curvature ametropia has been
further considered as :
1. Irregular. 6. Astigmatism against the
2. Regular. rule.
3. Symmetric. 7. Homonymous.
4. Asymmetric. 8. Heteronymous.
5. Astigmatism with the 9. Homologous.
rule. 10. Heterologous.
1. Irregular Astigmatism. This is usually located in
^the cornea, and is due primarily to some breach in the
continuity of one or more of its meridians ; for example,
the vertical meridian may appear regular, but the hori-
zontal meridian is not a uniform curve, but is irregular at
some point or points. Such meridians can not produce
clear retinal images, but, on the contrary, the resulting
retinal image is hazy or irregular.
2. Regular Astigmatism. In this variety the cornea
and lens are regular in their curvatures, from the maximum
to the minimum radius, and the retinal image can be made
clear with correcting glasses.
Before entering upon the study of the various forms of
regular astigmatism, the student's attention is called to two
126 REFRACTION AND HOW TO REFRACT.
important facts : (a) That, as a rule, the shortest radius of
curvature of the cornea is in the vertical meridian that is
to say, the vertical meridian has a stronger refracting power
than the horizontal.
(ti) The student should bear in mind that in the measure-
ment of curvature ametropia each millimeter of lengthening
or shortening of the radius of curvature is equivalent to a
6 D. cylinder. For instance, an eye which requires a -\-6 D.
cylinder axis 90 degrees has the horizontal radius of curva-
ture about one millimeter longer than the vertical radius ;
or an eye that requires a 6 D. cylinder axis 180 degrees
has its vertical radius of curvature about one millimeter
shorter than the horizontal. In axial ametropia, however,
it was shown that every three diopter sphere represented
about one millimeter in length, as measured on the axis.
Varieties of Regular Astigmatism. There are five
different forms of regular astigmatism :
(a) Simple hyperopic. (r) Compound hyperopic.
(fi) Simple myopic. (</) Compound myopic.
(e) Mixed astigmatism.
(a] Simple Hyperopic Astigmatism. Abbreviated As.
H., or H. As., or Ah. /About 5^ per cent, of eyes have
this form of refraction.) This is
a condition where one meridian
of the eye is emmetropic, and
the meridian at right angles to
it is hyperopic (see Fig. 101);
the vertical meridian focuses
parallel rays on the retina, and
the horizontal meridian would
focus back of it. The retinal image of a point is a line,
usually horizontal. (See 2, in Fig. 100.) The correcting
lens is a plus cylinder with its axis usually at 90 degrees,
ASTIGMATISM.
127
FIG. 102.
or within 45 degrees of 90 degrees. Example, -f- 2.00
cylinder axis 90 degrees.
(fr) Simple Myopic Astigmatism. Abbreviated As. M.,
or M. As., or Am. This is not a common condition. About
> I y 2 per cent, of all eyes have this form of astigmatism.
/This is a condition where one meridian of the eye is emme-
1 tropic, and the meridian at right angles to it is myopic (see
Fig. 102); the horizontal meridian
focuses parallel rays on the retina,
and the vertical meridian focuses
parallel rays in front of the retina
(in the vitreous), with the result
that they cross before reaching
the retina. /The retinal image of
a point is a line, usually vertical.
(See Fig. 100.) The correcting lens is a minus cylinder with
its axis at 180 degrees, or within 45 degrees of 180 degrees.
Example, 2.50 cylinder axis 180 degrees.
(r) Compound Hyperopic Astigmatism. Abbreviated
H. As. Co., or Comp. Has., or H-|-Ah (hyperopia com-
bined with astigmatism hyperopic). This condition repre-
~>sents nearly forty-four per cent, of all eyes ; it is the most
common of all forms of re-
fraction.
The retinal image of a
point is an oval ; never a line
and never a circle. (See I/
in Fig. 100.)
F, G . I03 , The correcting lenses are a
plus sphere and a plus cylin-
+ 3.00 cylinder axis 90 de-
der.
Example, -}-2.oo S.
grees. Compound hyperopic astigmatism is a combination
of axial ametropia (short eye) and simple hyperopic astig-
128 REFRACTION AND HOW TO REFRACT.
matism (curvature ametropia). In this form of astigmatism
both meridians have their foci back of the retina one
further back than the other. The retina intercepts the rays
before they can focus. Figure 103 shows this condition.
/ Usually the vertical meridian focuses nearer the retina than
the horizontal.
(d) Compound Myopic Astigmatism. Abbreviated M.
As. Co., or Comp. Mas., or M. H-Am. (myopia combined
with astigmatism myopic). This is by far the most com-
mon condition of all myopic eyes, and represents about
eight per cent, of all eyes.
The retinal image of a point is always an oval ; never a
line or a circle. (See 6, in Fig.
100.)
The correcting lenses are a
minus sphere and a minus cylin-
der. Example, i sph. O -
cylinder axis 180 degrees. A
combination of axial ametropia
FIG. 104. (long eye) and simple myopic
astigmatism.
Figure 104 shows that parallel rays have t\vo points of
foci in front of the retina one further front than the
other.
(c) Mixed Astigmatism. This form of refraction is
found in about 6^ per cent, of all eyes, and is abbreviated
in th/ee different ways :
I/ Ah-fAm. (astigmatism hyperopic with astigmatism
myopic).
2. H-f-Am. (hyperopia with astigmatism myopic).
/ 3. M-j-Ah. (myopia with astigmatism hyperopic).
/ The retinal image of a point is an oval or a circle ; never
line. (See 3 and 4 in Fig. 100.)
j^-i/'-'A *~^L*{ ^*-
.
ASTIGMATISM.
129
The correcting lenses are one of three combinations, and
spoken of as crossed cylinders. Examples :
1. +1.00 cyl. axis 90 degrees ^ 2.00 cyl. axis 180 degrees.
2. -j-i S. O 3 c )'l- ax ' s I 8 degrees (cylinder always^ stronger than the
sphere).
3. 2 S. ^ -f~3 cyl. ax ' 5 9 degrees (cylinder always stronger than the
sphere) .
' The condition of mixed astigmatism is one of simple
7hyperopic astigmatism, with simple myopic astigmatism :
one meridian focuses parallel rays in front of the retina and
the other meridian (at right angles) focuses parallel rays
FIG. 105.
FIG. 106.
back of the retina. Figures 105 and 106 show this arrange-
ment.
The remaining subdivisions of astigmatism are merely
classifications of the different forms already described, and
arise from a study of the axis of shortest radius of curva-
ture.
3. Symmetric Astigmatism. \Yhen the combined
values, in degrees, of the meridians of shortest or longest
radii of curvature in both eyes equal 180 degrees (no more
and no less), then the astigmatism in the two eyes is spoken
of as symmetric. For example, if the cylinder in the right
eye is at axis 75 degrees, and in the left eye at 105 de-
grees ; 75 degrees and 105 degrees added together will
130
REFRACTION AND HOW TO REFRACT.
make 180 degrees. (See Fig. 107.) Or if each eye takes
a cylinder axis at 90 degrees, they are also symmetric, 90
degrees and 90 degrees making 180 degrees. If both eyes
have axes 1 80 degrees, they are symmetric also, one
meridian being considered as zero (o).
4. Asymmetric astigmatism is the reverse of sym
H.JL/' metric, and is, therefore, the condition in which the com-
bined values, in degrees, of the cylinder axes do not make
1 80 degrees. For instance, if the right eye has a cylinder
aj^fxis 75 degrees and the left at 120 degrees, these
' ' / t/l
.".
93'
13
180
135
FIG. 107. Illustrating Symmetric
Astigmatism.
135
75 90
120
FIG. 108. Illustrating Asymmetric
Astigmatism.
added together would not make 1 80 degrees, but more than
1 80 degrees. (See Fig. 108.) Or, if the astigmatism in
the right eye was at 35 degrees, and the left at 90 degrees,
these added together would not make 180 degrees.
Symmetric astigmatism generally accompanies a regular
physiognomy, the center of each pupil being at an equal
distance from the median line of the face. Asymmetric
astigmatism usually accompanies an asymmetric physiog-
nomy, the center of one pupil being further from the
median line of the face than the other.
ASTIGMATISM.
13 1
Muscular insufficiency, hereafter to be described, is much
more common, and, in fact, should be looked for or antici-
pated in cases of asymmetric astigmatism. ^^*^ *flf
5 and 6. Astigmatism with the Rule and Astigmatism
Against the Rule. Astigmatism with the rule and astig-
matism against the rule refer to the condition already
) described as that in which the vertical meridian of the eye,
as a general rule, has the shortest radius of curvature.
Statistic tables on astigmatism show that most eyes
a plus cylinder at axis 90 degrees, or within 45
135,
45
FIG. 109. Illustrating Astigmatism
with the Rule.
135
FIG. no. Illustrating Astigmatism
aguinst the Rule.
degrees (inclusive) either side of 90 degrees (see Fig.
109); or a minus cylinder at axis 180 degrees, or within
45 degrees (inclusive) either side of 180 degrees. For
example, if an eye requires a plus cylinder at 45 degrees,
or at any axis from 45 degrees up to 135 degrees (inclu-
sive), taking axis 90 as the median line, then the astig-
--jnatism is u'ith the rule. But if an eye should require a
plus cylinder within 45 degrees either side of 180 degrees,
then the condition is one of astigmatism against the rn/e.
(See Fig. 1 10.) A plus or minus cylinder at 45 degrees
132 REFRACTION AND HOW TO REFRACT.
or 135 degrees is recognized as astigmatism with the
, lC ' &~-*9 >* * *V~, - ** ^
f 7. Homonymous astigmatism is the condition in which
/ the cylinder axis in each eye is the same.)
t ^ ' * _ f-C^_ - -*- 1
8. Heteronymous astigmatism is the condition in which
the astigmatism in one eye is with the rule, and in the other
eye against the rule. For example : *
O. D. 4-2 cyl. axis 90 degrees, and O. S. 4 2.00 cyl. axis 180 degrees.
?" Q. Homologous astigmatism is symmetric astigmatism
**with the rule /. e. :
O. D. 4 l.oo cyl. axis 60 degrees, O. S. -j-l-oo cyl. axis 120 degrees.
10. Heterologous astigmatism is symmetric astigma-
tism against the rule /. e. :
O. D. 4-I-OO cyl. axis 15 degrees, O. S. -\-i.O3 cyl. axis 165 degrees.
Meridians of the Eye. The various axes or meridians
of the eye are indicated by degree markings on the peri-
phery of the trial-frame, and by corresponding imaginary
lines drawn around the eyeball from the anterior pole or
apex of the cornea to the posterior pole.
Either eye (right or left) is exactly like its fellcnv, and is
numbered by starting from zero (o) on the left-hand side of
the horizontal meridian and counting downward to the right-
hand side until this same line is again reached. This makes
half a circle (hemisphere) of 180 degrees. (See Fig. 109.)
As the degrees in this half-circle are all carried across the
eye, they maintain their individual numbering, so that axes
5, 10, 15, etc., are the same whether above or below the
horizontal meridian. Hence there is no reason for having
a complete circle of 360 degrees. Some trial -frames have
the upper, while others have the lower, half numbered ;
this makes no difference in the exact numbering ; in one in-
stance the count is made from left to right, and in the other
ASTIGMATISM. 133
the count is made from right to left. The foreign trial-frame,
as represented on page 48, may be confusing if not studied.
Symptoms of Astigmatism. More aggravated symp-
toms of accommodative asthenopia are apt to be detailed
by the patient, but there are, in truth, no definite symptoms
whereby the presence of astigmatism can be positively, dif-
ferentiated from axial ametropia. The diagnosis of astig-
matism by the physiognomy is confirmed only because
most eyes are astigmatic ; the simple hyperopic eye squints
the eyelids together just the same as the eye that is astig-
matic, so that the writer would not diagnose astigmatism
by the patient's individual history of his eyes.
How to Diagnose Astigmatism. This is one of the
very early questions of the beginner in ophthalmology.
Astigmatism being the prominent factor in almost all refrac-
tive work, the writer feels justified in giving this part of
refraction extensive explanation. Of the various methods
of diagnosing astigmatism the writer would mention the
following :
1. Corneal reflex. IO. Chromo-aberration or cobalt-blue
2. Confusion letters. glass test.
3. Placido's disc. II. Thomson's ametrometer.
4. Stenopeic slit. 12. The ophthalmometer.
5. Astigmatic chart. 13. Direct ophthalmoscopy
6. The pointed line test. 14. Indirect ophthalmoscopy.
7. Perforated chart or disc. 15- Cylindric lenses.
8. Fray's letters. 1 6. Retinoscope.
9. Schemer's Test.
I. The Corneal Reflex Test. The cornea and under-
lying aqueous representing a spheric mirror, naturally fur-
nish a small image of surrounding objects. If the cornea
is astigmatic, the catoptric image must be correspondingly
distorted. To make the examination, the patient stands
facing a window, and the surgeon at one side observes the
134 REFRACTION AND HOW TO REFRACT.
image of the window-panes in the conical mirror ; these
will be broadened or lengthened, or they may appear in-
clined to one side, according to the axis and character of
the astigmatism. This test is not commonly used, is often
overlooked ; in fact, unless the astigmatism is of consid-
erable degree, is not a valuable test.
2. Confusion Letters. Letters on the card which is
used for testing distant vision are arranged in such order
that those which have a resemblance are placed next to
each other. (Fig. 74.) For example, X and K, Z and E,
O and D, C and G, P and F, S and B, V and Y, H
and N, A and R, etc. The patient, in deciphering these
letters in the line corresponding to his best vision, often
miscalls them, and can not tell an X from a K, or a Z
from an E, etc. These letters are, therefore, spoken of as
confusion letters. (This is a very good general test, but is
not infallible, as a patient with opacities in the media will
make similar mistakes./
3. Placido's Disc or Keratometer (see Fig. 1 1 1). To a
wooden handle is secured a round piece of thin sheet-iron
eight inches in diameter, and at its center is a small, round
5 mm. opening. On one side the disc is painted in alternate
concentric circles or bands in black and white ; these circles
are not equidistant, the radii of the several circles being cal-
culated according to the law of tangents, so that when re-
flected on a cornea of spheric curvature they appear equi-
distant in the image. On the reverse side is placed a slot to
hold a convex lens for magnifying purposes. To use this
disc, the patient is placed with his back to a strong light from
a window, or an artificial light may be placed over his head.
The surgeon holds the disc with the sight-hole close in front
of his own eye, and with the light illuminating the disc, the
patient is instructed to look into the perforation. The sur-
ASTIGMATISM.
135
geon then approaches the eye until the corneal image of
the outer edge of the instrument corresponds to the outer
edge of the patient's cornea. When this distance is
reached, a convex 2, 3, or 4 D. sphere may be placed in
the slot of the disc so as to magnify the corneal image.
(If the cornea is not astigmatic, then the black and white
circles will appear uni-
form throughout ; but if
there is astigmatism, the
circles will appear more
or less oval. If irregu-
lar astigmatism or conic
cornea is present, the cir-
cles will appear broken
or distorted in certain
parts. This test has be-
come almost obsol
FIG. in.
FIG. 112.
4. Stenopeic Slit (see Fig. 1 12). This is a round metal
disc of the size of the trial-lens, and contains a central slit
or opening about 25 mm. long and I or 2 mm. wide. The
stenopeic slits sold in the shops have various breadths of
openings, from ^ to 2 mm. ; that with the I mm. opening
136 REFRACTION AND HOW TO REFRACT.
is the one recommended. The purpose of the slit is to
cut off or exclude all rays of light at right angles to its
position in front of the eye. When placed at axis 90, all
rays in the horizontal meridian are excluded ; when placed
at axis 1 80, all rays in the vertical meridian are cut off, etc.
jTo use the stenopeic slit, place it in the trial-frame in front
I of the eye to be examined, the fellow-eye being covered.
The patient is instructed to read the letters on the distant
test-card, and as he does so, the slit is slowly turned
through the different meridians. If the vision remains the
same, no matter through which meridian the patient reads,
astigmatism may be absent ; but if the patient selects one
meridian in which he sees best, and another meridian at
right angles in which he does not see so well, astigmatism is
usually present. For instance, if the slit is at axis 75 degrees
and the patient reads ^, and at axis 165 he reads ~^, then
he is astigmatic in the 165 meridian. The amount of the
astigmatism can be calculated by placing spheric lenses
back of the slit and finding the difference in strength of
the spheres which bring the vision up to the normal. For
example, when the slit is at axis 75 and the patient reads
f^, if a -f 1.50 S. is used, and the vision becomes yj, then
1.50 corrects axis 75. Turning the slit to axis 165, and
proceeding in the same way, if -f- 2. 50 S. brings the vision
from ^ to ^}, then +2.50 corrects axis 165, the differ-
ence between the +1.50 and +2.50 being i D., and the
formula would be +1.50 sph. ^ -f i.oo cyl. axis 75
degrees. This test is not often used, and when resorted
to, the eyes should be under the influence of a cyclo-
plegic. This test is of special service in some cases of
mixed astigmatism, irregular astigmatism, presbyopia, and
aphakia.
ASTIGMATISM.
137
5. Astigmatic Chart. There is an infinite variety of
these cards (see Fig. 113), and the student is puzzled
FIG. 113. Astigmatic Charts of Dr. John Green.
which one to select. Ordinarily, the " clock-dial " will
answer every purpose. (Fig- H4-) This is a white
138
REFRACTION AND HOW TO REFRACT.
card * with peripheral Roman characters corresponding to
the characters on the clock-face, hence its name. From
these figures a series of three parallel and uniformly black
lines, with interspaces of the same width as the lines, cross
from XII to VI, III to IX, IIII to X, V to XI, VII to I,
and VIII to II. This chart should be so calculated that
FIG. 114.
the lines and interspaces will form an angle of 5 minutes in
width consistent with the distance at which the test is to be
/made : if at six meters, 8.7 mm. ; if at four meters, 5.7 mm.
In most charts the lines subtend an angle much greater
than 5 minutes for the distance at which they are used, and
in this way the true delicacy of the test for small errors or
* A black card with white lines is also used. (See Fig. 115.)
ASTIGMATISM.
139
amounts of astigmatism is sacrificed. The purpose of the
chart is to detect, by the patient's answer, whether astigma-
tism is present, and, if so, in which meridian.
The chart, illuminated by reflection from a steady artificial
light, is placed on a horizontal line perpendicular to the
patient's eyes ; it should never be hung at an angle, and
must always be perfectly flat. Each eye is to be tested
separately. Looking at such a chart, if all the lines appear
FIG. 115.
equally black, astigmatism of any considerable degree or
amount may often be excluded ; but if the patient selects
one series of lines as darker than others, then the presence
of astigmatism may be diagnosed. 'If the astigmatism is
of a very high degree, the patient may see the three lines
as one solid black line without interspaces.J
RULE i. The meridian of the. eye which corresponds to
the dark lines selected is the meridian of astigmatism.
Example. If the horizontal lines (from III to IX) appear
I4O REFRACTION AND HOW TO REFRACT.
darker than all the others, then it is the horizontal meridian
(o or 1 80 degrees) of the eye which is astigmatic. Or if
the lines from VI to XII are darkest, then the vertical mer-
idian of the eye is astigmatic. In other words, the series
of darkest lines indicates the meridian of greatest ametropia.
RULE 2. The axis of the cylinder in the prescription
will be opposite to the meridian of the dark lines.
Example. A patient who requires a plus cylinder at axis
90 degrees sees the horizontal lines (from III to IX) as very
dark, and the lines from VI to XII not so dark, and the axis
of the cylinder in the prescription will be opposite to 180
degrees i. e., at 90 degrees.
^According to the definition of "astigmatism with the
rule " and " astigmatism against the rule," it follows that,
with few exceptions, those patients who select a series
of lines at 180 degrees, or within 45 degrees either side of
1 80 degrees, as darker than other lines, have hyperopic
astigmatism, whereas those who select a series of lines at 90
degrees, or within 45 degrees either side of 90 degrees,
have myopic astigmatism. J
According to the definition of symmetric astigmatism, a
patient's right eye selecting the lines at 90 or 180 as
darker than those at right angles, will select the same series
of dark lines in the left eye. If the series of dark lines with
the right eye is from II to VIII, then the left eye selects
the dark lines from X to IV, etc.
The clock-dial is the form of chart in common use,
and as a test for astigmatism . is not without considerable
merit.
When the astigmatism is of small amount, it may not be
recognized by means of the clock-dial until after the spheric
correction has been placed before the eye or after a cyclo-
plegic has been instilled.
ASTIGMATISM. 14!
6. The writer's pointed line test, as shown in figure 1 16,
is a series of one-minute black squares, in three parallel
lines at right angles to each other, on a cream-colored card ;
the squares and adjoining spaces making a five-minute
angle for six meters. By means of a clockwork and bat-
tery, this dial may be revolved by pressing a button. The
principle of the test is the same as the perforated disc.
FIG. 1 1 6.
7. The Perforated Disc (Fig. 117). This is a modifica-
tion of the astigmatic chart. A piece of white cardboard
or metal, about ten inches square, has small, round perfora-
tions made in it of certain definite size. ^Each perforation is
separated from its neighbor by the distance of its diameter.^)
These openings are arranged in series of one, two, or three
parallel lines, exactly as in the pointed line test. This
chart or disc is hung on the window-pane, or an illumina-
tion is placed behind it. The patient, looking at the disc,
142 REFRACTION AND HOW TO REFRACT.
signifies which scries of perforations appear to coalesce and
form lines. This test is not commonly known or used. It
FIG. 117.
might be a valuable test if there was any convenient way
of uniformly illuminating it from behind.
8. Fray's Letters (Fig. 118).-
^zsJT ^^ a ?^ These letters are of the Old English
~ ^rz 5 5 55^^^
=~ "^? 5^8 type, and composed of strokes which
Ilium //i ///// run ^ n different meridians. The pa-
flliiill %/r J/ ft ^ ent > looking at these letters, selects
that letter which appears darker than
all the rest. The direction of the lines
j n the letter selected corresponds to the
meridian of greatest ametropia. (This
^"
test is very confusing to the patient,
FIG. 118. wno sees first one letter and then an-
other as darker than its fellows.^/
Schemer's Test. This is an old test for ametropia,
\\
\A\V
ASTIGMATISM.
143
good in theory, but really not sufficiently accurate for prac-
tical purposes. It is explained for the student's information,
and not with the idea that he will ever take time to use it.
The test is made with a small piece of metal (Fig. 119)
the size of the trial-lens, which contains two pin-point round
openings at its center, separated by an interval of two or
three millimeters. (One of these openings is covered with a
red glass, as suggested by Dr. Wm. Thomson.) This disc
is placed close to the eye, so that light may pass through
both openings into the eye at one and the same time. The
eye, if not presbyopic, should be under the influence of a
Fie. 119.
FIG. 120.
cycloplegic. The eye looks at a distant point of light The
principle of the test depends upon which part of the retina
is stimulated by the rays entering the eye through these
openings, all other rays being excluded. The student must
remember that rays which fall upon the temporal side of the
retina are referred to the nasal side ; those which fall upon
the nasal side of the retina are referred to the temporal side ;
those which fall upon the lower portion of the retina appear
to come from above ; and those which fall upon the upper
portion of the retina appear to come from below.
Diagnosis of Hyperopia ( Fig. 1 20). The disc is placed
with the red glass (R) above. The patient then sees a red
144
REFRACTION AND HOW TO REFRACT.
FlG. 121.
and a white light (W). The red appears below the white.
Gradually revolving the disc, the two lights move, and keep
the relative positions and distance apart. The greater the
distance between the two lights, the higher the refraction
or amount of the hyperopia. That plus sphere placed in
front of the disc which unites the two flames into one (pink)
flame is the approximate amount of the hyperopia.
Diagnosis of Myopia (Fig. 121). Placing the disc
before the eye as before, with
the red glass (R) above, the
patient sees the red flame
above the white (W). Gradu-
ally revolving the disc, these
two lights keep their rela-
tive positions and distance.
That minus sphere placed
before the disc which makes
the two lights appear as one (pink) light is the approximate
amount of the myopia.
Diagnosis of Emmetropia. This condition would give
but one light (pink in color), and unchanged by rotating
the disc.
Diagnosis of Simple Hyperopic Astigmatism. One
meridian appears the same as in emmetropia, and the meri-
dian opposite to the emmetropic meridian would show a
separation of the two lights, as in hyperopia. The plus cyl-
inder placed before the disc which unites the two lights in the
ametropic meridian represents the amount of the astigma-
tism.
Diagnosis of Simple Myopic Astigmatism. The
lights are red and white in one meridian, as in simple hyper-
opic astigmatism, but the red light is seen in the direction
of the red glass, and when the disc is rotated to the oppo-
ASTIGMATISM. 145
site meridian, only one light appears, and of a pink color.
The amount of the astigmatism is represented by the
strength of minus cylinder which brings the two lights
together in the ametropic meridian.
Diagnosis of Compound Hyperopic Astigmatism. All
meridians show two lights, the red light being in the direc-
tion of the clear opening in the disc, but one meridian will
show a greater separation of the lights than in the meridian
at right angles. To find the correction and the amount of
the astigmatism, proceed as in simple hyperopia, correcting
each meridian separately with a sphere.
FIG. 122.
FIG. 123.
Diagnosis of Compound Myopic Astigmatism. This
is the same as in compound hyperopic astigmatism, with a
reversal of the position of the lights, and the amount of the
ametropia is obtained with minus spheres.
Diagnosis of Mixed Astigmatism. One meridian
appears as in simple hyperopic astigmatism, and the meri-
dian opposite to it appears as in simple myopic astigma-
tism. The amount of the astigmatism is calculated as in
these two conditions.
10. Chromo-aberration Test. This is also known
as the cobalt-blue glass test. Cobalt is a mineral, and is
13
146
REFRACTION AND HOW TO REFRACT.
used as a coloring-matter by glass-blowers. Cobalt-blue
glass comes in two forms : one in which the glass is colored
throughout, and the other in which it is colored on only one
surface, known as " flashed." To the eye, cobalt -blue glass
appears dark blue, but contains a great deal of red. For
purposes of testing ametropia, a dark shade of blue should
be selected, or two or three pieces of a light shade may be
cemented together so as to give the desired dark shade.
This glass is cut round and fitted into a trial-cell. (See
Figs. 122 and 123.)
The power of cobalt-blue glass to exclude all but blue
FIG. 124.
and red rays gives this test its principle. Blue rays being
/more refrangible than red, naturally focus sooner than red.
~-\ Red rays will focus back of the blue. (See Fig. 1 24.)
There are several important details in the use of this
test which must be carefully executed if definite results are
to be obtained :
1 . The eye should be under the influence of a cydoplcgic.
2. By means of a light-screen, a small round area of
steady white light should be looked at from a distance of
four or six meters.
3. Each eye is to be tested separately.
ASTIGMATISM.
147
FIG. 125
FIG. 126
FIG. 127.
FIG. 128.
FIG. 129.
FIG. 130.
FIG. 131.
FIG. 132.
FIG. 133.
FIG. 134.
FIG. 135.
FIG. 136.
125. High hyperopia. 126. High myopia. 127. Low simple hyperopic as-
tigmatism. 128. High simple hyperopic astigmatism. 129. Low simple
myopic astigmatism. 130. High simple myopic astigmatism. 131. Low
compound hyperopic astigmatism. 132. High compound hyperopic astig-
matism. 133. Low compound myopic astigmatism. 134. High com-
pound myopic astigmatism. 135. Low mixed astigmatism. 136. High
mixed astigmatism.
148 REFRACTION AND HOW TO REFRACT.
4. The cobalt glass may be placed near the flame or,
better still, close in front of the patient's eye ; in every
instance it must be perpendicular to the front of the eye,
iand never at an angle.
5. All other lights except the one in use should be
excluded.
Diagnosis of Emmetropia. Patient sees a small circle
composed of two colors equally mixed ; purple. (See E
in Fig. 124.)
Diagnosis of Hyperopia (see H in Fig. 124). The
patient describes a red ring of light with a blue center.
Diagnosis of Myopia. The patient describes a blue
ring with a red center. (See M in Fig. 124.)
Diagnosis of Astigmatism. If astigmatic, then he will
describe one of the conditions as shown on page 147. If
the test is made as suggested, it will have three points of
recommendation :
1. The character of the refraction is quickly diagnosed.
2. It may lead to an early diagnosis of red-blindness, a
condition often overlooked.
3. Likewise it will show a central scotoma for red in
advanced toxic amblyopia, if the eye is made myopic with
a plus sphere.
ii. Thomson's Ametrometer (Fig. 137). This instru-
ment has two small gas-flames about five millimeters in diam-
eter, one stationary and the other movable on a metal arm,
which can be changed or revolved to any meridian. Each
eye is tested separately at a distance of twenty feet, and
preferably under a cycloplegic. The method of the test is
to move one flame along the metal arm until the two lights
appear to fuse. The scale, as marked on the arm, gives
the approximate strength of lens necessary to correct the
ametropia. By raising or lowering the arm any meridian
ASTIGMATISM.
149
may be tested. It is a most ingenious test, but not in
common use. {/
12. The Ophthalmometer (see Figs. 138 and 139).
This name literally means an " eye measure," but as the in-
strument measures only the different radii of corneal curva-
ture, a much better name would be keratometer, or measure
of the corneal radii v The object of the ophthalmometer is
h #- .
FIG. 137.
the measurement of corneal curves by means of catoptric
images viewed through a telescope.
The ophthalmometer consists of a telescope which con-
tains a Wollaston birefrangent prism placed between two bi-
convex lenses. Attached to the telescope is a graduated arc,
upon which are placed two white enameled objects called
mires (targets). (See Figs. 139, 140, 141.) The left mire
is stationary, and is made up of two_^cm. squares, separated
150
REFRACTION AND HOW TO REFRACT.
by a black line 2 mm. wide ; the right mire is movable and
graduated into steps, each 5 mm. wide ; a black line passes
through the middle of these steps. For purposes of focus-
ing, the telescope is mounted on a movable tripod. The
patient is seated with his chin and forehead resting in a
FIG. 138.
frame. At the side of the frame, and attached to it, are two
or four electric lights or Argand burners, which illuminate
the mires. The surgeon, looking through the eye-piece of
the telescope, focuses the center of the patient's cornea
until he sees two images of each mire clearly ; then he
selects the two central images for further study and ignores
ASTIGMATISM. I 5 I
the peripheral images. The next step is to move the
right-hand mire until these two images of the mires occupy
the center or pole of the cornea, so that their inner edges
just touch and the black line in each makes one continu-
ous black line through both (see Fig. 140) ; and to do the
FIG. 139.
latter, the barrel of the telescope may have to be gradually
revolved from left to right or right to left, but never more
than 45 degrees either way. When this position is ob-
tained, the axis or meridian is noted by the arrow, which
points to the figure on the dial at the back of the arc, or,
as in some old instruments, on the front of the dial. This
152 REFRACTION AND HOW TO REFRACT.
position of the mires is spoken of as the primary posi-
tion.
Revolving the telescope to the opposite meridian (mer-
idian at right angles), which is called the secondary posi-
tion, the observer notes any change which may have taken
place in the relative positions of the mires. If they have
not changed, but still maintain their edges in apposition, as
in the primary position, then the cornea has a uniform cur-
vature thoughout, and there is no astigmatism of the
cornea present. If, however, when the secondary position
is reached and the catoptric image of the mires with the
steps has encroached upon the catoptric image of the sta-
tionary mire, then the astigmatism is calculated by the
amount of this overlapping.
(See Fig. 141.)
Each step representing one
diopter of astigmatism, one-
half a step of overlapping
FIG. 140. FIG. 141. would represent half a diopter,
etc. If, in making the change
from the primary to the secondary position, the mires
should separate, then the surgeon would know that his
secondary position should have been his primary position,
and he will have to make a corresponding change.
As already stated, lenticular astigmatism is not a condi-
tion to be ignored, as only too often it will increase,
diminish, or even neutralize corneal astigmatism, so that in
point of fact the ophthalmometric findings are more often
useless than of real value in estimating the total refractive
error. Cylinders should never be prescribed from the
ophthalmometric findings until carefully confirmed by
other and much more reliable tests. As a keratometer,
the instrument can not be excelled, and, therefore, it has a
ASTIGMATISM. 153
place in testing the refraction in cases of aphakia. The
ophthalmometer as a means of diagnosis is suggestive
rather than positive.
13. Estimation of Curvature Ametropia (Astigma-
tism) with the Ophthalmoscope, Direct Method. The
presence of astigmatism is diagnosed by the direct method
from the fact that the vessels or details of the fundus are
not all seen clearly with one and the same glass in the
ophthalmoscope ; in other words, the vessels passing up
and down on the disc are seen clearly with a different lens
in the ophthalmoscope than is required to see the vessels
passing laterally or at right angles. (The amount of the
astigmatism is the difference in the strength of the respective
lenses used for this purpose) for instance, if the vertical
vessels are seen best with a -f-4 S., and the horizontal
vessels with a -f-2 S., then the amount of the astigmatism
would be -\-2 D.
When using the ophthalmoscope for making refractive
estimates in astigmatic eyes, /the student should remember
that the glass with which a vessel is seen distinctly in one
meridian represents the amount of the refraction in the
meridian at right angles to this vesseL) (In other words, /
each vessel in the eye-ground of an astigmatic eye is seen
clearest through the meridian at right angles to its course.J
This is a puzzle to the beginner, but he must remember that
cylinders refract opposite to their axes. In estimating the
refraction with the ophthalmoscope, the observer looks first (/? *
at the shape of the disc ; if it appears oval, this would be an
evidence of astigmatism ; secondly, if the upper and lower
edges of the disc are seen clearly with a different strength
glass than that required to see the inner and outer margins,
then this would be a further evidence of the presence of
astigmatism ; Xut the third and confirmatory test of the
presence of astigmatism should be the different strength
154 REFRACTION AND HOW TO REFRACT.
glasses required to see the vessels distinctly in the neigh-
borhood of the maculay An eye having an oval nerve,
whose edges can all be seen clearly with one and the same
glass in the ophthalmoscope is not usually astigmatic.
Examples of estimated refraction by the direct method.
Simple Hyperopic Astigmatism. Vertical vessels seen
with a -j- I S. and horizontal vessels seen without any lens
would equal + 1 .00 cyl. axis 90 degrees.
Simple Myopic Astigmatism. Vertical vessels seen
without any lens and horizontal vessels seen with 3 S.
would equal 3 cyl. axis 180 degrees.
Compound Hyperopic Astigmatism. Vertical vessels
seen with -{-4 S. and horizontal vessels seen with +38.
would equal +3.00 S. O + i.oocyl. axis 90 degrees.
Compound Myopic Astigmatism. Vertical vessels
seen with 2 S. and horizontal vessels seen with 5 S.
would equal 2.00 S. O 3.00 cyl. axis 180 degrees.
Mixed Astigmatism. Vertical vessels seen with
-(-28. and horizontal vessels seen with 3 S. would equal
3.00 S. O +5.00 cyl. axis 90 degrees.
14. Diagnosis of the Character of the Refraction by
the Indirect Method (see Fig. 142 and p. 99). There is
nothing exact about this method, and the refractive error,
to be recognized, must be considerable.
1. Gradually /withdrawing the lens (objective) from in
front of the eye, if the aerial image of the disc retains its
uniform size in one meridian, it signifies emmetropia for that
meridian ; but if it grows smaller in one meridian, that
meridian is hyperopic ; or if larger, then that meridian is
myopic.)
2. Ifthe image grows smaller, but more so in one meridian
than the other, it signifies compound hyperopia. If the
image grows larger, but more so in one meridian than the
other, then the condition is one of compound myopia. The
ASTIGMATISM.
155
image growing smaller in one meridian, while in the other
it grows larger, indicates mixed astigmatism.
FIG. 142. Companion picture to figure 89. Illustrating the indirect method.
Rays from (lie lamp (L) are reflected convergently from the mirror of the
ophthalmoscope, and, passing through the convex lens and into the eye,
produce a large retinal illumination, extending from I to I. TB are rays
from the edge of the disc, and, leaving the eye parallel, pass through the
convex lens and form an inverted aerial image of the disc at T/ B'. The
-f-4 S. in the ophthalmoscope magnifies the image T' B'.
15. The cylinder lens test for astigmatism is described
under Applied Refraction, page 251.
16. Retinoscopy is described in chapter vi.
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CHAPTER VI.
RETINOSCOPY.
Retinoscopy, or the Shadow Test. This may be de-
FIG. 143. The Author's Schematic Eye for Studying Retinoscopy.
fined as the method of estimating the refraction of an eye
by reflecting into it rays of light from ^a plane or concave
I 5 6
RETINOSCOPY.
157
mirror, and observing the movement which the retinal
illumination makes by rotating the mirror.
Suggestion. Before attempting to practise retinoscopy
upon the human eye, the beginner is advised to study the
method upon one of the many schematic eyes to be found
in the market.
The principle of retinoscopy is the finding of the point
of reversal, or myopic far point ; and when an eye is emme-
tropic or hyperopic, it must be given a myopic far point by
means of a convex sphere. (Fig. 144.)
i METER.
FIG. 144.
Advantages of Retinoscopy.
1. The character of the refraction is quickly diagnosed.
2. No expensive apparatus is necessarily required.
3. The refraction is estimated without the verbal assist-
ance of the patient.
4. The correction is quickly obtained.
5. The value of retinoscopy can never be overestimated
in the young, in the feeble-minded, the illiterate ; in cases
:?of nystagmus, amblyopia, and aphakia.
Axiom. \Yith an eye otherwise normal except for its
optic error, and under the influence of a reliable cycloplegic,
there is no more exact objective method of obtaining its
refraction than by retinoscopy.
158
REFRACTION AND HOW TO REFRACT.
The surgeon should wear any necessary correcting
glasses and have a vision of more than ; otherwise he
can never get satisfaction from this method. The surgeon
should keep his eyes wide open and not hesitate to use his
accommodation, as it does not have any effect on the result,
as in estimating the refraction with the ophthalmoscope.
FIG. 145. FIG. 146.
Author's Mirror with Folding Handle.
FIG. 145. Showing central light C, on small mirror B. This is the light the
patient sees when looking into the mirror, and corresponds in size to the
one-centimeter opening in screen. D is the folding cap handle to pro-
tect B when not in use. A is the metal disc.
FIG. 146. Shows the light moved to one side as a result of tilting the mirror.
The patient must have his accommodation under the in-
fluence of a reliable cychplegic ; this is imperative. Each
eye is tested separately, and if the patient has a squint, then
^^one eye should be covered while its fellow is being refracted.
The patient must be comfortably seated and told to look at
RETINOSCOPY.
159
the metal disc of the mirror or the observer's forehead
' above the mirror, and never into the mirror.
The Retinoscope, or Mirror. The plane mirror is 2
cm. in diameter on a round 4 cm. metal disc, with a 2 mm.
sight-hole at the center, made by removing the silvering,
and not by cutting a hole through the glass. (See Figs.
145 and 146.)
The concave mirror recommended has a 25 cm. focus
(ten inches) and is 3 y z cm. in diameter
on a metal disc of the same size as the
plane mirror. The sight-hole is simi-
lar in size and made /n the skme \\ ax-
as that of the plane, mirror.
The light should be steady, clear,
and \vhite, and secured to a movable
bracket. For general use, the Argand
burner is best.
The Light-screen, or Cover-chim-
ney. For the purpose of intercepting
the heat this is made of thin asbestos,
and the iris diaphragm attached to it
regulates the amount of light desired.
(See Fig. 147.)
The room for retinoscopy should be
darkened and all sources of light except
the one in use should be excluded.
Position of Light and Plane Mirror. These may be
as close together as 6 inches or as far apart as 6 meters.
It is a matter of choice with the surgeon himself where he
prefers to have them. /The writer recommends, however,
having the rays of light come from the 10 mm. opening in
the light-screen, at about 6 inches to the left and front of
the surgeon, so that the rays pass in front of the left eye
FIG. 147. Author's Iris
Diaphragm Chimney.
I6O REFRACTION AND HOW TO REFRACT.
and fall upon the mirror held before the right eye. Some
surgeons prefer having the light, with the 3 cm. opening in
the screen, placed over the patient's head or to one side of
it. (Fig. 148.) TJic distance between the light and mirror
will not alter the direction of the rays of light which comt
from the patient's eye.
Position of the Light and the Concave Mirror (Figs.
148, 149, 150). As the purpose of the concave mirror in
retinoscopy is to focus rays of light before they enter the
I METER
FIG. 148. Light over Patient's Head, and the Observer with Mirror at One
Meter Distance.
patient's eye, it is always necessary to have the light and
mirror widely separated. Usually, the light with the 3 cm.
opening in the screen is placed to one side or over the pa-
tient's head, and the surgeon with the mirror is seated about
one meter from the patient. This will place the focus of the
25 cm. mirror about 33 cm. in front of the mirror.
Distance of Surgeon from Patient. With the plane
mirror he may approach within a few inches of the patient's
eye to find the point of reversal, but with the concave mirror
RETIXOSCOPY.
161
he must remain at a sufficient distance to have the focus of
the mirror in front of the patient's eye.
How to Use the Mirror. It should be held firmly in
FIG. 149. Illustrating High Myopia with a Concave Mirror.
Rays of light from the lamp (L) are reflected by the mirror (//'), and form a
conjugate focus at L', and the rays from this focal point illuminate the
retina at L 1 . Corresponding effects result when reflection takes place
from the mirror at m". The eye (E) behind the mirror recognizes points
of reversal between the eye and mirror, moving in the same direction to
that in which the mirror is tilted.
FIG. 150. Illustrating Hyperopia with the Concave Mirror.
The eye (E) recognizes a virtual image behind the eye under examination, so
that when the mirror (/') is focusing the rays from the lamp (L) at I/,
the upper portion of the retina is illuminated, and vice versa, when the
mirror i m" \ is focusing the rays at L,, the lower portion of the retina is
illuminated. The retinal illumination moves opposite to that of the mirror.
the right hand before the right eye, so that the sight-hole
is opposite to the observer's pupil. The movements im-
1 62 REFRACTION AND HOW TO REFRACT.
parted to the mirror must be limited, though they may be
quick or slow, but never at any time should the mirror be
tilted more than 2 or 3 mm., otherwise the light will be lost
from the eye.
What the. Observer Sees, or the general appearance
of the reflection from the eye. The reflex from the pupil
varies in different patients, and is subject to many changes
as the refraction is altered by correcting glasses, by the
turning of the patient's eyes, by increasing or diminishing
the distance between patient and surgeon or the distance
between the light and mirror, or the strength of the
light. The amount of pigment in the eye-ground will
change the general appearance of the reflex, being dim
in some mulattoes, and very light in the blonde or albino.
If the refractive error is a high one, the reflex will appear
dull ; or if a low error, it will appear very bright. If the
>. media are not clear, the reflex will be altered accordingly.
The bright pin-point catoptric images seen on the cornea
and lens are not parts of the test, and should be avoided or
ignored. The I mm. bright ring of light sometimes seen
at the edge of the pupil should be avoided by the beginner
in retinoscopy, as it is an indication of spheric aberration,
which he will have to consider after mastering other details
of the method.
Facial Illumination. The rays of light reflected from
the mirror illuminate a portion of the patient's face, and
always move in the same direction as that in which the
mirror is tilted, no matter whether the mirror is plane or
concave.
Retinal Illumination, This corresponds to the portion
of the retina which receives the rays of light reflected from
the mirror. The retinal illumination is also called "the
image," "the light area," etc.
RETINOSCOPY. 163
The Shadow. This is the non-illuminated portion of
the retina immediately surrounding the illumination. The
illumination and shadow are, therefore, in contact ; if the
illumination changes its place upon the retina by a move-
ment of the mirror, then the shadow will move also. By
this change of illumination and shadow we speak of a
movement of the shadow.
Where to Look and What to Look for. Rotating the
mirror through the various meridians of the eye, the
observer makes a note of the (i)form, (2) direction, and
; (3) rate of movement of the retinal illumination as he
watches for them through a four or five millimeter area at
the apex of the cornea, as this is the portion of the refract-
ive media in the normal eye that the patient will use
when the effects of the cycloplegic pass away and the
pupil regains its normal size.
Point of Reversal. To find the point of reversal is the
underlying principle of retinoscopy. For example, having
determined with the plane mirror at a distance of one meter
that the retinal illumination moves with the movement of the
mirror and a -f~ 2 -5O S. stops all apparent movement (no
movement of the illumination being seen and the shadow
having disappeared), the observer knows that his eye is at
the point of reversal. Or with the concave mirror the
retinal illumination will move opposite to the movement of
the mirror and will stop with +2.50 S. before the eye.
The point at which all movement of the retinal illumina-
tion appears to have ceased is the point of reversal.
The real movement of the retinal illumination de-
pends upon the mirror whether it is concave or plane.
With the plane mirror the retinal illumination always moves
with the mirror and the light on the face ; whereas with the
concave mirror (focusing rays before they enter the eye),
164 REFRACTION AND HOW TO REFRACT.
the real movement of the retinal illumination is always
opposite to that of the mirror. The student should not
get the real and apparent movements confused, but pay
close attention to the apparent movement.
Direction of the Apparent Movement of the Retinal
Illumination. With the plane mirror, the apparent move-
ment of the retinal illumination will be with the mirror and
with the light on the face as long as the observer is within
the point of reversal ; but just as soon as the observer is
i beyond the point of reversal, the retinal illumination \\ill
appear to move opposite to the movement of the mirror
and opposite to the movement of the facial illumination.
F"
FIG. 151.
With the concave mirror the apparent movement of the
retinal illumination will be with the movement of the mir-
ror and the light on the face as long as the observer is be-
yond the point of reversal (Fig. 149) ; but just as soon as
the observer's eye is within the point of reversal, the retinal
illumination will appear to move against the movement of
the mirror and against the light on the face. (See Fig. 1 50.)
Rate of Movement of the Retinal Illumination. This
is influenced by several factors, but practice will teach the
observer that when the retinal illumination appears to move
slowly, the refractive error is a high one, and when it moves
fast, the refractive error is a low one.
Figure 151 represents a myopic eye with its far point
RETINOSCOPY.
i6 S
(point of reversal) at R/, and when rotating the mirror, this
point moves to R" ; but if the eye had its far point at F',
and the mirror was rotated to F", then the illumination at
R', having to move through a smaller arc in the same time,
appears to move slowly as compared with F', which ap-
peared to move fast. The same condition is shown in
figure 152, in which the observer appears to see an erect
virtual image back of the retina, and R' appears to move
slowly as compared with F', which appears to move fast.
Form of Illumination.-eA large, round illumination
may signify emmetropia, hyperopia, or myopia, with or with-
out astigmatism in combination.) Astigmatism is recognized
FIG. 152.
by the presence of a band of light, and this band of light
may be seen before any correcting lens has been placed be-
fore the eye if the astigmatic error is high ; or it will be
recognized during the process of neutralization if the error
is small /. e., if the astigmatism is of low degree. )The pres-
ence of astigmatism is known, therefore, by the band of light
or when the illumination appears to move faster in one
meridian than in the meridian at a right angle. The astig-
matism is in the meridian of slow movement.
The apparent difference between the plane and con-
cave mirror in the direction of movement of the retinal
illumination. \Yith the plane mirror the rays of light are
reflected as if they came from a point just as far back of
1 66
REFRACTION AND HOW TO REFRACT.
the mirror as the original source of light is in front of it.
The surgeon's eye behind a plane mirror is, therefore, in
the path of these rays, and sees that portion of the pupil-
lary area illuminated to which these rays are directed.
(See Fig. 153.)
With the concave mirror the reflected rays come to a
focus, forming an inverted image of the flame, which be-
comes the immediate source of light in front of the
observer's eye. When the concave mirror is tilted, the
immediate source of light goes in the same direction, but
with the result that the opposite portion of the pupillary
area is illuminated. (See Fig. 143.) This shows the
FIG. 153.
immediate source of light at L' and mirror tilted downward ;
the rays proceeding from L' diverge and illuminate the
upper portion of the pupillary area. Tilting the mirror
upward, the immediate source of light at L' moves upward
also (L 2 ), and the lower portion of the pupillary area be-
comes illuminated. (See also Fig. 149.)
Rule for Neutralizing Lenses with the Plane Mirror.
When the retinal illumination appears to move in the
same direction as that of the mirror, the observer is within
the point of reversal and a plus lens must be placed before
the eye to stop all apparent movement. When the retinal
illumination appears to move in the opposite direction to
RETINOSCOPY. 1 67
that in which the mirror is tilted, the observer is beyond
the point of reversal, and a minus lens must be placed
before the eye to stop all apparent movement.
Rule for Neutralizing Lenses with the Concave
Mirror. \Yhen the retinal illumination appears to move in
the same direction as that in which the mirror is tilted, the
observer is beyond the point of reversal, and a minus lens
must be placed before the eye to stop all apparent move-
ment. When the retinal illumination appears to move in
the opposite direction to that in which the mirror is tilted,
the observer is within the point of reversal, and a plus lens
must be placed before the eye to stop all apparent move-
ment.
Rule for neutralizing lenses, no matter whether the
mirror is plane or concave. When within the point of
reversal, use a plus lens, and when beyond the point of
reversal, use a minus lens.
Application of Retinoscopy in Ernmetropia (Fig.
153). Rays of light proceed parallel from an emmetropic
eye under the influence of a cycloplegic, and if a -f- 1 S.
is placed in front of such an eye, the rays will converge
and form a point of reversal at I meter distance, and the
observer at this point will not be able to see any move-
ment of the retinal illumination. The same result would
have been obtained at ^ of a meter if a -+- 3 S. had been
used, or at 4 meters if a -(-0.25 S., or at y 2 of a meter
if a +28. had been used, etc.
In taking the patient from the dark-room to test his
vision at 6 meters, an allowance must always be made for
the distance from the patient's eye at which the point of
reversal was found. If at ^ of a meter, 3 S. must be de-
ducted from the lens used ; if at ^ of a meter, 48.; if at
6 meters, nothing, or o. ij2._
i68
REFRACTION AND HOW TO REFRACT.
Application of Retinoscopy in Hyperopia. The
x'same conditions hold good in hyperopia as in emmetropia.
If a +4 S. gives a point of reversal at one meter, then
I S. must be taken from the 4 S. to give the eye parallel
rays of light, or infinity vision. If a +48. gave a point
of reversal at 2 meters, then 0.50 S. would have to be
deducted from the 4 S. for the infinity correction, which
would be -+-3.50 S.
Application of Retinoscopy in Myopia. Rays of light
from a myopic eye come to a focus at some point inside of
infinity, and if the surgeon so desires, he may approach such
an eye from a distance of six. meters, until he finds a point
where the retinal illumination ceases to move (where it does
not appear to move) ; and then, measuring this distance from
the eye under examination, he can quickly calculate the
amount of the myopia. This can not be done with the
concave mirror if the myopia is more than 2 S. If the
reversal point is at 4 meters, 3 meters, 2 meters, I meter,
*4 of a meter, ^ of a meter, or ^ of a meter, then the
myopia would be 0.25 S., 0.33 S., 0.50 S., I S., 2 S., 3 S.,
4 S., respectively.
If the surgeon will always refract the patient's eyes so
that he gets the point of reversal at I meter distance, he
will have the following rule to guide him /. c. :
., To add a I sphere to the dark-room correction, no
matter what that may be. For example :
Dark-room, o.oo 40.258. 40.508. -(-0.758. 4- I -S. -(-1.25 S.
Add, . . . i.ooS. 1.008. 1.008. i.ooS. i.ooS. i.ooS.
Infinity, . . i.ooD. 0.750. 0.50 D. 0.25!). o.oo 40.25 D.
Application of Retinoscopy in Astigmatism. If the
surgeon has mastered retinoscopy in hyperopia and myopia,
he should not have any difficulty in pursuing exactly the
RETINOSCOPY. 169
same course in cases of astigmatism. As already stated,
the presence of astigmatism is diagnosed by the presence
of a band or ribbon-like streak of bright illumination which
extends across the pupillary area.
(See Fig. 154.) This band of light
may be seen before any neutralizing
lens is placed in front of the eye,
if the astigmatism is in excess of
the spheric correction, as in the
following formula : I L
+0.75 sph. C +4-50 cyl. axis 105 degrees. Fl . G - . I 54-~ Band of Light.
Astigmatism Axis 90 de-
i.oo sph. ^j 5-OO cyl. axis 165 degrees. aree
Or the presence of astigmatism
may not be recognized until after a sphere has been placed
in front of the eye, as in one of the following formulas : ,.-,,
+ 4.50 sph. C +0.75 cyl- axis 75 degrees.
5.00 sph. ^ i-oo cyl. axis 180 degrees.
In refracting cases of astigmatism with the retinoscope,
[all the surgeon has to do is to refract the meridian of least-''
ametropia first, and then the meridian of greatest ametropia. ^*^
Taking the following formula :
-(-2.50 sph. ;3 -f i.oo cyl. axis 90 degrees ;
in the dark-room a +3-5O S. would make all movement
cease in the vertical meridian, at one meter distant ; but
when the mirror is tilted in the horizontal meridian, there
would be seen a band of light extending across the pupil
on axis 90 degrees. Then, substituting +4.50 S. for the
3.50 S.,all movement will cease in the horizontal meridian,
a +4.50 S. neutralizing the horizontal meridian. The
difference between these two spheres is I D., which is the
amount of the astigmatism. In neutralizing astigmatism
ppT-the writer advises using spheres, and after each meridian
I/O
REFRACTION AND HOW TO REFRACT.
has been refracted, to make the cylindric correction, and
prove it, if so desired.
Axonometer. To find the exact axis subtended by the
band of light while studying the retinal illumination, when
the meridian of least ametropia has been corrected, the
writer has suggested a small instrument, which, for want of
a better name, he has called an axonometer. This is a
black metal disc, with a milled edge, I ^ mm. in thickness,
of the diameter of the ordinary trial-lens, and mounted in
a cell of the trial-set. It has a central round opening, 12
FIG. 155.
mm. in diameter the diameter of the average cornea at its
base. Two heavy white lines, one on each side, pass from
the circumference across to the central opening, bisecting
the disc. To use the axonometer, place it in the front
opening of the trial -frame, and with the patient seated erect
and frame accurately adjusted, so that the cornea of the eye
to be refracted occupies the central opening. As soon as
that lens is found which corrects the meridian of least
ametropia, and the band of light appears distinct, turn the
h.
RETINOSCOPV. I/I
axonometer slowly until the two heavy white lines accu-
rately coincide, or appear to make one continuous line with
the band of light. (See Fig. 155.)
The degree mark on the trial-frame to which the arrow-
ead at the end of the white line then points is the exact
axis for the cylinder.
Application of Retinoscopy in Mixed Astigmatism.
Here the dark-room correction (after making deductions for
the distance of the point of reversal) will show one meridian
myopic" and the other, at right angles to it, as hyperopic. If
the astigmatism is more than one diopter, in each meridian,
the surgeon will diagnose in the dark-room the condition
of mixed astigmatism by opposite movements in the me-
ridians of minimum and maximum ametropia.
Application in Irregular Astigmatism. This condi-
tion is either in the lens or cornea, usually in the latter.
The reflex is more or less obscured by areas of darkness,
which make it extremely difficult to study the refraction,
and the observer will have to change his distance repeat-
dly to find clear spaces as close to the center of the pupil
as possible, as it is this portion of the pupillary area that
the patient will see through when the mydriatic effect passes
away. The kaleidoscopic picture obtained by moving the
mirror so as to describe a circle at the periphery of the
pupillary space is quite diagnostic of the corneal condition.
\Yhatever correction is obtained should be kept for reference
in a postcycloplegic manifest refraction, as it will not always
do to order the glasses while the eye has its pupil dilated.
The patient may choose a slightly different correction in
such cases, after the pupil regains its accustomed size.
Irregular Lenticular Astigmatism. This is often more
uniform than the corneal variety, and is characterized by
faint striae in the lens, pointing in toward the center. If
REFRACTION AND HOW TO REFRACT.
the striae are not very faint, they may be recognized with the
ophthalmoscope, even before any cycloplegic has been used,
issor Movement (see Fig. 156). This is a condi-
i^/*tion in which two bands of light are present, usually in
the horizontal meridian or inclined a few degrees therefrom.
^>v^* Tilting the mirror in the vertical meridian, a band of light
^^0. 3 is seen to come from above and to meet another band, which
comes from below ; while these two bands are approaching,
the dark space between them gradually disappears, until the
two bands unite and form one band
across the pupil in or approximat-
ing the horizontal meridian. This
movement of the bands is likened
to the action of the blades of a pair
of scissors, and hence the name.
To refract a case of this character,
the observer must proceed slowly
and endeavor to neutralize the
horizontal meridian first, and then
add minus cylinders with the avi^ ^ojp^ponrting tr> fhp
FIG. 156. Scissor Move-
ment
axis of
two
The resulting prescription should
also be a plus sphere with a minus cylinder, the cylinder
of less strength than the sphere, asthe condition is^notjme
of mixed astigmatism, the patient preferring this combina-
tion, as a rule.
Conic Cornea. In this condition the observer is im-
pressed at once with the' bright central illumination, which
usually moves opposite to the movement of the peripheral
illumination. ^The best way to neutralize a case of this
character is to proceed as in a case of irregular astigmatism.
The observer should also be on the lookout for a band of
light in this central illumination, as most of these cases are
astigmatic.
RETINOSCOPY.
173
Spheric Aberration. This is of two kinds positive
and negative. (See Figs. 157 and 158.) In the positive
form the peripheral refraction (A, A, that at the edge of the
pupil) is stronger than the central (B, B) ; the reverse of
is condition, negative aberration, is seen in conic cornea.
These two varieties of refraction should not worry the
observer, as most of the peripheral aberration is covered
up by the iris when mydriasis passes away, and, therefore,
is not of any great moment, except in conic cornea.
FIG. 157. Positive Aberration.
Fie. 158. Negative Aberration.
he Reisner Retinoscope (Figs. 159 and 160). This
instrument combines the mirror with an axis finder. The
metal disc, on which the mirror is secured by means of a
coiled spring at one point only, has a milled edge like that
of an ophthalmoscope. (See Fig. 159.) At the junction
of the metal disc with the handle is a small push-button
which has a short metal pointer which extends upward and
beneath the mirror. Figure 160 is a back view of the in-
strument, and shows the degree-marks of half a circle and
an index or pointer. When beginning the examination,
1/4 REFRACTION AND HOW TO REFRACT.
this instrument may be used just the same as any other
retinoscope, but as soon as the surgeon sees a band of light
tli en he presses the push-button to see if the mirror rotates
with its axis corresponding to the long measurement of the
band of light. If this is not so, then the mirror must be
turned by means of the milled edge, using the index-finger
as in turning the disc of an ophthalmoscope. When the
axis of rotation of the mirror corresponds with the long
FIG. 159. (One-half size.) FIG. 1 60. (One-half size.)
axis of the band of light, then the pointer on the back of
the instrument indicates the axis of the astigmatism, and
now all the surgeon has to do as he makes the changes in
the lenses is to press the button only. The handle of the
retinoscope is now held perfectly still and the upper portion
of the disc rests firmly against the observer's brow. The
merits of this ingenious instrument are as follows : The
RETINOSCOPY. 1/5
handle is held perfectly still, the mirror alone does the
moving; the sight-hole is always in front of the pupil; the
axis of the astigmatism is carefully recorded.
FIG. 161. (Two-thirds size. ) FIG. 162. (Two-thirds size.)
rer
Th
e Luminous Retinoscope (Figs. 161 and 162).
DcZcng Patent. The latest improvement in retinoscopes is
1/6 REFRACTION AND HOW TO REFRACT.
the luminous instrument here described. This instrument
is the author's plane mirror with the electric light attach-
ment. (Fig. 1 6 1.) The filament is contained in a tube
placed at an angle of 45 degrees with the handle and the
mirror is correspondingly tilted at an angle of 22 degrees.
The light from the filament passes divergently to a strong
convex lens which renders the rays less divergent as they
fall upon the mirror, and from the mirror the rays pass
divergently to the patient's eye. (See Fig. 162.) This in-
strument has innumerable points of merit: It does away
with any use of gas or lamp or cover chimney; the ob-
server is not annoyed with the heat from the gas or lamp ;
the observer does not have to move the light or bracket
when changing from one distance to another as when work-
ing with the gaslight close to the mirror ; the electric wires
(cords) carrying the current to the filament are of sufficient
length to give the observer two meters of space in which
to practise the method; the brilliancy of the illumination
can be made most intense or diminished very materially
with a convenient rheostat; the size of the divergent pencil
may also be controlled by adjusting the condensing lens at
the end of the tube.
CHAPTER VII.
MUSCLES.
EXAMINATION OF THE EXTERNAL EYE MUSCLES.
General Considerations. When the retinal image of an
object is situated exactly on the fovea, the eye is said to
"fix" the object.
Normally, when b^oth eyes "fix " the object, each eye
has an image of the object on its fovea, and these foveal
images or impressions are transmitted to the brain and fused
as one image in the visual centers. This condition is spoken
of as equipoise, or orthophoria, and the eyes are said to be
in equilibrium, or to balance. Whenever one eye alone fixes
an object, and the fellow-eye receives the image of the same
object on a part of its retina distant from the fovea, then
the brain takes note of two separate impressions, and this
condition is spoken of as double vision (diplopia).
(rt) The image of an object formed upon the retina above
the fovea is projected downward /. c. t objects situated
below the horizontal line of vision are recognized by that
portion of the retina above the fovea.
(&) The image of an object formed upon the retina below
the fovea is projected upward i. e., objects situated above
the horizontal line of vision are recognized by that portion
of the retina below the fovea.
(r) The image of an object formed on the retina to the
nasal side of the fovea is projected toward the temporal
side /. c., objects to the temporal side have their images
formed upon the nasal portion of the retina.
177
REFRACTION AND HOW TO REFRACT.
(d) The image of an object formed on the retina to the
temporal side of the fovea is projected toward the nasal side
/'. c., objects to the nasal side have their images formed
upon the temporal portion of the retina.
Homonymous Diplopia (Greek, ^xovurw-; from /*?,
same, and Zw<ia, name). Figure 163 shows the right
FIG. 163.
eye (R) fixing upon the object (O), but the left eye is
turned inward, so that rays from O fall upon its retina to the
nasal side of the fovea (M), and are projected outward to
the temporal side ; the result is that the left eye sees a false
object to the left of the real object. This condition of the
objects is spoken of as homonymous diplopia.
MUSCLES.
179
Heteronymous Diplopia (Greek, ere/?, other ; and ^
name). Figure 164 shows the right eye fixing the object
(O), but the left eye is turned outward, so that rays from O
fall upon the retina to the temporal side of the fovea and
are projected to the nasal side, with the result that the left
eye sees a false object
to the right of the real
object. This condi-
tion of the objects is
spoken of as heter-
onymous or crossed
diplopia.
H y p e r p horia
(Greek, "~ /^ over,
jiboj^g. ; ^<y>efv, to
tend). In the con-
sideration of vertical
diplopia, which is
always a condition
of crossed diplopia,
never homonymous
diplopia, h the eye
which is deviated up-
ward is spoken of as
the hyperphoric eye, x >
and necessarily its
image must be lower
than its fellow. For instance, if the left eye fixes an object
and the right eye is turned upward, the rays of light from
the object would fall upon the upper part of the retina of the
right eye, and would be projected downward below the true
object ; and this position of the right eye is spoken of as
right hypcrphoria. Or if the right eye fixes an object and
O'M
FIG. 164.
ISO REFRACTION AND HOW TO REFRACT.
the left eye sees a false object below, then the position of
the left eye is spoken of as left hyperphoria. Unfortunately,
in hyperphoria (unless from paralysis) the position of the
eyes does not tell whether the right superior rectus is too
strong and the left inferior rectus too weak, or the left supe-
rior rectus too weak and the right inferior rectus too strong.
Muscle Phorometry. Testing the power of the ocular
muscles.
Abduction. The power of the external recti muscles to
turn the eyes outward. The patient is comfortably seated
and told to look at a point of steady light at a distance of
about 6 meters, slightly below the level of his eyes, never
above the level. In this position prisms with their bases in-
! ward are placed in front of one or both eyes until the
patient says he sees two lights very close together. The
strength of the prism or prisms thus placed before the
eyes which will just permit the eyes to see one object
and if increased would produce diplopia, represents the
power of the external recti muscles. This is spoken of as
the power of abduction, and is abbreviated Abd. For
example, if with 7 centrads, base in, before the eyes there
are two lights, and with 6 centrads there is only one light,
then 6 centrads would represent the amount of the abduc-
tion. In other words, in the case supposed, as long as
there is less than 7 centrads before the eyes, base inward,
the external recti muscles can overcome their effect, but as
soon as a prism stronger than 6 centrads is used, then the
external recti muscles can not counteract the effect, and
diplopia is the result.
Adduction. The power of the internal recti muscles to
turn the eyes inward. The power of the internal recti is
tested in the same way as the external, except that the
prism is placed base outward. This is spoken of as adduc-
T' I I j, 1^ '\S-*-
) \S^\ X V C/i
. '
)
MUSCLES. iSl
tion, and is abbreviated Add. For example, if with 19
centrads, base out, before the eyes two lights are seen, and
with 1 8 centrads only one light, then 18 centrads represent
the power of adduction. In other words, as long as there is
a prism of 18 or less than 18 centrads before the eyes, base
outward in this case, the internal recti muscles can over-
come the effect ; but as soon as a prism stronger than 1 8
centrads is used, then the internal recti muscles can not
counteract the effect, and diplopia is the result. It must
be remembered that the internal and external recti are
antagonistic, and that the muscles of the two eyes are
tested together. The relative power of adduction to abduc-
tion has been variously estimated, but most authorities are
agreed that^adduction is about three times that of abduc-
tion, or about 3 to I that is to say, in eyes with normal
muscle balance, if adduction is represented by 18 centrads,
then abduction should be 6 ; or if adduction is represented
by 24 centrads, then abduction should be 8 centrads ; or if
adduction is 1 2 centrads, then abduction should be 4 cen-
trads, etc^
Sursumduction. This is the power of the eyes to fuse
AVO images when one eye has a prism placed base up or
down before it. For example, if a 3^ centrad prism is
placed base up or down before either eye and diplopia
results and persists, and then a 3 centrad is substituted and
there is no diplopia, then the eyes have overcome the effect
of the prism and the amount of the sursumduction is said
' to be 3 centrads. This test for sursumduction is made at
the same distance as in testing the lateral muscles. In
health the power of the superior and inferior recti muscles
is, as a rule, the same that is to say, they antagonize each
other equally. The power of the superior recti is spoken of
I as supraduction, snrsninvergencc ; and that of the inferior
; recti, as infraduction, dc.i>rsnnii'crgcncc.
1 82 REFRACTION" AND HOW TO REFRACT.
Muscular Imbalance. Whenever there is any disturb-
ance in the power, strength, or force of the ocular muscles,
the condition is no longer one of equipoise, or equilibrium,
or muscle balance, but is spoken of as muscular imbalance
(heterophoria). From this statement it must not be sup-
posed that the two eyes can not simultaneously " fix " an
object, any more than it must be supposed that a hyperoptc
eye can not see or have ..- vision ' without correcting
glasses.
Just as in hyperopia distant vision may be made clear
by the effort of accommodation, so in muscular imbalance
the visual axes can be directed to one point of fixation by
increased innervation. Muscular imbalance is subdivided
into two classes insufficiency and strabismus.
The following nomenclature of muscular anomalies, sug-
gested by Stevens, of New York, is in common use :
OrtJiopJioria, perfect muscle balance, equipoise, or binocular
equilibrium.
Orthotropia, perfect binocular fixation.
Hctcroplwria, imperfect binocular balance, or imperfect bin-
ocular equilibrium.
Hcterotropia, a squint or decided deviation or turning from
parallelism.
Hypcrplioria, ^tendency of one eye to deviate upward.
Hypertropia, a deviation of one eye upward.
Esophoria, a tendency of the visual axes to deviate inward.
Esotropia, a deviation of the visual axes inward.
Exophoria, a tendency of the visual axes to deviate outward.
Exotropia, a deviation of the visual axes outward.
Hypercsophoria, a tendency of the visual axis of one eye
to deviate upward and inward.
Hypercsotropia, a deviation of the visual axis of one eye
upward and inward.
MUSCLES. 183
Hypercxophoria, a tendency of the visual axis of one eye
to deviate upward and outward.
Hypcrexotropia t a deviation of the visual axis of one eye
upward and outward.
Insufficiency. Also called latent deviation, hetero-
phoria, or latent squint. This may be defined as the con-
dition in which there is a tending or tendency of the visual
axes to deviate from the point of fixation ; this may be slight
or transitory.
Causes of Insufficiency. The chief cause of insuffi-
ciency is some form of ametropia. Another cause may be
an anatomic defect of one or more of the ocular muscles
themselves, or a weakness of the muscle or muscles indi-
vidually, or as a result of some systemic weakness. The
ocular muscles often sympathize with the economy.
Symptoms of Insufficiency, or Muscular Asthenopia.
Accommodative and muscular asthenopia are intimately
associated, and the latter is so often the companion of the
former that they produce symptoms which are identical in
both and make it difficult to draw any sharp line of demar-
cation between the two. In muscular asthenopia, how-
ever, the patient complains that the eyes "become weak"
or "tired" after any prolonged use, and that this is espe-
cially apt to occur by artificial light ; that nearby
objects (reading, writing, or sewing) grow dim ; that the
words "seem to jump," or the "letters run together," and
in some cases occasionally, and in others more frequently,
objects appear double for a moment. Sometimes one of
the eyes feels as if it \vas turning outward or inward.
There are innumerable reflex symptoms, dizziness, nausea,
vomiting, fainting, and, in some instances, " all becomes
dark for a minute." Such patients often become very
anxious, fearing sudden blindness, etc.
184 REFRACTION AND HOW TO REFRACT.
Diagnosis or Tests for Insufficiency (Heterophoria).
Before taking up the individual tests for insufficiencies, it is
well for the observer to study the movements or excursions
of the eyes ; and to do this the patient, with his head erect
and steady in one position, fixes with his eyes the point of
a pencil held in the hand of the observer at about thirteen
inches distant. The pencil is moved from left to right and
from right to left, and upward and downward ; as this is
done, the surgeon should watch closely to see that each
eye has a normal mobility and the two eyes move together.
From a central point of fixation the eyes should move
inward about 45 degrees, outward 45 or 50 degrees,
upward about 40 degrees, and downward about 60 degrees.
The tropometer of Stevens will estimate the limit of motion
of each eye separately, but if there is a defect in mobility,
the surgeon may recognize it by comparing the distance of
the corneal edge in each eye from a certain definite fixed
point ; for instance, whether the lid margins encroach
equally upon the cornea or have equal intervals between
cornea and lid edges.
The Cover Test. The patient is told to look at the
point of a pencil held in the hand of the surgeon on a level
with the patient's eyes in the median line, and distant about
eighteen inches, or at an object six meters distant. While
the eyes fix the point of the pencil or distant object, the
surgeon covers one eye with a small card, and a moment
later quickly withdraws it and observes the position and
movement of the eye which he has just uncovered ; if it
moved inward toward the nose to fix the point of the pen-
cil, then there must have been an outward tendency of that
eye when under cover ; in other words, the external muscles
must have been strong or the internal weak. If the eye
thus released from the cover had moved outward toward
MUSCLES. 185
the temple to fix the point of the pencil, then the external
recti must have been weak or the internal strong. If the
eye released from cover goes up to fix. then the fellow-eye
deviates upward, and vice versa. This test is not always
reliable, and yet it may be a guide to further study.
The Fixation Test. Instead of covering one eye, as in
the previous test, the patient " fixes " the point of the pencil
as it is slowly advanced in the median line toward the nose,
'up to within four inches, if necessary. During this advance
of the pencil, if there is a weakness of the interni, the eye
with the weaker internus is the one which will usually
deviate outward.
To Determine Lateral Insufficiency. The condition
in which there is either a tendency for the visual axes to
deviate outward (exophoria), or a tendency for the visual
axes to deviate inward (esophoria). Proceed by producing
" vertical diplopia. Place a ten centrad prism base down
before one eye, for instance, the right eye, and have the
patient look at a point of light on a level with his eyes at a
distance of six meters. He will see two lights, one above
the other ; the upper light must belong to the right eye,
because the prism before the right eye bent the rays down-
ward. If one light is directly above the other, then the
condition is presumably one of equilibrium or equipoise.
If the upper light, however, is to the right, then the
visual axes deviate inward (esophoria). The amount of
the esophoria (insufficiency of the external recti) is repre-
sented by that prism placed base outward before the left
eye which will bring one light directly above the other. If
the upper light had been to the left, then there would have
been a tendency of the visual axes outward (exophoria,
insufficiency of the internal recti), and the amount of the
exophoria is represented by the strength of prism placed
1 j bh $ fl&Sif**^*** 1 *
~T3 &-
1 86 REFRACTION AND HOW TO REFRACT.
base inward which will bring one light directly above the
other.
To Determine Vertical Insufficiency (Hyperphoria).
Proceed by producing lateral diplopia. Place a ten centrad
-j prism base inward before the right eye, and have the patient
look at a point of light, as in testing for lateral insufficiency.
(It is always well to have the point of light just in front of
a large piece of black felt cloth tacked upon the wall.)
, If the two lights which the patient sees are on a horizontal
'I line, then the condition is presumably one of equipoise. But
(if the right light is lower than the left, there is a tendency
of the visual axis of the right eye to be higher than its
fellow. ) As to which muscle is at fault, this test will not
tell, and Stevens' tropometer will have to be used. The
amount of the deviation is represented by the strength of
prism placed base down before the right, or upward before
the left eye which will bring the two lights into a horizontal
line /. c., on a level.
To Determine Lateral Insufficiency at the Reading
Distance. Have the patient look at a black dot, with a
black line two or three inches long running perpendicularly
through it, at a distance of about thirteen inches. This is
> known as the line-and-dot test of von Graefe, and on a
larger scale may also be used in the previous tests. A prism
of seven or eight centrads is placed, with its base down, in
front of the right eye. If the patient sees two dots exactly
one above the other on one line, there is not supposed to be
any insufficiency. If, however, there are two lines and
two dots, and the upper dot is on the right, there is in-
sufficiency of the externi (esophoria) for near. The amount
of the insufficiency is represented by the strength of prism,
placed base outward, before the left eye which will bring
the two dots exactly on one line. If the upper dot is to
MUSCLES.
the left, then there is insufficiency of the intern! (exophoria)
for near, and the amount of the insufficiency is represented
by the strength of prism placed base inward over the left
eye which will bring the two dots, one above the other, on
one line.
Another method for testing lateral insufficiency at the
reading distance of 1 3 inches is to have a card about 6
inches square, and on this card to draw a heavy black line
about 3 inches long; this line is to be horizontal. At
the middle of the horizontal line draw a heavy black line,
one-half an inch long, extending vertically from the hori-
zontal line ; this short vertical line to be capped with an
arrow-point. The horizontal line is divided off into equal
spaces, each 3 l /$ millimeters apart and .numbered from I
to 1 5 each side of the arrow ; those to the left of the arrow
are marked "esophoria," and those to the right of the arrow
are marked "exophoria." (See Fig. 165.)
To use this method, a prism of 8 centrads is placed base
down before the right eye ; this doubles the scale vertically ;
the upper scale belongs to the right eye. The number
and the word in the upper scale to which the arrow in the
lower scale points, is the approximation in centrads of the
amount of the esophoria or exophoria. For instance, if
the lower arrow points to figure 9 in the upper scale to the
right of the upper arrow, that is, the word "exophoria,"
1 88 REFRACTION AND HOW TO REFRACT.
then there is approximately 9 degrees of "exophoria" at
this distance of 1 3 inches, the distance at which this scale
is intended to be used.
These tests for insufficiencies should always be made be-
s. fore estimating the refraction, and also after the correcting
lenses are carefully placed, with their optic centers, before
the eyes.
/ -To avoid confusion in making these tests, when a point
^ of light is used as the fixing object, it is customary to place
a piece of plane dark red glass before one eye, so that the
FIG. 166. Maddox Rod. FIG. 167.
> | red light always corresponds to the eye with the red glass.
Or a Maddox rod (Fig. 166), white or red, may be used
for the same purpose. This may be a series of rods (see
Fig. 167) placed in a metal cell of the trial -case, and the
eye, looking through it at the light, will see the image of
the flame distorted into a streak of broken light. A strong
-f- cylinder from the trial-case will answer the same pur-
pose. (As the rod refracts rays of light opposite to its axis, ;
the eye will see a streak of light in the reverse meridian to
MUSCLES. 189
that in which its axis is placed. To expedite the deter-
minations, the rotary prism of Cretes or the revolving
prisms of Risley may be employed. This latter apparatus
(see Fig. 168) is composed of
two superimposed prisms of I 5
centrads each, and mounted in a
milled-edged cell of the size of
the trial-lens. By means of a
milled-edged screw these prisms
are made to revolve so that in
the position of zero they neutral-
ize each other, and when rotated
over each other the prism j. 1( . l68
strength gradually increases un-
til the bases of the prisms come together and equal 30
centrads. The strength of the prism employed is indicated
by an index on the periphery of the cell.
Stevens' Phorometer. This is a very convenient appa-
ratus, composed of two 4-degree prisms placed in a frame
3 l / 2 inches from the eyes, which with an attached lever can
be rotated so as to test the strength of the vertical and
lateral muscles. Indexes and letters at the periphery of
the frame record the character and degree of the insuffi-
ciency. (See Fig. 169.)
Treatment of Insufficiencies. As ametropia is the
most common cause of insufficiency, the" first consideration
must be to select the proper correcting glasses. After this
has been accomplished, if the insufficiency still persists and
the patient is not comfortable, then the muscles should re-
ceive careful attention, and their condition be studied from
every point of view. The patient's general health should
be looked after, and if at all defective, must have remedies
prescribed for its improvement. In some instances the
190
REFRACTION AND HOW TO REFRACT.
patient may have to give up any close application of the
eyes for a time and pursue an out-door life. Operative
interference (tenotomy) must not be entertained until all
known means for the relief of the muscular asthenopia
have been exhausted.
The prescribing of prisms, as a fixed rule, for
use, which correct insufficiency, except in vertical erro/s, is
often a serious mistake on the part of the surgeon, as in
most instances they often do more harm than good by in-
creasing the difficulty. Internally, sedatives will frequently
-- Phorometer
Revolving
Maddox Rod
Revolving
Trial Frame
Revolving
Rotary Prism
FIG. 169.
give great satisfaction and permanent relief. The writer is
partial to the use of bromids with small doses of the iodid
of potash three or four times a day. The modus operand'
is not clear. The only guide that can be suggested is to
use sedative treatment and rest of the eyes whenever there
is a congestion of the choroid and retina and when the
ophthalmoscope shows the nerve edges hazy, the retina
woolly, etc. - "In another class of patients the internal use
of nux vomica is the treatment par excellence, and it acts
best in those cases where the nerve edges and the eye-
ground in general appear clear and free from irritation.
*s*r<?
To use nux vomica it must be given in the form of the
mcture and increased, one drop at each dose, until the
Datient becomes quite tolerant of it, taking as high as thirty,
ty, or even fifty drops three times a da}', and then the
dfcse is gradually diminished. Xux vomica does not seem
do well in cases in which the bromids are indicated as
viceyersa. (De Schweinitz.)
featment of Insufficiency of the Internal Recti.
Because the tests for heterophoria at 6 meters show an v
ability on the part of the patient to maintain equilibrium, it ^
must not be supposed that there may not be an insufficiency.
The normal ratio of adduction to abduction should be taken
nto consideration in every instance before coming to any .
such conclusion.
i^After the proper correcting glasses have been prescribed ^
-jand the patient's general health looked after, attention, if ^
necessary, should be directed to strengthening the weak
,muscles ; and to do this they must be given a certain amount
"rnatic exercise, known as ocular gymnastics. That
'success shall result from ocular gymnastics means perse-
verance on the part of the patient and the exercises system-
atically executed. There are two methods of procedure : in
cases of exophoria Dr. George M. Gould, " Med. Xews,"
-Nov. 18, 1893
1. Have the patient "fix" the point of a pencil, or the
end of his finger held at arm's length, and slowly draw
it toward the bridge of the nose. If diplopia results while
doing this, the exercise should cease, and be repeated from
the original distance. This, is a very convenient exercise., v,
rf <u *-IL< ^*-*~+*^ Ikdv'Vf rvyvon
and should be practised several times a day and a number/
etot6*^**^~t*ii^ y*> '>jL*fr. f \^^^ '^"Wt-rtAJM*- LA
oftimes at each sitting.
2. Prism Exercises. The patient is placed, standing ^
about a foot or two from a point of steady light, on a lev
or slightly below the level of the eyes, and told to look at
v
>
F92 REFRACTION AND HOW *O REFRACT.
it., and at ^nothing else. In this position a pair of weak
/ 4ff 4 Ufa fir l
prrsms DasefouT, in a trial-frame are placed in front of his
eyes. *
Then he is told to walk slowly backward as he keeps
his eyes fixed on the point of light. Should diplopia de-
velop at any distance short of 20 feet, then he is to raise
the prisms, go back to his original position, and start over
again. Repeating this a number of times in the surgeon's
office, it will be found, in most instances, that at the first
practice a pair of 5 A or 10 A can be overcome at a distance
of 20 feet. When the distance of 20 feet from the light is
reached without developing diplopia, the patient is instructed
to slowly count 20 or 30 (keeping the light single during
this time), then raise the prisms (gazing at the light), and to
slowly count 20 or 30 ag<yjy ^This-exercise is repeated
three or four times a day^ma a number of times at each
practice. A prescription is given for such a pair of square
prisms with a convenient frame to wear over the patient's
glasses. These exercises should, as a rule, be conducted
with the patient wearing his correction. Instead of the
prism-frame, the patient may hold the square prisms with his
hands ; but these are tiresome to hold, and for general use
the prism-frame, if not too heavy, is preferable. After a
few days' practice at home, the patient returns, and stronger
prisms which will permit the patient to maintain single
vision are ordered. This practice with stronger and stronger
prisms is repeated until the patient is able to overcome
prisms greatly in excess of the normal ratio of adduction
to abduction. It is often well to develop the power of the
internal recti to three or four times the strength of the ex-
ternal recti ; for when the exercises are stopped, some of the
strength of adduction will rapidly disappear.
It has been incidentally mentioned that prisms should
of class 2- ;ts in this class, rs: just stat-
, are usually . ; -epic r rC therefore require
to corrections . ; restyopes with exophoria hs
v reachec a stage of lif r when it is difficult
ost impossible t p'_:t new life irto old rt
or
ture
ruscl
ateneit. aro this fart becer.es more and nr.ore evider
th<! pt passes beyond fifty ;rc. Developing add
and it c!eer. seem as if the ar
ike the cil IT. were no exception to this st
by irpctisir.r foxation at 33 centimeters -ray
li ch a gecc i in some ycur.r ps,tut *
power
-tion with fixation or iris
is in Almost every instance '-nd with few
c cations a waste of time in ps after fifty fifty f
five of sixty yrs. These ps do not trke kincHJ
to such tr and in the Friters experience ,pts so tr
so jn seek assistance elsewhere.
In gyperphoria the full prismatic correc
tion( except in cases of presbyopia) is seldom erdei
ed-only alout two-thirds of it and this divided bet
ween the two eyes-base down in one and b-se u befi
re the othe r .
Testing the rusculsr condition at "3 c
centimeters with 'he presbyoric near correction be
fore the pt's eyes, there should be cbout ten degree
s of exophoria normally at this di:t&ncebut if fchi
there happens to be I2,I4,or 16 degrees ef exophori
then the presbyejsia is mnco~f or table ard complains
correspomdl when uring the eyes at near work fi
length of time. The treatment of
exophoria at the working cirtance in presbs* is to
add or prescribe prisms bases in to be made in the
near correction. The amt of prisn to be so ordered
is usually c-ivieed between the two eyes. As ! de-
grees of exophoria is normal for this distance of
^eteres as ^ust rentioned, then the amount
of escribed will be practically two-thirds
:he amount shown is excess of the normal it 1C
^ees. For example., the patient at fifty yi
'or each eye plus one sphere pe-riscopic
and has a vision of 6/6 'n each eye rith this cor-
rection snd does not reveal any insufficiency at 6
retere,but when p:us 2 sphere is added for near,*hi
then gt 33 cms there is found to be 16 degrees of
exophoris.
for a fe\v- days at
r.e teh patient returns cor.p"! sinin/g that a
close work the eyes pain,feel sore to th
uch,has occip hdrche , smarting of lids rnd
urred vision. The exophoria in this inst
"7 cgrees in excess of tfte norr:
amount. Ordering tv;o- thirds of this amour J?
ur degrees) divided t etween the two eyes,
e patient rill recve one cf the following
" i o n s ;
. . . . D.Ieriscopic
11 so' . . . C.T . '
00 n t
ase in.
fr near
. lus . . ctr.l -f - ed
.
lrer
rart
for
ef the
distance
fr
' -ed 2.: D
Base i?
o.s.
near.
. . i u c
cer.ert on
near the fol:
us 2
he Eair.e
t if or al i .
exop
"nethr Degrees of eop at 6
r-cterc rith the sams corert ion(I .OCCD leriscop
'n each eye) it would not be wise to give
ance correction, but to allow him to use hi^
relative . and at the car.e time to
the fixation exers if he becomes unc
e in the use *f his eyes at a distan
-.other vrry food tr in esses f exop to
courage convergence is to prescribe s weak so
the
m
ce
en
.on of eserin or
to the ounce , ST.
' lecarpin, quarter of
use ore dfop in each
a gr
eye r
MUSCLES. 193
not be prescribed in combination with the ametropic cor-
rection for the treatment of insufficiency, and yet there is
an occasional exception to this statement in cases which
must have prompt, though temporary, relief, Occasion-
ally, the relief may be permanent ; but this will not hap-
pen very often. When ordering prisms^ for such a case, it
is best to prescribe them in the form of hook fronts, so that
they ma}- be thrown aside at any time. In hyperphoria the
full prismatic correction (except in cases of presbyopia) is
seldom ordered, only about two-thirds of it, and this is
divided between the two eves base down before one, and
^m
base up before the other.T^jT^ ZZs% *%?*//*<***- <f
Treatment of Insufficiencyor the Extern! (Esophoria).
As esophoria is a tendency of the visual axes to deviate in-
ward, it will be found that patients with this form of insuffi-
ciency suffer very little, if at all, when using the eyes at near
work ; their chief discomfort arises from using the eyes for
distant vision. The " shopping headache," the " opera head-
ache," the " train headache," may be due to this form of in-
sufficiency, but it is not so apt to cause discomfort if the
ametropic correction is worn constantly. In other words, if a
hyperope does not wear his distance correction and accom- / y
modates at the same time that he endeavors to maintain equi- V *
poise (relative hyperopia), he may at times suffer severely. ^/^/>
If the symptoms of muscular asthenopia persist after pre- fa ' *+ "' V
scribing the ametropic correction, then prisms, bases out, ^n >
may be prescribed as hook fronts to be worn over the con-
stant correction when using the eyes for distance. It has
been the writer's experience that esophoria of two, three,
or four degrees seldom gives the possessor any discomfort
whatever, i Prism exercises for esophoria give very little
benefit, and are often a waste of time ; yet they should be ^^T
tried thoroughly if the case appears to demand it. y 7
194 REFRACTION AND HOW TO REFRACT.
Treatment of the Insufficiency of the Superior and
Inferior Recti. Having prescribed the ametropic correc-
tion, an attempt should be made to strengthen the weak
muscles by prism exercises prism base down before one
eye, and base up before the other eye. While this does not
; often give satisfactory results, yet it should be tried in each
instance. If prism exercises do not correct the difficult}-,
then prisms which overcome most of the insufficiency should
be prescribed for constant use. Failing in this second
attempt with prisms or with a full prismatic correction, then
tenotomy of the overacting muscle or muscles must have
consideration.
Tenotomy. As previously stated, tenotomy should never
be resorted to until every other known means of relief has
been tried, and even then no hard-and-fast rule can be
given for the amount of the insufficiency in degrees which
will prompt such a procedure. Some patients with as
much as four or six degrees of esophoria may never suffer
the least annoyance ; and yet other patients with the same
amount will estimate their sufferings as almost beyond en-
durance. And the same statement holds good in other
forms of insufficiency, especially exophoria. The question of
personal equation, the patient's nervous system, hysteric
tendencies, etc., must all be considered before undertaking
a tenotomy that may result in nothing but discourage-
ment.
If an operation has been deemed best, then it is for the
surgeon to decide whether he will divide the tendon of the
strong muscle or advance the weak muscle, or both. What-
ever operation or operations are performed, the amount of
the deviation should be estimated immediately before, as well
as during and after, the operation. When a simple tenot-
omy is performed, the eye is usually left open (unband-
J . ^oo. e^j^-^ ' ' ^ * I I** -
Y.'hen the esop ar.ts to 6,8,r 1C Octrees, then the
lent rill usually have narfte.^ s such as ecul
ar hdacheE,^ runring bfclrtwen the neck,&
seme tires the shoulders. They donot have am*
cor.fort when readn' A 8'r 8 t T f' flWg and in fact eft en
have t abandon any prolonged use of the eyes at
'. hear work . "hen the patiert h--s several
frees of esop f ~ :-i stance he r.ust use his din-
ce correct itn which usaally corrects any furthe
^ctmfort in the use of /his eyes for distance;
but he may continue to have disc at any near work
h disc as ocular pains, oc hdaches, pains running
o the neck end sorretimes frit between the shoul
s. Such pts do not have any corcf from thier
n rding or rriting or any close work vhich
requires the eyes to nove instead of chaining fix
For instance f a sewing werr.an will corr.e with
the story that she ca n sew with comfort with her
glssces on, but that she cannot read with cor.fort &
she crnnot undrrctand why. A stenographer who rur
the typewriter curing the day suffers from the s s
just described but she cm sit and sew in the ever
ing srd not get a hdache. THe trc-vble lies in w
v-eak aBducting i . ower. The treatment is net to
v;eal:en but t strenghten abduction. The w iter
' ethod vhich he believes to be o: iginalis to pract
aba or tu 1 ning outward of khz each eye seprrat
that is to tuf-n each eye outward while its fell
-s covered. The pt fixes his head in one pos
tion j?.nd not to turn it while practiseng ts follow
to cover the eye with a card ?s shown and in such
a rranner that the covered eye^ cannot see what th
other eye is io ing; then /ho Id ing the index finger p
;oint an a level with the eye, the finger is gradu-
ally r.ad eto describe a Tusrter circle to the sair.c
thre e being exercised; the eye fixes the
point of the finger to the Unit of external
rotation. Fi? st one eye and then t
minutes after each meal. Marked in
and the ?rr ean testify to remirkab]
ghting abduction. Occasionally t
pt rill also hav eto have pri
.After partial tenotomies TI95 r
after a tenotorr.y if any annoying ir
ways be successfully relievee
rade in the correcting glasses.
After par Ii page ?3I add -
Cummar. .
The f ol .owing sice is given ^s the \
tried after the correction is si
patient sees 6/6 rith the f !!?. in
is not to be ordered until the var5
this coorection*- to mak e sure that
on.
_/ 0.2 r sphere
O.?r sphere.
-fC.^ ^cl;: 2txfchfi5pm*xixiz. *.
-f" O I ^ at the opposite axis.
^,L^ st the same axis
^ , VT *PP axis
crosse r cyl lens
/^fter fecus p??0 4th line ac
their use is of advantage to pr
^uire lens of leesjff . v .eight anT als
formerly a plusI5.COS.B.contine vri
cyl next to the eye and the convex
and nweildy lens, so that nor vith
next to the eye and the outer aurfac
an and plus 8 in the hor.rr.er.
with very lone: laches appreciate to
The teric Q! so allow a r.uch neater
cemented on to the concave surface.
ther is exercired this v ay for 5 or ten
ovrrert rill follow this tr i: ew days
results by this very si~ le method of stren
sctice alone will not suffuce and the
s (bases in)
t is an interstir - rever the leas that
^ems-ins, it can Usually but not al-
cribing the necessary correcting prism to be
ios steps in refraction one the lens to br
d to be eorrect. For ir.sta- nce,if the
lus I. CO cer.b vrith rlus T.CC cyl . axisOO, this
s lens have been successively placed before
he eye will not accept some other coir.binsti-
he same axis
ic 1 ere ,-These have been described( p54)but
r^ and cases of aphakia,as these cases re-
ens which will enlarge the field of vision.
llurr.Cl at axis I6C ras mede v:ith the 2
ras placed outward, this r.sde a very thick
e toric "lens this vrould equal a plusT .r.
rould have a plus 10 c;-l in the vrrt.mefcidi-
I res en o; toric bee of enl. field. I eo-ple
^ ere than the old cphero-r; :r lens,
focrl by alloving the scale e- segment to be
Vc le with sen Itive r etir!aa do no
t as a rule ftnjty t tries as they per- ' '
pheral rays to fall upon j ortions of the ret
which do not tolerate the inc eased stirulatii
n, I rest as a rule are not sir.ilarly
disturbed.
I age?94 add . Tused bifocal s(Kryptok) *-This
variety is also known as "invisible" . It is not
unlke the bifocal shown in Fig. 185. It is made
by taking a small circular piece tf flint glass
nd by greet heat fusing ne surface t a crtwr
glass- piece; the cr^n glsss is square in she
this is the form in vhich the Krypttk Sales C
supply the opticians, who then grind the v
curves to meet the conditions of the oculist
prescr: ptions.
After trifocals : age 295 add. One-ptece bifocl
s.-This is rot unlike the solid or ground bifo-
cals but it nor ncde in the toric rurve and
neater and more delicate than the previaus col-
id biffccal. This one-piece bifocal is frequent-
ly also called"invisible tf . Close insp I Tle(
ted light will generally sho^r where the two ed
ge-s come together. Its think they will av
seeing where the upper and lower corr come to-
gether, lut they will see^/it in any bifocal arc
the term invisible applies t the friends of U
petient who cannot always see that '.if teals are
being wor n. latients of unknown age wearing
bifocals of the "inv" variety en . oy the fact
that their friends still think they are youngt'
MUSCLES. 195
aged) so that visual fixation is maintained, and the muscle
balance tested frequently to see that, by subsequent con-
traction, the insufficiency does not return. To avoid such a
misfortune it may be necessary to use prism exercises dur-
ing the healing process. The writer is not an advocate of
partial tenotomies. V" *Z^-4Ji 'jfty- 'Vt**' * ,
Strabismus (arpl<fta, " to turn as'ide ") ; also called heter-
otropia, " cross-eye " or " squint," or manifest squint. This
is a condition of the eyes in which the amount of the
insufficiency is so great that it can not (always) be over-
come by muscular effort ; and, in fact, inspection often shows
the manifest condition. Or strabismus may be defined as
the condition in which the visual axis of jone eye is deviated
from the point of fixation. The eye which has the image
of the object on its fovea is spoken of as the fixing eye,
while the other eye is termed the squinting or deviating
eye. The squinting eye does not always have normal
visual acuity ; and, in fact, correcting lenses will not
always produce such a result.
Varieties of Strabismus. Convergent, divergent,
vertical, monolateral, alternating, periodic, concomitant,
and paralytic.
Convergent squint (con, "together," and vergere, "to
incline or approach ") ; also called internal squint (strabis-
mus convergens), esotropia. This is the condition in which
the visual axis of one eye is deviated inward, the other
fixing the object ; or one eye fixing an object, the visual
axis of the other eye crosses that of the fixing eye closer
than the object. (See Fig. 163.) This is thePmost common
form of squint. Both eyes have some form of hyperopia,
as a rule, the squinting eye usually being the most ame-
tropic. The diplopia as a result of this condition is
X
homonymous.
196 REFRACTION AND HOW TO REFRACT.
Divergent squint (di, ''apart," and vergere, "to in-
cline ") ; also called external squint (strabismus divergens),
exotropia. (See Fig. 164.) This is the condition in which
the direction of the visual axis of one eye is directed out-
ward, the other eye fixing the object ; or one eye fixing
an object, the visual axis of the other eye can cross it only
by being projected backward. JYhe diverging eye is usually
myopic.
Monolateral (one-sided) squint ; also called constant.
It may be either convergent or divergent, but the squint is
a constant condition of one eye.
Alternating Squint. This is the condition in which
at different times the right eye fixes and the left eye squints,
or the left eye fixes and the right eye squints. The vision
in one eye may be as good as that of its fellow.
Periodic squint ; also called intermittent. This is the
condition in which the visual axis of one eye occasionally
deviates. It may eventually become constant, and is often
the first indication of a beginning convergent or divergent
squint.
Vertical Squint. This is the condition in which the
visual axis of one eye is deviated upward. Also called
hypertropia.
Concomitant Squint. This is the condition in which
the squinting eye has freedom of movement and will follow
its fellow, and yet one eye deviates (inward or outward)
because of an inability to " fix."
Paralytic Squint. This is the opposite condition from
concomitant, in which there is a restriction in the move-
ment of one eye in a certain direction, due to a palsy of
one or more of the muscles.
Causes of Squint. These are many and various. The
chief causes, however, are : (i) Ametropia, which may pro-
MUSCLES. 1 97
duce a change in the normal relationship between accom-
modation and convergence ; (2) anatomic anomalies ; (3)
mechanic anomalies ; and (4) amblyopia.
I. Ametropia produces a change in the normal relation-
ship between accommodation and convergence. While it is
possible for accommodation to take place without conver-
gence, or convergence without accommodation, yet there is
an affinity between the two processes which, if materially in-
terfered with, will produce diplopia and eventually squint. In
speaking of relative hyperopia, it was shown that the accom-
modative effort was accompanied by contraction of the inter-
nal recti muscles (convergence) ; so that in hyperopia of, say,
four diopters, accommodating for infinity convergence would
be stimulated to a proportionate degree at the same time ;,
and if accommodating for a near point, the hyperope must
accommodate and converge just that much more. The
result is that a person with a hyperopia of any considerable
amount frequently squints inward in the effort to maintain
! binocular vision. If, now, one eye is more hyperopic than
the other, the difficulty of adjusting convergence to accom-
modation is increased. Say that the right eye has 3
diopters and the left 4 diopters of hyperopia ; then the two
eyes each exert 6 diopters to fix at 1 3 inches ; the left eye
still has i diopter of its hyperopia remaining, and with the
result that the retinal image of that eye is not clear, and
accommodation is still further taxed, stimulating at the
same time the internal rectus, so that the left eye deviates
inward and ultimately remains convergent. This act of
convergence explains the presence of convergent squint in
hyperopia, and also shows why the squinting eye usually
has the higher refractive error. It must not be supposed
that all hyperopic eyes have a squint, as some of these can
198 REFRACTION AND HOW TO REFRACT.
accommodate without converging in a proportionate de-
gree, and this is especially so when the amount of the
hyperopia is the same in both eyes.
^ Myopic eyes, in contradistinction to hyperopic eyes, can
not accommodate beyond their far points, but must con-
verge. If the myopia is 8 diopters, then these eyes would
have to converge 8 meter angles to fix an object at that
distance (5 inches) without any accommodative effort. It
must also be borne in mind that myopic eyes are long eyes,
and that to converge 8 meter angles means a great effort
on the part of the internal recti muscles, and this force can
not be continued for any length of time without discomfort ;
the result is, convergence is relaxed, and, one eye remaining
fixed, the other is turned outward. This is much more
likely to happen if one eye is more myopic than the other.
This explains the presence of divergent squint in cases of
myopia. But it must not be supposed that all myopic eyes
necessarily have squint, as some of them have roomy orbits,
strong internal recti muscles, and a short interpupillary
distance.
2. Anatomic Anomalies. This applies especially to the
breadth of the face (skull) and the size of the eye and
orbit. The broad face, which naturally gives a long inter-
pupillary distance, predisposes to greater convergence than
the narrow face. The long, myopic eye would not have
the freedom of movement that the short eye possesses in
the same-sized orbit.
3. Mechanic Anomalies. This refers especially to the
length and strength of the extraocular muscles. Short
and strong internal recti would predispose to convergent
squint, whereas strong external recti would develop diver-
gent squint.
MUSCLES. 199
4. Amblyopia. Statistics show that from thirty to
seventy per cent, of all squinting eyes are amblyopic. The
cause of the amblyopia may be that the eye was born
defective in its seeing quality /. e., the cones at the fovea,
the optic nerve, or the visual centers in the brain may be at
fault. Or if born perfect and having its visual axis deviated
by one of the many causes above mentioned, it may be-
come amblyopic from not being used (amblyopia exanpp-
sia). This consideration of cause and effect is most impor-
tant from a prognostic point of view.
Among other causes of squint must be mentioned opaci-
ties of the media, as nebula of the cornea, or any want of
transparency in the cornea at or near the visual axis, or
polar or nuclear cataract. Temporary or intermittent
squint may result from vitreous opacities, or from the rem-
nant of a hyaloid artery passing in front of the fovea. Parents
occasionally delude themselves with the idea that the
child's squint is the result of whooping-cough, measles,
teething, sucking the thumb, or imitating a companion,
etc., and are slow to believe that there can be any refrac-
tive error, forgetting that the supposed causes they men-
tion may be but coincidences.
To Estimate the Amount of the Strabismus or
Squint. This is not always easy at the beginning of the
examination, for the reason that the squinting eye has long
since learned to ignore the false object ; and if the angle of
the strabismus is large, the surgeon will have to reduce it in
part with a prism, so that the patient can see the false object ;
and if this is a point of light, a piece of dark red glass will
have to be placed in front of the fixing eye. The strength
lof the prism required to bring the two lights together will
/ I be the prismatic estimate of the deviation. Or the amount
of the squint may be roughly determined with the strabis-
2OO
REFRACTION AND MOW TO REFRACT.
mometer. (See Fig. 170.) This is a piece of bone or ivory
hollowed on one side so as to fit the curve of the eyeball.
Its edge is graduated in millimeters. This device is held
gently against the lower lid of the squinting eye, so that
the zero (o) mark corresponds to the center of the pupil as
the eye fixes a distant object, the fellow-eye being under
cover. When the cover is removed,
the squinting eye again deviates,
and the amount of the deviation is
again noted by the position of the
center of the pupil of the squinting
eye over the millimeter line on the
instrument. Each millimeter of
j deviation is supposed to represent
j 5 degrees of deviation. This device
is not reliable, and is not in common
use.
A more reliable estimate is ob-
tained by measuring the deviation
on the arc of the perimeter. (See
Fig. 171.) To do this, the patient
is seated with the squinting eye
FlG I70 opposite to the fixation point (R)
and instructed to look at a distant
/object (R) across the room, so that the object, the fixation
point, and the squinting eye (R) are in line ; this line repre-
sents the direction which the eye would take normally.
The observer, taking a lighted candle, places it at the fixa-
tion point and gradually moves it outward along the inner
surface of the arc until his own eye, directly back of the
flame, sees an image of the flame at the center of the pupil
of the squinting eye. The degree mark on the arc from
which the flame was pictured represents the amount of the
MUSCLES.
2O I
deviation or angle of the strabismus ; this angle being
formed by the visual axis with the direction of the normal
visual line. The degree mark on the arc is in front of the
optic axis and not the visual axis, but for purposes of
approximation they are considered as the same.
Treatment of Strabismus. As ametropia is the chief
factor in the cause of squint, this cause must be promptly
2O2 REFRACTION AND HOW TO REFRACT.
removed by the use of correcting glasses. The correction
of the ametropia means four essentials : '
1. In young subjects the eyes must be put at rest, and
kept at rest for two, three, or four weeks, with a reliable
cycloplegic and dark glasses. Preference is given to
atropin in each instance, the writer considering it folly to
use homatropin' in such cases.
2. During the use of the cycloplegic, the lenses which
correct the ametropia are selected with care and the greatest
precision, by every known means to this end ; and just
here is the place of all places to use the retinoscope, as
most cases of strabismus appear in children, and, too, the
squinting eye often being amblyopic, can not assist in the
selection of the glass.
3. The correcting glasses are ordered in the form of
spectacles, and are to be worn from the time of rising until
going to bed. The strength of the glasses should be as
near the full correction as it is possible to give.
4. The " drops " are continued for a day or two after
the glasses have been obtained, and in this way, while the
drops are still in the eyes, and as their effect slowly wears
away, the eyes gradually become accustomed to the new or
natural order of accommodation and convergence. After
the cycloplegic has entirely disappeared, the patient should
be carefully restricted in the use of the eyes for near-work
for several days or weeks. j
As hyperopia and astigmatism in combination are gener-
ally congenital conditions, it therefore follows that conver-
gent squint appears quite early in life, as soon as the child
begins to concentrate its vision on near objects. The
squint, at first periodic or intermittent, finally becomes con-
stant. Such eyes should be refracted at once, and before
amblyopia exanopsia can be established. It is interesting
i
NS
MUSCLES.
to note that the eyes in many young children begin to fix'
or lose their squint as soon as cycloplegia is established.
The prognosis is favorable for good vision with glasses
when this occurs. It will also be observed in other sub-
jects that while the drops are in the eyes and glasses,
worn constantly, the squint disappears entirely ; but as soon
as the cycloplegia passes away and near vision is attempted,
the squint returns, and vision falls back in the squinting
eye to almost the same point that it had before the cyclo-
plegia. This occurs in cases where the amblyopia is becom-
ing established, or where there is a strong muscle devi-
ating the eye. If the squint is due to amblyopia exanopsia,
then the vision may be improved in one of two ways, as
FIG. 172.
suggested by Dr. G. M. Gould, " Amblyopiatrics," " Med.
News," Dec. 31, 1892. One way is to use drops in the
fixing eye, and thus compel the squinting eye to do the
seeing ; or the other way is to cover the fixing eye with a
blank over the glass (see Fig. 172), and have the patient
practise in this way for one or tw/o hours each da^y using
the squinting eye alone^ *^~ ^^
Worth's Amblyoscope or " Fusion Tubes. 1 " To
cultivate or develop binocular vision Worth has given us an
instrument which he calls an amblyoscope. (See Fig. 173.)
This instrument consists of two halves joined by a hinge.
Each half consists of a short tube joined to a longer one
at an angle of 120 degrees; at the junction of the tubes is
2O4 REFRACTION AND HOW TO REFRACT.
an oval mirror. A translucent glass object slide is placed
at the distal end of each tube. At the hinged ends are
lenses whose focal length equals the distance of the re-
flected image of the object slide ; in front of these lenses are
grooves into which additional lenses of the trial case may
be placed to correct the refractive error of the patient. The
two halves of the instrument are united by an arc, having a
long slot at one end and an adjusting screw at the other.
The object slides can be brought together to suit a
convergence of 60 degrees, or a divergence of the visual
axes of 30 degrees. When the adjusting screw is used an
additional movement of 10 degrees is obtained.
FIG. 173. Worth's Amblyoscope. (Reduced size. )
At the far end of each tube there is also a square slot
into each of which may be placed half a pictured object;
for instance, a picture of the right side of a man, showing
his arm and leg extended, may be placed in the left tube,
and in the right tube is placed a picture of the same size,
of the left side of the man with his leg and arm similarly
extended. When the patient looks into the tubes, the sur-
geon (or the patient) may adjust the tubes until the two
half pictures unite and form one complete picture. Or the
picture in one tube may be a picture frame, and in the other
tube is a picture of an animal or an object, the idea being
MUSCLES. 2O5
to have the patient so fuse the two pictures that the object
is placed in the frame. There are many different pictures
accompanying the instrument so as to give variety to the
daily exercises and thus maintain the patient's interest.
This instrument is certainly a valuable one and in many
instances accomplishes its purpose.
Cases that are cured by correcting the ametropia must
wear their glasses constantly. Glasses in such cases can
seldom be abandoned. In young children the squint re-
turns almost at the instant the glasses are removed. The
earliest age at which glasses can be prescribed is three^
years or thereabouts, as it would be unreasonable in most
cases to expect a child to appreciate the glasses as anything
but a toy before this age.
The younger the patient when glasses are prescribed, the
more favorable the prognosis and less likelihood of a ten-
otomy. The older the patient when glasses are ordered,
the less the likelihood that glasses will cure the squint and
the greater probability of a tenotomy being necessary.
This is explained from the fact that the squint having per-
sisted for a long time, the muscle which held the eye in the
deviated position has grown strong and the opposing mus-
cle weak.
The correction of squint by glasses applies particularly
to cases of the concomitant (convergent or divergent) form.
Vertical squint is* seldom cured by correcting glasses alone.
Prisms will occasionally substitute for an operation.
Monocular and alternating squint are greatly relieved
by the correction of the ametropia, and may or may not
be cured with glasses alone.
Periodic or intermittent squint, if due to permanent opaci-
ties in the media, can not, as a rule, be cured by any form
of treatment.
2O6 REFRACTION AND HOW TO REFRACT.
Paralytic squint is not a part of the subject-matter of this
work. Cases of concomitant squint are generally amenable
to operative treatment, whereas cases of paralytic squint are
not.
It may be stated as a good rule to follow that no case
should ever be operated upon until the glasses u'Jiicli cor-
rect tlic ametropia have been worn constantly for several
weeks after all apparent improvement has ceased. If cases
for operation can be selected, the best age is about puberty,
when the muscles have reached a fair state of development.
If the squint is due to an anatomically short muscle, then
there need not be any great delay in operating after glasses
have been ordered.
Whenever a tenotomy has been performed, the eyes
should again be carefully refracted, as it is a well-estab-
lished fact that tenotomy often relieves a tension that will
materially change the radius of corneal curvature ; and
hence the amount of the astigmatism and the cylinder axis
will be altered.
Tenotomy. For convergent squint, if of moderate
degree, division of the tendon of the internus of the con-
verging eye may be sufficient ; but if the squint is consider-
able, the tendons of both interni may have to be divided.
Occasionally, it is necessary to divide the internus and
advance the externus.
For divergent squint, if of moderate degree, division of
the tendon of the externus of the diverging eye may be
sufficient ; but if the squint is considerable, the tendons of
both externi may have to be divided. Occasionally, it is
necessary to divide the externus and advance the internus.
For vertical squint, tenotomy of the stronger superior or
stronger inferior rectus, or both, may be necessary.
It is good practice in every instance, before " rushing "
MUSCLES. 2O7
into an operation for squint, to take the field of vision and
search carefully for a central scotoma, which, if present,
should put the surgeon on his guard against operative
interference with the hope of obtaining any result other
than cosmetic ; and even then there is grave danger that
the case will soon lapse into the former state of deviation,
or possibly deviate in the opposite direction.
CHAPTER VIII.
CYCLOPLEGICS. CYCLOPLEGIA.ASTHEN-
OPIA. EXAMINATION OF THE EYES.
A cycloplegic (from the Greek, y.u*h>s, "a circle," /. c.,
the ciliary ring, and ^tyry, " a stroke ") is a drug which
will temporarily paralyze the action of the ciliary muscle.
A mydriatic (from the Greek, nuSptaats, " enlargement
of the pupil ") is a drug which will temporarily dilate the
pupil.
Atropin will dilate the pupil and also cause a paralysis
of the ciliary muscle. Cocain will cause a dilatation of the
pupil, but will not paralyze the action of the ciliary muscle.
A cycloplegic is also a mydriatic, but a mydriatic is riot
necessarily a cycloplegic.
The Uses of a Cycloplegic. (i) To temporarily sus-
pend the action of the ciliary muscle, or to put the eye in
such a state of rest that all accommodative effort is for a
time suspended while the static refraction is being estimated.
(2) The retina and choroid are given an opportunity to
recover from irritation and congestion incident to eye-strain
(" eye-stretching "). There are many different cycloplegics
employed for estimating the static refraction, and each has
particular qualifications for individual cases. Cycloplegics
may be classed as of three kinds : (i) those the effect of
which passes away slowly ; (2) those the effect of which
passes away moderately fast; and (3) those the effect of
which is very brief.
:/The first effect of a cycloplegic is its mydriatic quality,
208
> after which the accommodative effort is suspended. The
paralysis is not permanent. The following table, from Jack-
son, shows the length of time paralysis persists and the
time it takes for the ciliary muscle to fully recover :
J^Atropirt, effect begins to dimi
Daturin, ' ' 3 "
3 "
2 "
12 hours.
Hyoscyamin,
Duboisin,
Scopolamin,
7 Homatropin,
ish in 4 days ; complete recovery, 15 days>
lo
8
8
6
2
>\
If a solution of one of the above-mentioned cycloplegics be
instilled into the conjunctival sac of a healthy eye, it will be
carried by the blood- and lymph-vessels at the sclerocorneal
junction into the ciliary muscle and iris, where it acts
directly upon the nerves and ganglia of these structures,
and the aqueous humor also receives some portion of the
.drug. If cautiously used, the action will be limited to one
I eye, showing that the drug does not pass through the car-
diac circulation ; otherwise, the pupil and ciliary muscle of
the fellow-eye would be similarly affected.
Some conjunctivas are very sensitive to any of these
drugs, and develop an inflammation so severe in individual
instances as to resemble ivy poisoning of the lids. f Duboisin
especially, and hyoscyamin, by absorption, may develop
hallucinations and even a loss of coordination./
Any cycloplegic, in fact, when carelessly used, may pro-
duce very unpleasant symptoms, such as dizziness, dry
throat, flushed face and body (mistaken for scarlatina), rapid
pulse, a slight rise of temperature, and delirium. To avoid
such an annoyance, which is apt to reflect discredit upon
the physician and upon the profession in general, the patient
should always be given definite instructions how to use the
drug in each instance. Stopping the use of the drug and
18
2IO REFRACTION AND HOW TO REFRACT,
applying cold compresses will relieve the conjunctivitis, and
if constitutional symptoms manifest themselves, a dose of
paregoric, cooling drinks, a darkened room, and stoppin
/the use of the ,drug will soon restore the patie
~CSy fc^-v
, FORM OF
Name, MR. BROWN.
K. Atropin. sulphatis, ........... gr. j
Aquae dest., .............. ^S 1 }' '
M. Ft. sol. Label, poison drops ! !
SlG. One drop in each eye three times a day, as directed.
R. Dropper. DR.
Date, Tuesday, March 14, 1899.
The reason for labeling this prescription "poison drops"
is not to frighten the patient, but to caution him against
leaving the medicine where children may get hold of it,
and at the same time to let him understand that it is to be
used and handled with care.
Mr. Brown is told to have one drop put. in each eye
' jthree times a day, after meals, and to report at the office on
Thursday (the prescription is given on Tuesday in this case).
The reason for using the drug for this length of time is to
)? insure complete paralysis, and also to give the eyes a physi-
ologic rest. In having these " drops" put in the eyes, the
patient should tip his head backward and turn his eyes down-
ward, and as the upper lid is drawn up, one drop (from the
dropper) is placed '(not dropped) on the sclera at the upper and
outer part. After the drops are placed in the eyes, as far away
s from the puncta lachrymalia as possible, the patient holds
the canaliculi closed by gently pressing with the ends of the
index fingers on the sides of the nose at the inner canthi
for a minute or two. If more than one drop enters the eye,
/ it will run over on to the cheek, and should be wiped off.
With children, these instructions are not so easy of exe-
CYCLOPLEGICS. 2 1 I
cution, and the writer has seen a few such clinical subjects
flushed and delirious from gross carelessness on the part of
parents in dropping the medicine into the inner canthi,
where it soon passed into the nose, or else the drug is
allowed to flow over the cheek and into the child's open
mouth. } Ordinarily, there need never be any discomfort
from the use of these drugs beyond a slight dryness of the
fauces.
Caution. Cycloplegics should never be used when
~~^ Jp*'
there is the least suspicion of glaucoma in one or both (j/y'
eyes. Cycloplegics should not be used in the eyes
of nursing women; such patients are peculiarly suscep-"'
tible to the action of these drugs, and the mammary
secretion may thereby be diminished in amount. After the
./ age of forty-five or fifty years, or in the condition known
I as presbyopia, it is seldom necessary to use a cycloplegic.
If a cycloplegic is necessary in presbyopia, one of the
weaker drugs is generally employed.
^In the selection of a cycloplegic the surgeon must be
guided by the patient's occupation, age, the character of
the eyes, and the refraction,' From the foregoing table it
will be seen that atropin and daturin are slow in passing
from the eye, making their employment on this account
very objectionable in many instances. The accommodation
returns sooner after the use of hyoscyamin and duboisin
than from atropin, but not so promptly as from scopolamin
and homatropin. The effect of the latter is very brief. A
patient who might lose his business position if he remained
away from work for more than a week could not afford to
have atropin or daturin used in his eyes, whereas a school
child might accept atropin as a luxury. The man of busi-
ness, the cashier in a bank, the storekeeper, and others
must, in many instances, have their eyes refracted in at
212 REFRACTION AND HOW TO REFRACT.
> least two days ; and this latter time means, of course, the
use of homatropin. The nearer the age to forty years, the
jless need for one of the stronger cycloplegics, as the power
of accommodation has markedly diminished at this period
of life, so that hyoscyamin or scopolamin will answer every
purpose. After thirty-five years homatropin can, as a rule,
be relied upon as a cycloplegic.
In hyperopic eyes of young subjects it is useless to em-
ploy homatropin, as the active ciliary muscle requires a
strongly acting cycloplegic to stay the accommodative
power. In myopic eyes one of the stronger cycloplegics
may be used to advantage, for the following reasons :
Myopic eyes have large pupils, as a rule, and do not mind
the mydriasis ; myopic eyes are often in a state of irritation,
and the drug gives them a much-needed rest ; the myope's
distant vision is not disturbed by the cycloplegic, as in the
case of the hyperope.
Whenever a cycloplegic is prescribed, the patient should
be ordered a pair of smoked-glass spectacles to wear dur-
ing the mydria^sis. Of the two forms of smoked glasses,
r coquilles and*' plane, the latter should always be preferred t
as they are without any refractive quality, whereas coquilles
have some form of refraction that may act very injuriously.
Another reason for ordering the plane glass is that the
patient will often wish to wear them with his prescription
glasses, which he could not do so well if they were coquilles.
Dark glasses are of four shades of " London smoked " A
B, C, and D, A being the lightest shade and D the darkest.
The prescription would be :
For MR. BROWN :
^ > R . One pair plane London smoked " D."
SlG. For temporary use. DR.
March 14, 1899.
CYCLOPLEGICS. 2 I 3
The cycloplegics above mentioned for purposes of re-
fraction are ordered in the following strengths :
Atropin. sulphatis, gr. j to aq. dest., . . f^ij.
Duboisin. sulphatis, gr. ss " " " . . f 3 ij.
Hyoscyamin. sulphatis, gr. ss " " " . . f 3 iss.
Daturin. sulphatis, gr. ss " " " . . f 5 ij.
Scopolamin. hydrochlor., . . .
All these, except scopolamin, are ordered to be used
'"7 three times a day, preferably after meals ; but scopolamin
being a very powerful drug, the surgeon should ^place it in
the patient's eyes himself in the office, and not give a pre-
scription for it. Only two drops are necessary, and are
instilled a half-hour apart, the static refraction being esti-
mated one hour after using the second drop.
How to Use Homatropin. This drug is expensive, and
j it is never necessary to prescribe more than one grain for
any one patient. Personally, the writer has found the fol-
lowing most satisfactory, though the strength of the homa-
tropin may be increased if desired :
For Miss ROBINSON :
*~1 .> t
li . Homatropin hydrobromate, gr. j
Aq. dest., n\, xl.
M. Ft. sol. Label, poison drops ! !
SlG. One drop in each eye, as directed.
R. Dropper. DR.
March 14, 1899.
One drop of this solution instilled into a healthy eye will
produce mydriasis in a few minutes, but its action on the
ciliary muscle is so trifling that the near point will be but
slightly changed. It is thus shown that this drug is a
decided^fnydriatic, and only becomes a cycloplegic under
definite usage.
To produce cycloplegia with homatropin, the patient is
214 REFRACTION AND HOW TO REFRACT.
given the above prescription and told to use it as fol-
lows : *,
! To place one drop in each eye at bedtime the first night.
"This one drop dilates the pupil and establishes a change in
the circulation of the blood-supply to the iris and ciliary
body a very important matter for the patient's comfort, and
at the* same time preventing a tendency to spasm of the
ciliary muscle. The next morning one drop is to be placed
in the eye every hour, from the time of rising until leaving
( ; home to go to the surgeon's office. At the office one drop
is placed in each eye about every five minutes, until six
drops have been used ; then, after waiting half an hour!
' (for the cycloplegic effect, which will last for one hour), the|
refraction is carefully estimated. After a short interval the
cycloplegic effect will begin to rapidly disappear, so that the
/patient will be able to read within forty-eight hours' time
!with his correcting glasses.
Occasionally, a busy patient will insist upon having his
eyes refracted during his first visit, and can not take time
to use the drops in the manner above suggested. The
surgeon must, therefore, start and use the drops in his
office. This is forcing the ciliary muscle into a state of
paralysis that does not always give ultimately satisfactory
results. " Forcing " homatropin into an eye in this way
will always produce a " bloot-shot " eye (hyperemia of the
conjunctiva, etc.) that does not improve a patient's appear-
ance ; and it often produces severe neuralgic headache that
may result in nausea or vomiting in occasional instances.
'Furthermore, it is possible, with a drug like homatropin, if
7 not properly used, to have some of the sphincter-fibers
become paralyzed while others may remain free to act. In
this way a spasm of the ciliary muscle may be produced
that will give a false astigmatism. Personally, the writer
CVCLOPLEGICS. 215
is not partial to this method of forcing the ciliary muscle
into repose.
To somewhat obviate the "blood-shot" condition of the
eye, and also to assist the action of the " forcing" process,
one drop of a two or four per cent, solution of cocain
maybe instilled while the homatropin is being used. This '
also diminishes the danger of spasm. But cocain is objec-
tionable...^ that it will, in some cases, "haze" the cornea.
The retinoscope will show this, and the patient will state
that, u;hji|e he can see the letters on the test-card, yet they
have a "mist" over them. Instead of using the hom-
atropin alone, a small amount of cocain may be added to the
solution for the purpose mentioned. Or, homatropin may
be combined with cocain and chlorid of sodium in the form
of a disc/andone of these, placed in the conjunctival sac, is
allowed to dissolve, and in this way paralyze the accommo-
dation. Or, homatropin may be used in a solution of dis-
"'tilled castor oil. It is claimed that when the drug is used
in this form, it remains in contact with the tissues and acts
more energetically.
Homatropin as a cycloplcgic should be held in reserve
for individual cases, and not used as a routine practice. It
>is a good, reliable paralyzer of the accommodation in many
eyes at the age of thirty-five, or thereabouts ; but in a young
hyperopic eye it is a waste of time to attempt successful
paralysis with it, and the danger of producing a false "astig-
matism should certainly deprecate its use in these cases.
Another very serious objection to its use is that before the
eyes can become accustomed to the prescription glasses,
the ciliary muscle recovers and begins to accommodate,
with the result that the patient says he can see better at a
distance without his glasses than he can with them, and
has no small amount of mistrust of the surgeon's ability,
*k
REFRACTION AND HOW TO REFRAC
as he will have to wear his glasses a long time before his
ciliary muscle will relax its accustomed accommodative
efforts. This is not nearly so likely to occur if one of the
slowly acting cycloplegics is used. X
The method of refracting with one of the slowly acting
cycloplegics, and then endeavoring to counteract the effect
with a solution of eserin, is not recommended. Temporarily,
eserin may overcome the cycloplegic ; but as its action is
only transitory, the paralysis reasserts itself and will not
disappear until the specified time.
Refracting one eye at a time with a cycloplegic while the
patient pursues his occupation with the other eye is not a
method to be considered. This means a great amount
of discomfort, headaches, eye-strain, and even diplopia at
times, during so prolonged a treatment.
If a hyperopic patient must occasionally use his eyes for
near work while he has drops in them, a pair of +3 or
+4 spheres may be given for temporary use.
CYCLOPLEGIA.
Cycloplegia is a paralysis or paresis of the ciliary muscle.
This condition may be monocular or binocular ; it may be
partial or complete. /Mydriasis may or may not accompany
the cycloplegia, though the two conditions usually occur
together ; and when they both exist, the paresis is spoken
of as ophthalmoplegia interna} Th,e ciliary muscle and
sphincter of the iris are controlled by branches from the
third nerve ; but these branches are from independent cen-
ters ; the fibers going to the ciliary muscle arise beneath the
floor of the third ventricle, in front of the fibers which go to
control the sphincter of the iris.
Causes. Temporary paralysis of the ciliary muscle and
iris, as already stated, will result from the external or internal
CYCLOPLEGIA. 2 I 7
administration of a cycloplegic. It is interesting, in many
cases, to find the cause and relieve the patient's anxiety
when the paresis is due to one of the cycloplegics. Aside
from the use of eye-drops, the question of external '
medication (liniments, ointments, and plasters) should be
inquired into, as also whether rectal or vaginal supposi-
tories containing a cycloplegic have been used.
Other causes of this form of paralysis are tonsillitis, quinsy,
diphtheria, Bright's disease, rheumatism, gout, exhausting
diseases, blows upon the eye, etc. Other and more serious
causes, as controlling a guarded prognosis, are intracranial
hemorrhage, meningitis, syphilis, brain -tumor, etc. In
some instances the cause can not be definitely ascertained.
Symptoms and Diagnosis. Photophobia, dilatation of
the pupil, and loss of accommodative power consistent with
the optic condition of the eye.
A myopic eye retains its vision at the far point only ; an
emmetropic eye or a hyperopic eye wearing correcting
glasses has good distant vision and absence of a near
point ; an uncorrected hyperopic eye has poor distant and
near vision.
Prognosis. This depends upon the cause.
Treatment. This must be symptomatic and expectant,
with a removal of the exciting cause, if possible. As many
cases of cycloplegia are the result of, or follow, an attack of
diphtheria, or a disease which has reduced the system below
par, tonics, fresh air, etc., must be ordered. When brought
on by syphilis, mercury and iodid of potash must be
prescribed. Dark glasses for the photophobia should
always be ordered, and lenses for near-work may be worn
as a temporary expedient. The use of eserin locally will
occasionally do good work, but is not advised for constant
use or for every case. Faradism may be used if the cyclo-
19
2l8 REFRACTION AND HOW TO REFRACT.
plegia is very persistent, but the best results may be ex-
pected from systemic treatment. The use of strychnin or
nux vomica are recommended in certain instances.
OF THE ClLIARY MUSCLE.
'Cramp of th ciliary muscle is the opposite condition to
that of cycloplegia, just described. Ciliary cramp may
occur in one or both eyes, usually in both ; it may occur
in any form of ametropia or in emmetropia. Ciliary
cramp is of two kinds clonic and tonic.
Clonic cramp is an occasional and temporary condition
which comes on while the eyes are in use, and passes away
soon after the eyes have had an opportunity to rest, and
may not occur again for several days.
- Tonic cramp, also calledf ' spasm " of the accommodation^)
is a permanent condition as compared with the clonic form,
and occurs whenever the eyes are used for distant or near
vision. The patient can not use his eyes for any length of
time, or with any considerable concentration, without suffer-
ing as a consequence.
Causes. Clonic cramp may occur as one of the early
symptoms of presbyopia. Ametropia is a very common
cause, and especially in cases of low amounts of hyperopia
or myopia. Emmetropia, or eyes made emmetropic with
glasses, may develop clonic or even tonic cramp if the eyes
are used to excess or in a bad light. Such cases have been
-called " hyperesthesia of the retina." Tonic cramp may
develop from the same causes which bring on the clonic
I form, and is usually seen among young hyperopic children,
or the "pseudo-myope " already described. It also occurs
occasionally in hysteric patients or those recovering from
some severe or long illness. The writer has seen this form
CRAMP OF THE CILIARY MUSCLE. 2IQ
of cramp precede or antedate by several weeks a collapse
of the nervous system /. c., nervous prostration.
Symptoms. Naturally, ciliary cramp means ocular
pains and headaches. Opera headache, " train headache,"
" shopper's headache," " bargain-counter headache," etc.,
are some of the many names given to cramp of the ciliary
muscle, and are, no doubt, the result of accommodative effort
in a bright light or watching moving objects, these symp-
toms being a part of the history of accommodative astheno-
pia (already described) and accompanying insufficiency of the
muscles. Symptoms of myopia are very evident during the
cramp.^ In the tonic cramp the ocular pains and headache
may be so excruciating in individual cases as to make the
family physician and patient dread cerebral disease until the
immediate cause is found out.
Treatment. As the cause is usually one of ametropia,
this must be corrected by the careful selection of glasses
\while the eyes are undergoing a prolonged rest with a
t cycloplegic and dark glasses. Later on the patient must be
cautioned against any overuse of the eyes. The general
health* should have any necessary attention. Sedatives,
alteratives, and tonics have their place in individual cases.
Reflex causes must be looked for and, as far as possible, re-
moved. Insufficiencies should always be carefully searched
Ifor, and frequently prism exercises to develop the strength
lof the weak muscles may give marvelous results. Un-
fortunately, there are occasional instances of tonic cramp
that persist in spite of any treatment, and such cases obtain
relief only when presbyopia definitely asserts itself.
Asthenopia (from the Greek, a. priv. ; aOlw;, " strength ";
an}> ) " eye ") means a weakness or fatigue of the eye, applying
especially to the retina, the ciliary muscle, the extra-ocular
muscles, or a general weakness of any one or two or all
2O
2 2O REFRACTION AND HOW TO REFRACT.
of these structures in one and the same eye. Asthenopia^
is a disease, and is often spoken of as "weak sight," " eye-
jtrain." or " eye-stretching/'
Varieties. For purposes of study, differential diagnosis,
and treatment, asthenopia, or eye-strain, has been divided
into the following varieties : Retinal, muscular, accommo-
dative, and asthenopia due to a combination of any two or
all three varieties.
Retinal Asthenopia. This is the rarest form of asthen-
opia, and usually occurs in females. It is brought about
by overuse of the eyes in too dim or too bright a light,
and may result from a too prolonged use of the eyes at any
kind of work or in any kind of light. It may result from
exposure to the sun's rays, to electric lights, or to light-
ning, or by reflection from bright objects, such as snow,
etc. Retinal asthenopia may occur as a symptom of hys-
teria, or in a patient whose nervous system is peculiarly
susceptible to vibrations, sounds, and lights ; in a patient
whose nervous system is an uncertain quantity. Such pa-
tients are very unsatisfactory to treat or even to examine ;
they often imagine that the reflected light from the ophthal-
moscope or retinoscope is "very hot," etc.
Symptoms. The chief symptom is a dread of light
"/ (photophobia), or photophobia and lacrimation together.
Treatment. The first thing to do is to remove the
cause, if this can be found ; otherwise the treatment should
be very conservative. Ametropia must be corrected and
the eyes be given some regular work ; in other words, it is
r\ot good practice to restrict all use of the eyes. The treat-
ment with "tinted glasses," made so much of by the char-
latan to " gull " the innocent public, should not be ordered,
as the patient grows accustomed to them and they event-
ually become an absolute necessity on all occasions. Care-
CRAMP OF THE CILIARY MUSCLE. 221
ful attention to the general health is certainly indicated ;
tonics, out-door sports, etc., should be prescribed in
individual cases. The shade of the trees is to be recom-
mended in preference to the seashore and bright reflection
from the sand and water.
Muscular Asthenopia. This is due to weakness or
fatigue of one or more of the extra-ocular muscles, most
frequently the interni (cxophoria). Muscular asthenopia
of the exophoric kind is the result, as a rule, of a want of
power to maintain convergence. The symptoms are in
keeping with a cramp followed by a relaxation of converg-
ing power. Ocular pains, eyeballs tender to the touch (per-
chance the internal recti themselves become sore to the
touch or feel sore on movement of the eyes), and in some
cases the conjunctiva and subconjunctival tissues overly-
ing the muscles become hyperemic during or after the use
of the eyes, simulating rheumatism of these structures. In
other cases dim vision and diplopia will be occasional mani-
festations. Patients with muscular asthenopia occasionally
find that they can continue at near work by using one eye,
but this does not occur very often.
Treatment. This resolves itself into the correction of
the ametropia, exercise of the weak muscles, etc. (See
chapter on Muscles.)
Accommodative Asthenopia. This is by far the most
common form of asthenopia, and is due to fatigue of the
"-Iciliary muscle ; it is, therefore, to be expected in hyperopic
eyes. It is caused in various ways : from overuse of the
eyes in too bright or too dim a light, or from using the eyes
for too long a time in any kind of a light. The best pair
of eyes, if overtaxed, may suffer from accommodative
asthenopia, even when wearing the ametropic correction.
Or accommodative asthenopia may result from a weakness
222 REFRACTION AND HOW TO REFRACT.
of the ciliary muscle as a part of- the general condition of the
whole body, and this may come on after or during some long
illness, such as typhoid fever. Accommodative asthenopia
x^s often present in the early months of presbyopia.
Symptoms. The principal symptom is headache fron-
tal, frontotemporal, or fronto-occipital ; or this pain or dis-
comfort may extend into the neck or between the shoulders.
The headache develops during the use of the eyes, and
grows worse if the effort is prolonged, and usually ceases
after the eyes are rested. See chapter on Hyperopia and
Myopia.
Treatment. When glasses are necessary, they should
be ordered by the static refraction. The general health of
the patient should receive careful attention. An out-of-
door life will often be necessary, and in certain cases the
time for using the eyes at any near-work will have to be
very much restricted.
Accommodative with Muscular Asthenopia. This
variety of asthenopia embraces the two forms just men-
tioned, and its description and treatment are included in both.
Reflexes Due to Eye-strain. Among the symptoms
of the various forms of asthenopia described on the previous
pages, the writer has avoided any decided reference to reflex
symptoms, preferring to speak of these reflexes in a general
way under one heading. Many patients who suffer from
headaches, ocular pains, etc., during the use of their eyes,
also very frequently suffer from constipation, indigestion,
heartburn, nausea, or even vomiting. Other patients
may have nervous attacks, a fear of some impending
calamity, or they are irritable or despondent ; they may
suffer from insomnia, or, if they sleep, it is not a restful
sleep. Others may have epileptic attacks, nervous twitch-
ings, etc. To just what extent eye-strain is responsible for
EXAMINATION OF THE EYES. 223
these and many other reflexes the writer is not prepared
to say, though every ophthalmologist has certainly seen
some cases of accommodative and muscular asthenopia with
gastric symptoms, or nervous symptoms, or epileptic
attacks, or irritable tempers, or insomnia, or enuresis, etc.,
in which these reflex symptoms entirely disappeared after
the eye-strain was properly treated.
EXAMINATION OF THE EYES.
A systematic method should be pursued in the examina-
tion of the eyes, and the results recorded in a book or on
a card prepared for that purpose. The student should be
a careful observer, and also be able to question the patient
intelligently for short and definite answers. The following
is an excellent method of making records, but there is no
arbitrary rule, arid in this respect each surgeon may follow
his own desires :
Date,
Name,
Occupation, ......................................
Age, ................... Sex, .................. - Diagnosis, ...................
ACCOMMODATION. ASTIGMATISM. MUSCLES.
O. D. V ............ p. p.
O. S. V., .......... p. p.
History, ...............................................
S. P. (status prasens, " present condition"). Inspection, ..................
Ophthalmometer, O. D .......................... O. S ..........................
Ophthalnuacopic examination, O. D .......................... O. S ............. - ......
Manifest refraction. Fields. Color sense. R .
224 REFRACTION AND HOW TO REFRACT.
The above record is rilled out as the examination pro-
ceeds, but it is not always advisable to follow the exami-
nation in the order given ; on the contrary, it is better, after
getting the patient's name and address, to ask certain other
questions which may appear in keeping with an individual
case.
1. Occupation. This is a very important question, as
bearing directly upon the amount and character of work
done by the eyes ; for example, writing, reading, sewing,
music, engraving, weaving, drafting, surveying, painting,
typewriting, typesetting, sorting colors, etc.
2. Age. This is of the utmost importance in comparing
the range of accommodation (near point) with the emme-
tropic condition. Knowing the patient' sage and near point
^x\vill often give a diagnosis of the character of the refraction.
3. The name tells the sex, but the question really is
whether the patient is married, single, widow, or widower.
/If a young married woman, whether she is nursing a young
' I child.
4. History. Under this heading the questions should
bear directly upon the eyes. " In what way do the eyes
/cause trouble?" The usual answer to this question is
\ tf headache." To get a complete history of the headache,
and be able to differentiate it from headache due to other
I causes, the succeeding questions seem appropriate :
What part of the head aches ? Is it frontal, occipital,
temporal, interocular, vertex, or all over the head ?
When does the headache come on during or after
the use of the eyes ? Does it cease after resting the eyes ?
Is the headache worse when using the eyes by artificial
light ? Is the headache constant ? Is it periodic ? Is it
worse at a certain hour of the day? Is the headache pres-
ent when first waking in the morning ? Does the head ache
EXAMINATION OF THE EVES. 225
during or after attending a place of public amusement or
when shopping ? If a female, is the headache only monthly ?
The ophthalmologist must not think because a patient has
^a headache that it is surely and always due to the eyes, and
that glasses are going to cure it. It is for the ophthalmolo-
gist to find out just what part the eyes take in causing the
patient's discomfort, and not always expect to cure with
glasses headaches that have no direct relation to the eyes.
One of the most common causes of headache which
may be mistaken for ocular headache is the " brow ache "
due to malaria, but a history of previous malarial attacks,
chills and fever, a residence in a malarious district, and the
fact that it is periodic in character, should certainly give a
clear differential diagnosis.
Other patients may not consult the ophthalmologist on
account of headache, but for a pain in or back of the eyes,
or back part of the head, or between the shoulders, which
comes on after any effort of vision. Others may complain
of a feeling of sand in the eyes, or a burning in the lids, or
a smarting or itching in the lid margins, or excessive lacri-
mation, or a feeling of drowsiness as soon as the eyes are
used for any length of time, or a feeling as if the eyelids
would stick to the eyeballs.
The patient's seeing qualities may develop the history of
poor distant vision and good near vision, or vice versa ; this
should be inquired into very carefully, and it may be well
to ask about other members of the family, if they have the
same condition. Or a history of the vision gradually fail-
ing or of a sudden loss of sight may be obtained, and pres-
byopic symptoms should be referred to, if the patient is
over forty years of age.
If the patient wears glasses, it is well to inquire whether
the}' were ordered by an ophthalmologist or if they are
226 REFRACTION AND HOW TO REFRACT.
the patient's own selection. In the former instance a record
should be made of the character and strength of the lenses,
and whether the lenses were ordered with or without
/'drops" in the eyes; and if "drops" were used, if the
effect lasted for t\vo days or longer (slowly or quickly
acting cycloplegic). Ask how long the glasses have been
worn, and if the same symptoms are present that existed
when the lenses were previously ordered.
Having made a note of the patient's history, it is next in
order to study the present condition (status prcescns) :
1. Breadth of face, its symmetry or asymmetry; inter-
pupillary distance.
2. The eyelids, whether swollen, discolored, or having
red margins.
3. The eyelashes (cilia), whether regular, irregular, or
absent. If there are chalazia, styes (hordeola), inflammation,
moist or dry secretion at the roots of the cilia (blepharitis).
4. Inspect the inner surface of the lids and ocular con-
junctiva for inflammation or growths.
5. Inspect the lacrimal apparatus in all its parts.
6. Inspect the cornea for its polish, transparency, and
regularity.
7. Depth of the anterior chamber.
8. Iris, its color and mobility.
9. Pupil, its size, shape, and position.
10. Color of reflex from the pupillary area.
1 1. Palpate to measure the intraocular tension.
^ 1 2. Use the cover test at 1 3 inches for any muscular
anomaly.
Following this record of the history and present condi-
ition, the distant vision and near point are taken for each
eye, one or more tests for astigmatism are made, the mus-
cles are tested for distance (six meters), and the ophthal-
EXAMINATION OF THE EYES. 22"J
mometric measure of corneal curvature may be recorded.
Finally, and most important of all, the ophthalmoscopic
examination is made, and the cornea, aqueous, lens cap-
sule, lens, vitreous, nerve (shape, size, color, cupping, and
vessels), conus, macular region, etc., and periphery of the
eye-ground are studied.
Lastly, fields and color sense, dynamic or manifest re-
fraction.
CHAPTER IX.
HOW TO REFRACT.
General Considerations. Before placing lenses in front
of an eye, the surgeon should be acquainted with at least
five important facts :
1. The Patient's Age. This tells at once, from the
table on page 70 (which the surgeon should commit to
memory), what the near point will be if the eyes are emme-
tropic or standard.
2. The Near Point. This will usually indicate hyper-
opia if beyond, and myopia if closer than, the emmetropic
near point for the age.
3. The Distant Vision in Each Eye. If very defective,
or if less than and near point closer than the age calls
for, myopia is indicated. Good distant vision and near
: removed indicate hyperopia.
4. The distant vision, if recorded with question
marks, usually indicates astigmatism.
5. The Results of Testing with the Astigmatic
Chart. Darkest lines from XII to VI, or I to VII, or XI
to V, indicate astigmatism (myopic) with the rule; or
darkest lines from IX to III, or VIII to II, or X to IV,
indicate astigmatism (hyperopic) with the rule.
It is well to remember that about four patients out of
js/vt/f^ (five have hyperopia, or one patient in five has myopia, and
the minus sphere selected almost invariably requires a
cylinder in combination. Remember, also, that astigma-
; tism is usually with the rule and symmetric, and that plus
228
HOW TO REFRACT. 2 29
cylinders are generally selected at axis 90 or within 45
degrees either side of 90, and minus cylinders are gen-
erally selected at axis 180 or within 45 degrees either side
of 1 80.
The Placing of Trial-lenses. i. These should always
be placed as close as^ ]3ogg^g^o_the_eYes without interfer-
ing with the lashes ; and to accomplish this, the trial-frame
should be easy of adjustment.
2. The center of the trial-lens must be opposite- to the
center of the pupil.
3. If the distant vision is very defective, -,
--, or ~l., a strong lens of 2, 3, or 4 D. will often be re-
quired ; whereas, if the vision is ^^ or ~~, a weaker lens
would be called for.
4. When a spheric lens placed before an eye improves the
vision, it should not be changed for another unless the vision
is made better by having its strength increased or dimin-
ished by placing in front of it another sphere (plus or
minus) of less strength. For instance, if a -f 2 sph. has
been placed before the eye and the vision is improved from
xY to vHIs ' ^ s + 2 s Pk- snou ld not be changed until a
.50 or 0.50 sph. has been held in front of it and the
patient states whether he can read more with it or less without
it. When a vision of -^j- is approximated, then its accuracy
must be determined by placing first a +0.25 and then a
0.25 in front of the correction, so as to learn from the
patient which one, if either, of these lenses improves the
vision. Or if the correcting lenses selected are weak ones,
then o. 12, plus and minus, may be used in place of the
0.25.
/ 5. Spheric lenses should always be tried before using
cylinders, and the vision brought as low as possible with
LAA/^-A-" *"*>- '"\
J^U>WA^ WAJUX IM<L -f 4 -
(
23O REFRACTION AND HOW TO REFRACT. U\
a sphere before combining a cylinder, and, in fact, after
the vision has been improved as much as possible with
a sphere, the pointed line-test for astigmatism may be
brought into use, as very often low errors of astigmatism
are not recognized until this point in the refraction has
been reached. ^Advocates of the ophthalmometer place
the cylinder before the patient's eye and then add the
spheric correction. The writer is not partial to this method
or way of refracting^)
6. When a patient miscalls one or more. letters in a cer-
tain line, the surgeon must not hurry on until these are
corrected by the patient with a suitable glass, and in
this way the refraction is gradually worked out until the
vision is brought to the greatest acuity possible. It is
never wise to stop with a vision of -^j- t as we are often able
to get a visual acuity of ~, or occasionally ~.
7. Cylinders. When a plus cylinder is employed, it is
placed with its axis at 90, and then slowly revolved (if nec-
essary) to an axis where the patient says he can see better.
A minus cylinder is placed at axis 180, and revolved in the
same manner. The rule (4) for changing spheres also ap-
plies to cylinders i. e., to increase or decrease the strength
of the cylinder by placing in front of it a plus or minus
1 cylinder of less strength at the same axis.
8. Axis of the Cylinder. When a patient is not sure
about an exact axis, though he is sure that the cylinder
improves the vision, then the surgeon(may employ a sphere
of the strength of the sphere and cylinder combined, and
use a cylinder of the same strength as before, but with
opposite sign and at about the opposite axis\ For example,
with +2.25 sph. O +0.75 cyl., the patient is not sure if
the vision is best with the axis at 35, 40, or 45, the sur-
geon must then use a +3.00 sph. O 0.75 cyl., when the
HOW TO REFRACT. 23 I
exact axis (at right angles) will usually be selected without
any hesitancy or doubt.
9. Proving the Correction. All tests at the trial-case,
when a cycloplegic is used, should be confirmed with the
retinoscopc.
10. Crossed Cylinders. This is a trial-lens that has
one meridian minus and the opposite meridian plus. They
are made of any strength, but for general use the 0.25 cyl-
inders are employed /. e., 0.25 sph. O +0.50 cytl The
purpose of the crossed cylinders is to increase the refrac-
tion in one and diminish it in the opposite meridian. For
example: if +2.00 sph. O+i-OO cyl. axis 90 gives a
vision of yggg, and the crossed cylinder lens is placed in
front of this combination with 0.25 at axis 180, and the
-(-0.25 at axis 90, and the vision comes down to , it
shows that the vertical meridian was 0.25 too strong, and
j'the horizontal 0.25 too weak, and the result would be
, vf'V*^'
^ (- 1.75 sph. ^ +1.50 cyl. axis 90 degrees. Or, if 3.00
sph. has brought the vision to , and the crossed cylinder
*!$*/' lens is placed before it and rotated to axis 15 for the
* *
minus cylinder and axis 105 for the plus cylinder, and the
vision comes to , the result would be 2.75 sph.
C 0.50 cyl. axis 15. C6^ v^ ^ V? ~^vvx.|J^Lit\ fc l^ H r
Methods of Estimating Refraction. To determine "7"M
the refraction of an eye it may or may not (as in presby- l>~^^
opia) be necessary to employ a cycloplegic. When the f-vb
refraction is estimated without, a cyclonlegic., it is spoken of t-t^jJLt
r < r ,' \ lU~ t^?"^ .-
as manifest or dynamic (dr., ^jvi/^, power ) refraction. /".
When a cycloplegic is used, the refractive estimate is spoken ^
of as static (Gr. frrar-xn-, from, '--n^a:, " to stand at rest "). In
\ ^.^-*- '
one instance the ciliary muscle is permitted to act, and in ^
the other it is at rest. A third method is to obtain the /
static refraction and then to estimate the strength of the
232 REFRACTION AND HOW TO REFRACT.
glasses to be prescribed after the effect of the cycloplegic
has passed out of the eyes ; this is spoken of as post-
cycloplegic refraction. Eyes for refraction are divided into
general classes, according to the age of the patient. In
^,-those under forty-five years of age a cycloplegic is usually
employed, but after this age a cycloplegic is often dispensed
with. (See Presbyopia.)
Fogging Method. This method simulates the static or
cycloplegic method, as the ciliary muscle is in great part
(if not entirely) placed artificially at rest by having in front
of the eye under examination a phis sphere of sufficient
strength to more than overcome any ciliary muscle poivcr tJiat
the eye might otherwise use ^vhen looking at a distance of six
meters. The "fogging" method is so called because dis-
tant vision is made obscure or "foggy." The eye under
examination with this strong plus sphere in front of it is, to
all intents and purposes, for the time being, at least,
myopic. This fact should be carefully borne in mind, as the
method to pursue in estimating the refractive error is to
proceed the same as in any regular case of myopic refrac-
tion. Therefore, this method is only of service in estimat-
jing the refraction of eyes that have some form of hyperopia
'or simple myopic astigmatism, and is not of service in
myopia or compound myopia.
How to Proceed in Hyperopia. Having estimated
with the ophthalmoscope that the eye is hyperopic about
2.50 D., then place a plus 4 D. sphere before the eye and
have the patient look at the card of test letters at a distance
of six meters. This has the immediate effect of making
distant vision very "foggy," but by waiting a few seconds
this foggy vision will clear slightly and the ciliary muscle
will relax a part, if not all of its effort to accommodate.
Then proceed as if refracting a myopic eye by placing a
*
^|
>
HOW TO REFRACT. 233
0.50 sphere in front of the +4 1). sphere, and gradually
increase the strength of the minus sphere until the patient
reads ^.|. If the minus sphere is 1.25 D., then the amount
of the hyperopia will be -f 2.75 D., which is the difference
between the +4 D. and the 1.25 D.
How to Proceed in Simple Hyperopic Astigmatism.
Having estimated with the ophthalmoscope or retinoscope
or ophthalmometer or any of the various ways that the eye
has hyperopic astigmatism of about 2.50 D., proceed as in
hyperopia by making the eye myopic by the addition of a
+ 4 D. sphere. Have the patient look first at the card of
test letters and then at an astigmatic clock-dial next to the
letters. Proceed by adding minus spheres as in the previ-
ous case, and as soon as the patient recognizes one series
of lines on the clock-dial as much darker than those at
right angles to these dark lines, then place minus cylinders
in position with their axes at right angles to the darkest
lines, and as soon as the lines are uniformly black on the
dial then have the patient look at the test letters again and
increase the strength of the minus sphere if necessary until
the eye can read ^j or ^. Presuming that 2 sphere and
2 cylinder at axis I To degrees were used, then the dif-
ference between these and the -j- 4 sphere would give -j- 2
cylinder, axis 90, as the amount of the hyperopic astigma-
tism.
How to Proceed in Compound Hyperopic Astigma-
tism. If the meridian of highest refraction is 4 D., then
place a -f 5 D. sphere before the eye, wait a few seconds,
then begin by adding minus spheres until a series, of lines
on the clock-dial show up very black, then add minus
cylinders until all the lines become uniformly black, then
turn to the test letters and increase the strength of the minus
sphere as necessary until the vision is brought to normal.
234 REFRACTION AND HOW TO REFRACT.
With -f- 5 D. sphere before the eye and if I sphere and
3 cylinder at axis 165 were employed, then the com-
pound hyperopia would be -f- 1 sphere with -}- 3 cylinder at
axis 75.
How to Proceed in Mixed Astigmatism. Estimating
the refraction approximately as -f- 2 O 3, cylinder; place
-(- 5 D. sphere in position as before, then find the meridian
I of darkest lines on the clock-dial, add minus cylinder with
the axis at right angles to the darkest lines, then when all
lines are equally black, diminish the strength of the plus
sphere until vision comes to the normal, if it is possible to
get it to this point. The result may be -f- 5 sph. O
2.50 sph. O 3- SO cyl. X 1 80, which would equal -{-2.50^
C 3.50 cyl. X 1 80.
Advantages of the Fogging Method. It is one of the
best approximate ways of estimating the refraction if for
any reason a cycloplegic can not be employed, as in cases
of glaucoma, nursing n>otners, or of an individual who may
'A i TI r *u A
have an idiosyncrasy for a cycloplegic. 1 he fogging method
also has the advantage, like the cycloplegic method, of un-
covering or bringing out most if not all the latent hyper-
opia.
Manifest or Dynamic Method. This method is the
very reverse of the "fogging" method, in that the refraction
is estimated without first making the eye artificially myopic,
or placing the ciliary muscle at rest. It is, therefore, a
method thatns liable to give all sorts of erroneous results
if the subject is hyperopic and less than forty years of
o estimate the refraction by the manifest method, the
patient is told to look first at the test letters and then at the
clock-dial at 6 meters distant ; the astigmatism is corrected
and then plus or minus spheres added as necessary to bring
HOW TO REFRACT. 235
the vision to the normal. The rule is to employ the
strongest plus lenses or the weakest minus lenses which
will give normal vision.
Advantages of the Manifest Method. This is the
method by which the eyes of patients past forty-five years
of age are refracted ; a fairly good method in cases of
compound myopia in young subjects if drops can not be
employee!.
Objections to the Manifest Method. It is not a method
to be used as a rule in young subjects. If a young subject
must be refracted without drops, then the fogging method
should be followed.
The habit of prescribing glasses from the manifest or
"fogging" method without any knowledge of the ophthal-
moscopic findings is not a method that merits the attention
of the conscientious physician. Such work is very unsatis-
factory, often leading to gross errors, ultimate dissatisfaction
on the part of the patient, or injury to the eyes.
Postcycloplegic Refraction. The ordering of glasses
after the static refraction has been recorded and the effect of
the cycloplegic has left the eyes. For instance : While the
ciliary muscle is paralyzed with atropin the static refraction
is found to be -f- I-5O sph. O +2.OO cyl. axis 90 degrees,
which gives a vision of -. The atropin is then stopped
>dhd the patient told to report in fifteen days, when the ciliary
muscle will have regained its original strength and gone
back to its old habit of accommodating for distance. The
static refraction is placed before the eyes, and the strength of
the sphere is gradually reduced until the vision just equals
-, as it was when the " drops " were in the eyes. What-
ever this correction with the glasses may be, is ordered.
Occasionally the strengtlvof the cylinder as well as its axis
is also changed
236 REFRACTION AND HOW TO REFRACT.
Objections to the Postcycloplegic Method. The pa-
tient is annoyed by the long delay to which he ie subjected
before getting his glasses. But the principal fault lies in
the fact that the eye is not placed in the emmetropic condi-
tion ; it is allowed to retain more or less of its accommo-
dative power for distance. This, however, can not always
be avoided.
Static Refraction. By this method the glasses are pre-
scribed while the ciliary muscle is under the effect of the
cycloplegic. In hyperopia allowance must be made in the
strength of the sphere for the distance at which the test is
made. At 6 meters 0.25 is deducted from the sphere
f\ without any change in the cylinder. The only possible
^objection to this method is in cases of hyperopia, in which,
after the effect of the cycloplegic passes away, the ciliary
muscle may endeavor to accommodate for distance with
the glasses in position, and with the result that the pa-
tient can not see clearly except near at hand. To avoid
any such contingency the surgeon will have to make a
deduction in the strength of the plus sphere to meet such
cases. The rule for ordering glasses by the static refrac-
tion in hyperopia is to deduct 0.25 from the sphere and
/ have the glasses worn at once and constantly while the
\effect of the " drops " is gradually leaving the eyes. In
this way the eyes grow accustomed (slowly) to seeing at a
distance without exerting the ciliary muscle ; the eyes are
thus placed in an emmetropic condition. If this effect of
the cycloplegic passes away before the patient can obtain
the glasses, it will be necessary to use the drops for a day
or so after the glasses arc received. Unfortunately, how-
lever, some hyperopic eyes, in young subjects especial ly,
'with vigorous ciliary muscles, will develop a spasm of the
accommodation for distant vision which will make the
HOW TO REFRACT. 237
glasses very annoying on account of distant objects look-
ing "dim." Such patients should be advised of this fact
at the time the glasses are ordered, and if dim distant
vision does develop, that it will be transitory, and to per-
sist in wearing the glasses. There are two ways of reliev-
-ing this " dim " vision if it should occur :
1. To prescribe a weak solution of atropin (^ of a grain
to i fluidounce), I drop in each eye once or twice a day,
the idea being to slightly relax the accommodation ; this is
accomplished, but, unfortunately, the mydriatic effect is a
disturbing element which the patient will not submit to
long enough, as a rule, to obtain relief.
2. The better way is to make a compromise in the
strength of the sphere. An eye which has been in the habit
of accommodating 3, 4, 5, or 6 diopters for distance, does
not often give up this habit very gracefully, even if assisted
by a slowly acting cycloplegic, /so that when the static
refraction calls for more than 3 diopters, the surgeon is fre-
quently compelled to make a deduction of more than 0.25. )
There is no hard and fast rule as to just hm*.' innck shall be
deducted, and very few surgeons agree on this point.
Glasses may be ordered as follows, the surgeon being
guided in great part by thejpatient's age anoK>ecupation ;
also as to whether there is esophoria or exophoria. It
will be found that cases of esophoria will accept almost
a full correction, whereas cases of exophoria will require a
\\-ry liberal deduction in the strength of the glass, the
patient being allowed to use his relative hypcropia :
Static refraction at 6 meters :
-j-l.oo sph. or less deduct o. 12 or 0.25.
From -fl-oo sph. up to 3.00 sph. " 0.25 or 0.50 or 0.75.
" +3.00 sph. up to 6.00 sph. " 0.50 or 0.75 or i.oo or 1.50.
" -f 6- s ph- U P to 8- and above " I. or 1.50 or 2.00 up to 3.00.
238 REFRACTION AND HOW TO REFRACT.
It is true that glasses ordered in this way do not leave
the eyes in an emmetropic condition, and that, later on, when
asthenopic symptoms redevelop, the strength of the glasses
will have to be increased. But this method has two advan-
tages : first, it gives the patient his glasses without any long
delay, and the eyes have an opportunity to become accus-
tomed to them while the effect of the " drops " is passing
away ; and, second, the patient accepts a much stronger
glass in this way than by the postcycloplegic method, which
is a decided advantage.
The ordering of lenses in low errors for distant vi-
sion depends entirely upon the condition of the patient's
eyes and symptoms. It is not unusual to find the most
distressing asthenopia, headaches, blepharitis, etc., disap-
pear as if by magic when corrections are ordered for small
defects, especially if there is astigmatism. In other instances
slight ametropic errors may not produce any unpleasant
symptoms, and such a patient need not wear the correction
for distance.
The Ordering of Glasses in Myopia. There is no
fixed rule for prescribing glasses in myopia. Each case is a
law unto itself, and should receive the most careful consid-
eration from every point of view. But as the student must
have some idea as to how to proceed, the writer would sug-
gest the following subdivisions :
J. Myopic eyes which can with safety use one pair of
glasses for distant and near vision.
2. Myopic eyes which require two pairs of glasses one
for distant vision, and another pair for near-work, reading,
writing, etc.
3. Myopic eyes which should have the near correction
only.
CLASS i comprises those cases in which there is an active
HOW TO REFRACT. 239
ciliary muscle, and the ophthalmoscope shows but little, if
any, change in the eye-ground indicative of stretching (chil-
dren, or in beginning myopia). Glasses carefully selected
by the static refraction may be ordered in such instances
for constant use, but with the distinct understanding that
if any discomfort arises at any time they will be subject
to change.
CLASS 2. Adults who have not previously worn their
myopic glasses. In these cases the power of the ciliary
muscle is weak, deficient in sphincter fibers, and, if forced
into activity, the patient would be very uncomfortable, the
eye would stretch, and the myopia increase, the tissues in
these eyes yielding more readily than in class I. The
glasses selected by the static refraction may be prescribed for
distance, but a second pair, I, 2, or 3 diopters weaker, must
be ordered for the reading, writing, or working distance,
that the accommodative effort may in part be kept in
abeyance. Class I, if not carefully watched, may pass
into class 2 and class 2 may pass into class 3.
CLASS 3. These cases require unusually strong lenses,
and it is to these especially that the term " sick," or
"stretched" eye particularly belongs. The ophthalmo-
scope may show vitreous opacities, areas of retinochoroid-
itis, macular choroiditis, a broad myopic conus, and even
posterior staphyloma. The eyes are prominent, occupying
much of the orbital space. Eyes of such length are limited
in their power of comfortable rotation, and hence it is com-
mon for one eye to diverge, the patient stating that lie uses
only one eye for near vision. The diverging eye is usually
more or less amblyopic, due to want of use or pathologic
changes, or to both. Such eyes have lost almost or quite
all the power of accommodation. These eyes must be
placed in such a condition that the desire to converge and
24O REFRACTION AND HOW TO REFRACT.
accommodate is at a minimum. The prescribing of glasses
for these long eyes must be limited to the one pair for near-
work, and yet the patient may, by bringing the glasses
closer to the eyes, improve the distant vision for the time
being a sort of artificial accommodation. To appreciate
what is meant by this statement it is necessary to reconsider
the optics of a myopic eye. A myopic eye of 20 D. has a
far point of 5 cm., and the minus lens required to make
such an eye receive parallel rays of light at a focus upon
its retina should be of such strength that the rays passing
through it would have a divergence as if they came from
this far point (5 cm.). Such a lens would be a 20 D.
This means, of course, that the 20 D. would have to be
placed with its surface against the surface of the cornea,
which is an impossibility. The usual distance for a lens in
front of the eye is I or I ^ cm., so that this distance must
be subtracted from the distance of the far point. In this
instance I cm. from 5 cm. would leave 4 cm., and this would
represent 25 D. As just stated, the glasses for this class
of patients are limited to the one pair for near-work, and
therefore it would be necessary to reduce the strength of
these lenses 4 diopters and thus prevent, as far as possi-
ble, any accommodative effort. The patient using this
21 D. for near, can, if he wishes, improve his distant
vision at any time by pressing the lenses closer to his eyes.
The strength of concave lenses increases as they are
brought closer to the eyes, and diminishes as they are re-
moved from the eyes.
Caution. The great danger in any refraction at the trial-
case, but especially in myopia, is an overcorrection, and
this is very likely to occur if the surgeon is not extraor-
dinarily careful in having his lenses placed as close to the
eyes as possible while making the test.
HOW To K I. TRACT. 24 I
Prophylaxis. The prescribing of glasses for myopic
eyes is only a part of the general treatment to which these
"sick" eyes are entitled. If the treatment stops at this
point, then the glasses may be an injury instead of a bless-
ing. Myopia once established may pass through the
various classes already described, and eventuate in greatly
reduced vision or total blindness if certain limitation of
their use is not insisted upon.
1. Light. A good, clear, and steady light is always essen-
tial ; it should come from the left side, never from in front.
2. Time. The length of time that myopic eyes may be
used should be restricted as much as possible, consistent
with their condition ; that is to say, they should never be
used after they become the least fatigued, and any use of
the eyes should be counteracted by life in the open air.
3. Attitudes. The head should have as little inclination
as possible in reading, writing, or close work, as so faulty a
position invites a congestion of the intraocular tissues. At
school or at home the book should be inclined, and its dis-
tance from the eyes be regulated by the size of the patient.
4. Print. The use of small print or minute objects
must be forbidden. English or Gothic type should be sub-
stituted for Greek, German, and other characters. Fine
needle-work, embroidery, etc., must be abolished. If nec-
essary, music notes must be given up entirely.
5. Health. The health of the patient must be looked
after, and all irregularities corrected constipation, etc.
These are a few of the major considerations to which the
patient's attention must be drawn, the surgeon being limited
in his remarks to the exigencies of the individual case.
In conclusion it may be well to know how myopia is
produced, since it has been stated that the condition is rarely
seen in young children. It is well known that astigmatism
242 REFRACTION AND HOW TO REFRACT.
(hyperopic) is a congenital defect, and with this in mind it
is very easy to appreciate the succeeding steps which lead
to the compound myopic condition, showing at the same
time the reason why simple myopia is so rare an anomaly.
Take a child six years of age who has a compound hyper-
opia of say +0.50 sph. O +0.75 cyl. axis 90 degrees ; this
child enters upon its course of study without any correcting
glasses, and is subjected in its pursuit for knowledge to a
faulty school desk and chair, possibly facing a window.
The print is defective in many ways. The artificial light
for home study in the evening may be of poor quality, and
so placed that but few of its rays fall upon the child's book.
With these and other hindrances the eyes are strained
(stretched). The tissues are very yielding in their growing
state, so that at the age of ten years the refraction may
show +0.75 cyl. axis 90. The +0.50 sph. (the axial
ametropia) has disappeared by an elongation of the optic
axis. The vertical meridian is now emmetropic. The same
conditions exist for the next three years, during which the
number of studies is multiplied and the hours for study are
prolonged and the child reaches the age of puberty ; the
refraction is now found to be 0.50 sph. O +0.75 cyl.
axis 90 degrees, mixed astigmatism. In two years more the
refraction is found to be 0.25 sph. O 0.75 cyl. axis
1 80 /. e., compound myopic astigmatism. From this time
forward these eyes progressively stretch and are subject to
the stretching process unless the progress is stayed with
glasses and prophylactic treatment.
In the brief detail of this one case the student will fully
appreciate another important fact that the vertical meridian
of the cornea, as a rule, maintains throughout the shortest
radius of curvature. This is abundantly demonstrated by
statistics.
HOW TO REFRACT. 243
The following summary of refractive errors and direction
of meridians of shortest radius of curvature in 2500 pairs
of eyes, 1300 in private and 1200 in hospital work, pre-
pared by Dr. Risley and the writer, shows the correctness
of the above statements : *
PRIVATE. HOSPITAL.
Monocular astigmatism, 70 $<>% 94 7-8J&
Binocular astigmatism, 1151 88.5 828 69.0
Total cases, 1300 1200
Binocular symmetric astigmatism, . . . 694 60.2% 613 74.4%
Binocular asymmetric astigmatism, . . . 310 26.8 158 19.8
Heteronymous astigmatism, 123 10.6 40 5.2
Homonymous astigmatism, 24 2.1 17 1.2
Total binocular astigmatism, . . . . 1151 828
Symmetric astigmatism :
(a) According to rule (homologous), . 543 78.2^ 559 97.7^0
(6) Against rule (heterologous), . . . 151 21.8 54 2.3
Total symmetric astigmatism, . . . 694 613
Asymmetric astigmatism :
(a) According to rule, 223 71.8% 126 79.1$
(t>) Against rule, 87 28.2 32 20.9
Total asymmetric astigmatism, ... 310 158
DIRECTION OF THE MERIDIAN OF SHORTEST RADIUS IN ALL CASES OF
SYMMETRIC ASTIGMATISM.
Meridian at 90, 57--f $
Meridian inclined 15 or less on each side, 19-74"
Meridian inclined from 15 to 30 on each side, 4.0
Meridian inclined from 30 to 45 on each side, I.O
Meridian at 180, 12.
Meridian inclined 15 or less on each side, 4.0
Meridian inclined from 15 to 30 on each side, 2.O
Meridian inclined from 30 to 45 on each side, 0.5
* This report was read in the Section on Ophthalmology at the forty-sixth
annual meeting of the American Medical Association, at Baltimore, Md., May
7 to 10, 1895.
CHAPTER X.
APPLIED REFRACTION.
In estimating the refraction of any eye the surgeon will
do good work if he will make it a rule never to be satisfied
until each eye has a vision of -^j- or more, and if this visual
acuity is not attained, to understand the reason why :
whether it is his fault or the fault of the eye itself. It is
most essential in every instance to have the good-will of
the patient.
The following cases are detailed so as to demonstrate
each form of ametropia in all its phases :
CASE I. Simple Hyperopia. This is a common form
of ametropia, occurring about 20 times in 100 cases :
January 3, 1899. JOHN SMITH. Age, twenty. Single. Stenographer.
- D - -VT p - P '= type ' 5 D ' at H cm '
O. S. ^1. p. p. = type o. 5 o D. at 14 cm. ^f\
Add. 22 degrees ; abd.= 6 degrees.
History. Frontotemporal headaches almost constantly,
but much worse when using eyes at near-work. Had
severe headaches when a school -boy. Never liked to
study ; preferred out-of-door sports.
5". P. Face symmetric, but narrow. Blepharitis mar-
ginalis. Irises blue. Pupils round, 3 mm. in diam. Eyes
fix under cover.
Oplitlialnwinctcr. Negative.
Ophthalmoscope. Both eyes the same. Media clear.
244
APPLIED REFRACTION. 245
Disc small and round, with physiologic cup. All vessels
near the disc are seen clearly with +38. Eye-ground
flannel -red and accommodation very active.
Manifest or dynamic refraction :
O. D. +i. 25 S. = ^f.
O. S. +i.2 S s.= ^L.
R . Atropin and dark glasses for refraction.
January 5, 1899. Patient seated at 6 meters from test-
card and small point of light. O. D. V.= j^. = O. S.
V. ~y. Cobalt-blue glass shows (each eye separately)
blue center and red halo. (See Fig. 125.)
Retinoscope, with -(-48., developed point of reversal at
I meter for each eye.
With trial-lenses, O. D. and O. S. each select +3 S.,
which gives a vision of - y -, and they positively refuse to
see v v ' clearly with an addition of +0.25 S. In other
words, this + 3 S. is the strongest lens which each eye will
accept and maintain clear distant vision. Tlic rule for
refraction in hypcropia is to employ the strongest lens
which the eye will accept without blurring the distant
vision,
To prove that the ciliary muscle is at rest, and that the
glass selected is correct, add a +4 S. to the distance cor-
rection, and the rays of light emerging from the eye must
focus at the principal focus of the added +4 S., at 10
inches (25 cm.); and if the patient can read fine print at
this distance, the ciliary muscle is at rest and the glass cor-
rect. If a +3 S. had been added instead of a +48., then
the principal focus would be at 13 inches ; if +5 S., then
at 8 inches, etc.
The question is, What glasses shall be ordered ? The
246 REFRACTION AND HOW TO REFRACT.
writer would give the following prescription, and instruct the
patient to stop the drops and wear the glasses constantly :
For MR. SMITH.
K. O. D. +2.75 sph.
O. S. -f 2.75 sph.
Sic. For constant use.
January 7, 1899.
January 8th : Glasses from the optician neutralize ; are
centered and accurately adjusted.
January 2ist : Add. = 18 degrees. Abd. = 6 degrees.
Near point in each eye = 10 cm. No headache or dis-
comfort of any kind.
Considerations. The static refraction as represented by
.00 sph. means that rays of light which pass through
this lens and focus at the fovea diverge from six meters'
J distance, which heretofore we have considered for purposes
Jj& of calculation as parallel ; but when glasses are ordered,
^ allowance must always be made for this small amount of
/divergence, and so 0.25 is deducted from the +3 sph., that
I/ the eye may have parallel rays focusing on its retina when
lit *" looking beyond a distance of six meters. To have been
^ mathematically exact, -f-o. 12 should have been deducted in
i * JL lace of +0.25.
^ e P ur P ose ' m a ^ cases of refraction is to place the eye
an emmetropic condition, though this is not always advis-
able in every instance. The hyperopic eye naturally accom-
modates for distance, and the emmetropic eye does not ; then
the hyperopic eye is made emmetropic when a spheric lens
permits parallel rays to focus upon its fovea without any
assistance whatever from the ciliary muscle.
Advantages of Atropin in This and Similar Cases.
The glasses are ordered while the ciliary muscle is at rest.
The accommodation returns gradually. The eye becomes
APPLIED REFRACTION. 247
accustomed to seeing at a distance without the assistance
of the ciliary muscle. Atropin produces a physiologic rest,
which the overacting ciliary muscle, disturbed choroid, and
irritated retina require. None of these good results can be
expected in a case of this kind from the use of a " quick "
cycloplegic like homatropin.
SUMMARY. Age of patient, twenty years. Amplitude of
accommodation is 10 D. for this age.
Near point is 14 cm., which shows only 7 D.
Facultative hyperopia (Hf.) equals difference between 10
and 7 D., which is 3 D.
Manifest hyperopia (Hm.; equals 1.25 D.
Total hyperopia, or static refraction (Ht.), equals 3 D.
Latent hyperopia (HI.) equals the difference between the
manifest and total, 3 D. and 1.25 D., making 1.75 D.
The far point, or conjugate focus, is negative or virtual,
and lies back of the retina, where the emergent rays (diverg-
ing) from the eye would meet if projected backward ; this
point corresponds to the principal focus of the lens which
corrects the hyperopia :'. c., in this instance, -j~3 D., and
the negative far point is therefore at 1 3 inches.
The -(-2.75 makes the eye practically emmetropic ; the
near point, after the effect of the atropin passes away, is 10
cm., which is the emmetropic near point for the patient's
age. The plus sphere selected represents a shortening of
the eye of I mm., as measured on the optic axis.
CASE II. Simple Myopia. \Yith the one exception of
lemmetropia, it will be found that myopia, plain and simple,
r>|whhout astigmatism, is one of the rarest conditions of the
eye which the surgeon will meet in careful refractive work.
About one and one-half per cent, of all patients, by careful
refraction, are found to have simple myopia. Therefore
the condition is not common.
**-"'
V
ck
REFRACTION AND HOW TO REFRACT.
January 3, 1899. Miss RARE. Age, twenty-five years. Single.
O. D. V. = L p. p. = type 0.50 D. at 9 cm.; p. r. at 33 cm.
O. S. V. = -. p. p. = type 0.50 D. at 9 cm.; p. r. at 33 cm.
i\. i-
Add. 1 6 degrees. Abd. 6 degrees. Exophoria, 3 degrees at 13 inches.
History. Does not suffer much from headache, but eyes
ache after any prolonged use at near-work. Never able to
see well at a distance. Always stood high in her class at
school, though she had to have a front seat to see the fig-
ures and writing on the blackboard. Has excellent vision
for near-work and does fine embroidery. Has been accused
of passing friends on the street without speaking to them.
If she drops a pin on the floor, has to get on her knees to
find it. Parsnts do not wear glasses. Grandfather had
"elegant" sight, had "second sight," and never wore
glasses. Patient has postponed getting glasses because
parents objected.
>S. P. Face symmetric. Interpupillary distance, 65 mm.
Irises dark. Pupils large, round, 5 mm. Eyes out under
cover.
OpJithalmometer. Negative.
OphtJialmoscope shows each eye the same. Media clear.
Disc large and round. Shallow physiologic cup. Narrow
myopic conus at temporal side of disc. Choroidai vessels
seen throughout periphery of eye-ground and extending
almost to nerve margin. All vessels near nerve-head seen
with 3. S.
Manifest Refraction. Each eye 3.50 sph. gives vision
<"-
Cobalt-blue glass gives red center and dark blue halo.
(See Fig. 126.)
R. Atropin and dark glasses for refraction.
APPLIED REFRACTION. 249
January 5, 1899: Patient seated at 6 meters from test-
card. O. D. and O. S. vision equals Xl . Retinoscope with
2. S. develops point of reversal at I meter for each eye.
'It will be noticed that the vision in hyperopia with and
without drops is decidedly different, whereas in myopia
there is little, if any, change. With trial-lenses each eye
selects separately 3 sph., which gives a vision of -^-. If a
2.75 sph. is substituted, the vision falls to 5. If a
3.25 is employed, the vision remains ~, but the letters
look small, black, and " far away." The rule for refraction
in myopia is to employ the weakest lens through which the
eye can still maintain clear, distant vision.
What Glass to Order. The writer would give the fol-
lowing prescription and instruct the patient to stop the
drops and wear the glasses constantly :
January 7, 1899. For Miss RARE.
U. O. I). 3.00 sph.
O. S. 3.00 sph.
SIG. For constant use.
January 9th : Glasses neutralize ; are centered and accu-
rately adjusted. Add. 18 degrees. Abd. 10 degrees.
Patient is delighted with glasses.
January 2 1st: Near point, 12 cm.
Considerations. As a rule, concave lenses are ordered
without any deductions for the slight amount of diver-
gence of the rays of light for the distance (6 meters) at
which the estimate is made. To be exact, 0.25 should
:be added to the 3.00 sph. in this case ; but the surgeon
must avoid the danger of tfrrrcorrecting the myopic eye,
and, to be on the safe side, 1(ne glass is usually ordered as the
patient selects and the retinoscope confirms j^ These lenses
make the eyes, to all intents and purposes, emmetropic.
f\
25O REFRACTION AND HOW TO REFRACT.
Advantages of Atropin. The choroid and retina are
given a physiologic rest that they could not obtain in any
other way. (/The patient will not^sgjfifctiUtflr strong a glass. .
as was the case in this very instance w Hwfc"" m aiMgyfcefrft ^ >
Myopic eyes usually have large pupils, and do Jiot suffer,
therefore, from mydriasis to the same extent as^Tr^K^s
eyes. The far point remains unchanged. The power of con-
vergence is somewhat relieved by the glasses, \\vhich at
the near working distance are of the nature of prisms,
bases in.
SUMMARY. Age of patient is twenty-fiye years. Ampli-
tude of accommodation at this age is 8 ^D. /'Near point, 9
cm. = 11 D., and far point 33 cm. =30. Difference be-
tween near point and far point in diopters = 8 D., which is
the amplitude of accommodation for the patient's age^/
I Difference between the near point in diopters (i i D.)
( and the amplitude (8 D.) is 3 D., which is the amount of
|the distant correction needed. With glasses on, the near
point, after the effect^ of the atropin passes away, is 12 cm.,
which represents the emmetropic near point for the age.
This myopia of 3 D. represents an eye i mm. longer than
the standard eye, as measured on the optic axis.
CASE III. Simple Hyperopic Astigmatism. Not an
uncommon condition. About 5.5 percent, of all eyes have
this form of refraction.
April 3, 1899. Miss ROBINSON. Age, twenty-four years. Single. Dress-
maker.
VI
O. D. V. =-jjj-???. p. p. = type 0.50 at 18 cm.
O. S. V.=-j5Jr?? ?. p. p. = type 0.50 at 18 cm.
Add., 23 degrees. Abd., 5 degrees. At 6 meters, esophoria 4 degrees ;
at 13 inches, I degree of esophoria.
Astigmatic clock-dial shows darkest lines from X to
IV with O. D.
APPLIED REFRACTION. 251
Astigmatic clock-dial shows darkest lines from VIII to
II with O. S.
History. Headache every day ; seldom entirely free from
ocular discomfort. Distress begins in the forehead and
extends to the back of the head and into the neck. After
a hard day's sewing, has to go to bed and bind the head
with a handkerchief. Once a week has a " sick headache,"
when she has to give up work entirely and take headache
powders. Sick headache often ceases after emesis.
S. P. Face symmetric. Blepharitis well marked, with
many cilia missing. Edges of lids thickened. Irises light
blue in color. Pupils apparently oval in vertical meridian.
Corneal reflex shows axis inclined from vertical in each eye.
OpJitlialmomctcr. O. D., 3.50 ; axis, 75, with the rule ;
O. S., 3.50; axis 105 degrees, with the rule.
Oplithalnwscopc. O. D., vertically oval nerve axis, 75
degrees. Accommodation very active. Underlying con us
down and out. Vessels at 75 best seen with -f- 2.50 ; and at
axis 165, without any lens. O. S., same general conditions
as in O. D. Vertically oval nerve axis, 105. Vessels at 105
degrees seen with -f- 2. 50, and at axis I 5 without any lens.
Manifest Refraction.
VI
O. D., +2.50 cyl. axis 65 degrees = -yj- ? ? ?.
O. S., same cylinder with axis 125 degrees.
R. Hyoscyamin and dark glasses for refraction.
A/
April ;th : Six meters from test-card and point of light, *^
/ t **>
O. D. '' ^ <*
'<
Clock-dial shou^-flie same as at first examination. A <f
^^^"^^
Cobalt-blue"gTass before O. D. gives blue center and red
on each side at axis 165 degrees. O. S. the same at axis
15 degrees. (See Fig.
.
.7" / / -4-> *^i i^t &>4 n/ &>*-*
*nr***>*
'
252 REFRACTION. AND HOW TO REFRACT.
Stenopeic Slit. O. D., axis 75 with +0.25 S., V. = ~
and at axis 165 with +2.508., V. = ~. O. S., axis 105
with +0.25 S., V. = ; and at axis 15 with +2.50 S.,
V - --^
" vi '
Rctinoscope at I meter shows : O. D., at axis 75 degrees
-(-1.25 S., and axis 165 degrees, +4.25 S. O. S., at axis
105 degrees, +1.25 S., and at axis 15 degrees, -f~4- 2 5 S.
At Trial-case.
O. D. +0.25 sph. O -f 3.00 cyl. axis 75 degrees, V. = _Y_I -f-
O. S. +0.25 sph. O +3.00 cyl. axis 105 degrees, V. = . vt . -\-.
April 6th : Same result as April 5th. Add., 20 degrees.
Abd., 6 degrees. Esophoria, 2 degrees at 6 meters.
For Miss ROBINSON.
R. O. D. -(-3.00 cyl. axis 75 degrees.
O. S. -(-3.00 cyl. axis 105 degrees.
Sic. For constant use.
>
April 7th : Glasses neutralize ; are centered and accu-
rately adjusted.
April 1 6th : Perfectly comfortable. Free from headache
since the first day she used the " drops." Add., 20 de-
grees. Abd., 6 degrees. Esophoria at 6 meters, 2 de-
grees ; and at 13 inches, o. Near point, 12 cm.
Considerations. Apparently, the static refraction in this
case would indicate compound hyperopic astigmatism ; but
~*\ when 0.25 is deducted to produce parallel rays, then the
prescription becomes one for simple hyperopic astigmatism.
General Rule for Prescribing Cylinders. Order the
cylinder just as found, without any change in its axis or
strength.
The vision in each eye at the different visits, before
lenses were placed in front of the eyes, was always uncertain,
APPLIED REFRACTION. 253
the patient miscalling certain letters, and hence it is that
the vision is recorded with as many question marks as
there are mistakes in the line of letters /'. e., ?? ?.
IX
In taking the vision at the first visit, the patient could
read part of ^^ if not closely watched. In other words,
if she was permitted to tilt her head to one side and nar-
row the palpebral fissure by squinting the lids together, and
making, as it were, a stenopeic slit out of her eyelids, the
vision was improved. But when told to open the eye wide,
she could read only part of . This is explained by
the fact that when the lids were drawn together, the verti-
cal meridian was partly excluded, and then, by accommo-
dating, the vision was improved through the horizontal
meridian. Astigmatic eyes often take advantage of this
condition when the nature of the astigmatism is suitable,
but only at the expense of frowning and straining the
accommodation.
It will also be noticed that the stenopeic slit was not used
as a test at the first visit. This is also explained for the
same reason that the patient would draw his lids together
and therefore annul the virtue of this test. The stenopeic
slit is to be used in these cases only when the ciliary muscle
is at rest.
SUMMARY. When the hyoscyamin has passed out of
the eyes and the glasses are in position, the near point be-
comes 12 cm., which is quite consistent with the patient's
age. < Before using drops, the near point with the eyes wide
open was only 18 cm., representing about 5.50 D. ; and
this, subtracted from the amplitude for twenty -four years of
age, would leave 3 D. for distance uncorrected./
As every 6 D. cylinder represents I mm. of lengthening
or shortening of the radius of curvature of the cornea,
then this patient, taking a -f- 3 cyl. at axis 7 5 in the right
254 REFRACTION AND HOW TO REFRACT.
eye, has the 165 degree meridian l / 2 of a mm. too long as
compared with the 75 meridian, which is supposed to have
the normal radius of 7.8 mm.
The same is true of the meridians of the left eye.
CASE IV. Simple Myopic Astigmatism. Not a com-
mon condition. About 1.5 per cent, of all eves have this
form of refraction. ^^4^-
April 10. Miss JENKS. Age, eighteen years. Single.
O. D. V. = ~ ? ? ? - P- P- 9 on-
VI
O. S. V. = ^y-j ? ? ?. p. p. 9 cm.
Add., 20 degrees. Abd., 5 degrees. Esophoria at 6 meters = 3 degrees ;
and I degree at 13 inches.
Pointed Line Test. Each eye selects the series of points
from XII to VI as coalescing and appearing as dark
lines.
Cobalt-blue Glass. O. D. and O. S. each show blue
above and below the red. (See Figs. 1 29 and 1 30.)
Stenopeic Slit. Axis 90 degrees V. = ~~ ; axis 1 80 V.
VI
VI*
History. Never had good distant vision. Has occa-
sional headaches. Comes to find out if glasses will im-
prove vision.
S. P. Face symmetric. Irises dark in color. Pupils
apparently round, 4 mm. in diameter. Eyes out under
cover.
Ophthalmometer. Each eye 2 D., axis 90.
Ophthalmoscope. O. D., media clear. Disc large and
round, with underlying conus out. No physiologic cupping.
Choroidal circulation everywhere recognized, characteristic
a stretching eyeball. Horizontal vessels seen with
2 S. ; vertical vessels seen without any correcting lens.
O. S., same general conditions as in O. D.
APPLIED REFRACTION.
Manifest Refraction.
O. D., -2.50 cyl. axis 180 = ^| .
O. S., 2.50 cyl. axis 180 = ^ [ .
R . Atropin and dark glasses for refraction.
April 1 2th : Six meters from test-card and point of light.
Retinoscopy at one meter. Vertical meridian +1.25 S.
Horizontal meridian i.oo S.
Stenopdc slit at axis 1 80 with -f- o. 2 5 == x ' ; at axis 90 =
so, >:;.
Cobalt-blue glass and pointed line test show same results
as at first visit :
VI
O. D. V. = ~y ? ? ?.
O. S. V. = -^V ? ? ? -
At Trial -case.
O. D., -j-o.25sph. O 2.00 cyl. axis 180 degrees = V1 .
VI
O. S., -f 0.25 sph. ^ 2.00 cyl. axis iSodegrees = -=77-.
April 1 3th: Same results as yesterday. Add. = 20 ;
Abd. = 6. Esophoria, 2 degrees.
For Miss JENKS.
R. O. D., 2.00 cyl. axis 180
O. S., 2.00 cyl. axis 180.
April 1 4th: Glasses neutralize. Centered and properly
adjusted.
April 28th : Comfortable. Enjoys good distant vision.
Near point, each eye, 9 cm.
Considerations. Apparently, the static refraction would
indicate mixed astigmatism, but when -(-0.25 is deducted
to produce parallel rays, the prescription resolves itself into
one for simple myopic astigmatism.
The general rule for ordering cylinders is the same in
256 REFRACTION AND HOW TO REFRACT.
myopia as in hyperopia /. e., no change in the strength or
in the axis of the cylinder.
A cycloplegic is always necessary in such cases, as is
shown by the different results obtained by the manifest and
stenopeic slit.
The vision was always uncertain before lenses were placed
before the eyes, as is indicated by the question marks.
At the first visit the vision was taken with the eyes -wide
open. If allowed to narrow the palpebral fissure by squint-
ing the eyelids together and making a stenopeic slit out of
them, the patient could read a part of y^g. When the
lids were thus drawn together, the myopic vertical meridian
was excluded in part and the horizontal meridian was util-
ized. cThe stenopeic slit was of some assistance before
drops were used, as the accommodation could not be
exerted, as in the case of the hyperopeJ
SUMMARY. After recovery from the cycloplegic, small
type was clear at 9 cm., which was in keeping with the
patient's age. The near point before " drops " were used
was also 9 cm., but not constant, nor was the type clear.
The 2. oo cyl. at axis 180 represents ^ of a mm. of
shortening in the vertical radius of curvature as compared
with the normal radius of 7.8 mm. in the horizontal.
The astigmatism is regular, symmetric, with the rule.
CASE V. Compound Hyperopic Astigmatism. The
most common form of all refraction. It is a combination
of simple hyperopia with simple hyperopic astigmatism.
About 44 per cent, of all eyes have this form of refraction.
April 1 2th. MR. COMMON. Age, twenty-eight. Married. Bookkeeper.
VI
O. D. V. =-^-??. p. p. = type 0.75 D. = 22 cm.
VI
O. S. V. = "x~??- P- P- = type 0.75 D. = 22 cm.
Add., 23 degrees. Abd,, 7 degrees. Esophoria, 2 degrees; at 13
inches, o.
APPLIED REFRACTION. 257
Astigmatic Clock-dial. O. D. and O. S. each selects
darkest series of lines from IX to III.
Phicido's disc shows each corneal image as a horizontal
oval. Schemer's test shows two lights, separated in all
meridians : the vertical have the least separation and the
horizontal the most.
Ophthalmometer. 2.25 D. with axis 90 in each eye.
History. Family physician has tried in vain to stop the
headaches, which he said were from biliousness. Headache
develops as soon as the patient commences to use his eyes,
and gets worse toward noon ; and from that time on, during
the rest of the day, he is cross and irritable, and feels dizzy.
Unable to read in the evenings as he did a few years
ago. Is wearing a pair of " rest " glasses, which he received
from an optician ; they were of some benefit for a very short
time.
5. P. Lid margins red and excoriated. Many fine scales
(looking like dandruff) adhering to the cilia. Irises gray
in color. Pupils round, 3 mm. Eyes in, under cover.
Ophthalmoscope. O. D. and O. S. each medium clear.
Disc small, vertically oval. Shallow physiologic cup.
Venous pulsation on disc. Narrow conus to temporal side.
Nerve-head prominent and edges somewhat hazy. No path-
ologic conditions recognized. Vertical vessels best seen
with -f- 3.00 and horizontal with -f-i.oo.
U . Duboisin and dark glasses for refraction.
April 1 4th : Six meters from test-card and point of light.
VI
O. D. V. = j*x ? ? ? -
O. S. V. = ^???.
Retinoscopy develops point of reversal at one meter in
each eye; vertical meridian with +2.25 S., and horizontal
meridian with +4.00 S.
2$S REFRACTION AND HOW TO REFRACT.
Stenopeic slit axis 90 degrees with -}- 1.25 = -^j- ; at axis
1 80 degrees with -f 3-OO - v ,-j-.
Cobalt-blue glass, blue center and red all round, more
conspicuous on the right and left sides. (See Figs. 131
and 132.)
At Trial-case.
O. D., +1.25 sph. O -fi-75 cyl. axis 90 = -TT?-.
O. S., -)-l.25 sph. O -(-1.75 cyl. axis 90 = -J-.
April 1 5th : Same results as April I4th. Add., 22. Abd.,
7. Esophoria, I degree.
For MR. COMMON :
R. O. D., -f i.oo sph. O 4 I -75 cyl. axis 90 degrees.
O. S., 4-1.00 sph. O +1.75 cyl. axis 90 "
SlG. For constant use.
April i /th : Glasses neutralize ; are centered and ad-
justed.
April 24th : Has been perfectly free from headaches
ever since getting glasses. Never realized what a blessing
glasses could be. Near point with each eye is now 14 cm.
Considerations. The prescription for glasses was the
same as the static refraction, with the exception of the re-
duction in the strength of the sphere. No change in the
cylinder. A cycloplegic as a means of obtaining a prompt,
correct, and satisfactory result in such cases can not be dis-
pensed with.
The decided change in the vision before and with the
cycloplegic is quite diagnostic of compound hyperopic as-
jtigmatism. When the cylinder and sphere are of any con-
siderable strength, the patient can often overcome (faculta-
tive hyperopia) the spheric but not the cylindric correction.
SUMMARY. After recovery from the cycloplegic the
APPLIED REFRACTION. 259
small type becomes clear at about 13 cm., which is the
near point consistent with the patient's age. The far point
before using drops was really two points, both negative
that of the vertical meridian being about I meter, and the
horizontal meridian about ^ of a meter back of the retina.
The form of the astigmatism is regular, symmetric, and
with the rule.
CASE VI. Compound Myopic Astigmatism. A com-
bination of simple myopia with simple myopic astigmatism.
This is the usual form of refraction in myopic eyes. About 8
per cent, of all eyes have compound myopia. The writer's
experience is such that he never refracts a case of myopia
without searching carefully for a cylinder in combination
witli the sphere.
April 1 2th. MRS. USUAL. Age, thirty years. Married. Housewife.
VI
O. D. V., --jj- ? ? ?, type 0.50 = 7 to 14 cm.
VI
O. S. V., -- ? ? ?, type 0.50 = 7 to 14 cm.
Add., 1 6 degrees. Abd., 6 degrees. Exophoria at 6 meters, 2 degrees.
History. Suffers from ocular pains, as if knife-points
were sticking into the eyes, which come on as soon as near-
work is attempted or continued. Says that she constantly
sees fine dust particles floating before her vision. Has been
wearing glasses from an optician ( 3 sph.). Has all the
symptoms of near-sightedness. Family history of father
and two sisters wearing glasses for "near sight."
S. P. Face symmetric. Eyeballs prominent. Irises
dark in color. Pupils small (for a myope) and round,
3 mm. Eyes markedly out under cover.
Ophthalmoscope. O. D., many fine floating vitreous
opacities. Nerve large and round, with broad underlying
conus down and out. Choroidal vessels seen throughout
eye -ground. Vessels at about axis 1 20 degrees best seen
26O REFRACTION AND HOW TO REFRACT.
with 2, and vessels at axis 30 degrees best seen with 3.
O. S., same general conditions as in O. D., except the prin-
cipal meridians are about 60 degrees and 1 50 degrees.
Indirect method shows a vertically oval nerve, with the
conus to the nasal side of the aerial image (as the eye-
ground and nerve-head have undergone vertical and lateral
inversion). Withdrawing the lens, the nerve grows larger
in all meridians, but more so in the vertical.
Cobalt-blue glass shows O. D. red center, blue all around,
more pronounced on the sides in the 120 meridian. O. S.
shows red center, blue all around, more pronounced on the
sides in meridian of 60 degrees. (See Figs. 133 and 134.)
Stenopeic slit before O. D. at axes 1 20 V. ^^^jFjnT ? ? ? > a t
axis 30 V. =" xx ~ ? ? O. S., the same with axes 60 degrees
and 150 degrees.
Astigmatic Chart. O. D. selects the lines from V to XI
as darkest. O. S. selects the lines from VII to I as
darkest.
Ophthalmometer. O. D., I D., axis 35 degrees. O. S.,
I D., axis 145 degrees.
Manifest.
VI
O. D., 2.5osph. C 0.75, cyl. axis 35 = - vn ? ? ? ?.
O. S., 2.50 sph. O 0.75 cyl. axis 145 = - VI[ ? ? ? ?.
1 . Atropin and dark glasses for rest and refraction.
April 1 3th: At six meters from test -card and point of
light :
O.D.V. =1 -?. .S.V. =] g-?.
O. D., 2.00, sph. O i. oo cyl. axis 30 ss -yj- ? ? ?.
VI
O. S., 2.00 sph. O I. oo cyl. axis 150 = - v j- ? ? ?.
Retinoscope confirms this trial-case result. Retinoscope
also shows a general cloudiness of the media (vitreous),
APPLIKI) REFRACTION. 26 1
which, of course, will account in part for the vision not
VI
being - v - in each eye with correcting glasses.
April 1 4th : Same result as on the ijth.
For MRS. USUAL.
R. O. D., 2.00 sph. O i.oo cyl. axis 30 degrees.
O. S., 2.00 sph. O i.oo cyl. axis 150 "
SlG. For distance, as directed.
April i 5th:
K . Tonics. Rest of eyes. Attention to general health.
April i /th : Glasses neutralize ; are centered and ad-
justed.
April 2pth : Add., 16. Abd., 6. Exophoria, 2.
Vision in each eye read slowly.
Considerations. The static refraction was ordered just
as found, and no deduction whatever was made in the
sphere. The rule is to prescribe in the same way as in
/simple myopia. But all cases of myopia can not and must
not be prescribed for by rule. Each case of myopia is a
law unto itself. See description under General Considera-
tions, page 238, also pages 227, 228, and 229.
SUMMARY. The near point is now 14 cm., which is
perfectly consistent with the patient's age. Fourteen centi-
meters represents an accommodative power of 7 D., and
this was the difference between the near and far points
before the drops were used. The astigmatism is regular,
symmetric, and with the rule. Vision is not brought up to
normal on account of the changes in the vitreous and dis-
turbed eye -ground, due, no doubt, to the want of a proper
correction the cylinder. The choroid and retina are both
in a stretching condition.
CASK VII. Mixed Astigmatism. Not an uncommon
condition. About 6^ per cent, of all eyes have this form
262 REFRACTION AND HOW TO REFRACT.
of refraction. This is a combination of the simple hyper-
opic and simple myopic astigmatisms, with their axes oppo-
site or at- right angles to each other, as a rule.
April 8th. MR. CROOK. Ag, twenty-one years. Single. Clerk.
VI
O. D. V. = -J(L. p. p. = 14 cm. with type 0.75 D.
VI
O. S. V. = -j^. p. p. = 14 cm. with type 0.75 D.
Add., 20. Abd., to.
History of poor sight all his life, but thinks it was better
as a boy. Has frequent frontotemporal headaches, which
are worse after using eyes at any prolonged near-work.
Father has good sight, but his mother and her family have
all been near-sighted ; has one aunt that developed cata-
racts. Has been to several "stores," but could not get
fitted with glasses that would improve his vision.
5. P. Face broad and symmetric. Long interpupillary
distance. Irises dark in color. Pupils large, 5 mm. ;
round. Eyes out under cover.
Ophthalmoscope. O. D., media clear. Disc vertically
oval, axis 105. Macular region shows changes. Vessels
at 105 best seen with 4-2, and at right angles with 2.
O. S., same conditions found, except that the meridians are
at 75 degrees and 165 degrees.
OphtJialmometer. O. D., 4 D. axis 100. O. S., 4 D.
axis 75.
Cobalt-blue Glass. O. D. violet center, blue above and
below in meridian of 105 and red at the sides in meridian
of 15. O. S., violet center, blue above and below in
meridian of 75, and red on sides at axis 165. (See Figs.
135 and 136.)
Stenopeic Slit. O. D. axis 15 with +2 sph., V.=
VI
VI
IX
at axis 105 with 2 sph., V. = . O. S. axis 165 with
4-2 sph., V.=r -y^- ; at axis 75 with 2 sph., V.= .
APPLIED REFRACTION. 263
Indirect Method. Each eye shows a lengthening of
the vertical meridian as the condensing lens is withdrawn
from the eye, and at the same time the horizontal meridian
grows narrower. As the lens is advanced toward the eye
the vertical meridian grows shorter and the horizontal
meridian grows broader.
,/ The astigmatic chart does not show any difference in the
/shading of the lines ; they all appear about the same.
Retinoscope at 1 meter distance shows myopia in the verti-
cal meridian and hyperopia in the horizontal.
R . Atropin and dark glasses for refraction.
April loth : At six meters from test-card and point of
light. O. D. and O. S. V. = ^.
LA
Cobalt-blue glass shows the same as at first visit.
Retinoscope at the distance of one meter shows point of
reversal in O. D. at axis 105 degrees with 1.500., and
axis 15 with +3 D. O. S., axis 75 with 1.50 D., and
axis 165 with +3 D.
Stcnopeic Slit. O. D., axis 15 with -\-2 sph., V. =
j N x ! ; axis 105 with 2.50. sph., V. = -^. O. S., axis
165 with +2 sph., V. = ^ ; at axis 75 with 2.50 sph.,
y - .-.XL
' ix ;
At Trial-case. O. D, 2.50 cyl. axis 15 degrees O
-f-2 cyl. axis 105 degrees, V. = [ l s - O. S., 2.50 cyl.
axis 165 O +2 cyl. axis 75, V. =
VIISS*
Or,
VI
O. D., 2.50 sph. O 4 4- 50 cyl. axis 105, V. ylfsg.
VI
O. S., 2.50 sph. O -(-4-5 cyl. axis 75, V. ^ ss> .
Or,
VI
O. D. -f- 2 sph. O 4.50 cyl. axis 15 degrees, V. = yjjgg -f-
VI
O, S. -j-2 sph. O 4.50 cyl. axis 165, V. = V1ISS -\-.
264 REFRACTION AND HOW TO REFRACT.
April nth: Same results as April loth. Add., 2O.
Abd., 8.
For MR. CROOK.
B. O. D. -\-2 sph. O 4.50 cyl. axis 15 degrees.
O. S. -f-2 sph. O 4.50 cyl. axis 165 "
SlG. For constant use.
April 1 2th : Glasses neutralize ; are centered and adjusted.
April 26th: Near point 10 cm., which is consistent with
age of patient.
Considerations. The ophthalmoscope, retinoscope,
cobalt-blue glass, indirect method, and stenopeic slit were
direct guides to the character of the refractive error.
Emphasis is placed upon these different methods, as so many
beginners in ophthalmology have a fear or dread of the re-
sult in refracting cases of mixed astigmatism.
The stenopeic slit shows a difference of 4.50 in the two
principal meridians ; bearing this fact in mind, if a 4.50
cylinder at axis 1 5 be placed before the right eye, then all
meridians would be made equally hyperopic 2 D. Com-
bining -f- 2 sph. with the 4. 50 cylinder at axis 1 5 in the
right eye or at axis 165 in the left, the refraction would be
corrected.
Or, if a +4.50 cylinder at axis 105 be placed before the
right eye, then all meridians would be made myopic
2.50 D. Combining a 2.50 sph. with this +4.50 cylin-
der at axis 105 in the right eye or at axis 75 in the left eye,
the refraction would be corrected.
For a further consideration of combination of lenses see
page 51.
The rule for ordering cylinders is the same in mixed
astigmatism as in other cylindric corrections without
change.
SUMMARY. The character of the astigmatism is regu-
APPLIED REFRACTION. 265
lar, symmetric, and with the rule. The near point returns
to the normal for the age. ,. Eyes with such errors do not,
VI
as a rule, obtain a visual acuity of , for the reason that
changes have taken place in the eye-ground, especially at
the macula. '
CASE VIII. Irregular Astigmatism.
April ad. MARY SMILES. Age, ten years. Scholar.
VI
O. D. V. = xxx slowly. No p. p. obtained.
VI
O. S. V. = LX~. No p. p. obtained.
History of poor sight ever since an attack of measles
when two years of age, at which time was kept in a dark
room for six weeks. Eyes were never strong afterward ;
always very sensitive to light. Child was sent home from
school with a note from the teacher : " Mary is near-
sighted and should see a doctor."
6". P. Eyelids appear normal. Excessive epiphora.
Corneas nebulated, especially O. S., which has a decided
leukoma at the pole. Anterior chambers of normal depth.
Pupils 3 mm., round. Corneal reflex very irregular.
Ophthalmoscope. No view obtained of the eye-ground
through the small pupils on account of corneal opacities.
Homatropin mydriasis shows O. D. cornea faintly nebu-
lated in scattered areas ; rest of media clear. Nerve small
and round. Vessels at axis 35 degrees best seen with -f-2
D. O. S., there is a 3 mm. area of opacity at the pole of the
cornea ; no clear view of the eye-ground. Indirect method
shows a small nerve and refraction hyperopic.
R . Atropin and dark glasses for refraction.
April 22d : Retinoscope at I meter shows band of light
at axis 35, indicating hyperopia. Other meridians very
irregular. O. S., nothing definite made out.
23
266 REFRACTION AND HOW TO REFRACT.
Placido's disc shows irregular, distorted circles.
With Put-hole Disc.Q. D. V. = Y v '-. O. S. V. = ^-.
A A 1 , \
With Stcnopeic Slit. Axis 45 degrees before O. D. and
with +2.25 S., V. = ~ ? ?. O. S., can not improve
vision with any glass.
At Trial-case. O. D., +2.00 cyl. axis 145 = ^??.
O. S., no glass accepted.
April 2$d : At trial-case O. D., +1.75 cyl. axis 35 = ~.
For MARY SMILES.
R. O. D., 0.25 sph. O +1-75 cyl. axis 35 degrees.
O O. S., plane glass.
SIG. Constant ^use.
Considerations. This case shows the advantage of the
stenopeic slit and the use of the pin-hole disc. The near
point could not be obtained on account of the poor visual
qualities and the child's inability to appreciate what was
wanted.
CASE IX. Tonic Cramp or Spasm of the Accommo-
dation.
MRS. L. Age, twenty-four years.
VI
O. D. V. = xxv ??. p. p. =9 cm. (?) Add., 24 degrees. Abd., 6
degrees. Esophoria, 4 degrees.
VI
O. S. V. = "xxv ^ *** P- P- 9 cm - (**) ^ v e rt i ca l deviation.
History of having had glasses changed on three different
occasions during the past year. Drops were used each
time, and the three prescriptions were all different. Glasses
were always satisfactory for the first w r eek, but after this
time she was always able to see better at a distance with-
out them. Has pains in her eyes and all over the head
whenever she attempts to use the eyes with or without any
glasses. Headaches nearly set her " wild" if she tries to
AITI.IKI) KK1 KACT1ON. 267
concentrate her vision on a distant or near object. Has not
been able to read or write or sew for the past two years.
Has been under the care of the gynecologist and neurologist,
and they each pronounce her physical condition as normal.
The neurologist suggests a diagnosis of " hysteria."
Patient sleeps well and has a good appetite, but will suffer
from nausea and vomiting if she uses her eyes for any length
of time. Patient has been married five years. Has one living
child. No miscarriages. Is apparently in the very best
of health, and is provoked with her apparent good health
as not being consistent with her suffering, and hence she
does not receive any sympathy from her family or her
friends.
5. P. No external manifestations of any ocular irregu-
larity.
Manifest Refraction.
VI
O. D., 0.50 S. O +1.00 cyl. axis 90 degrees = -yj-.
O. S., the same as O. D.
Ophthalmoscope. O. D. and O. S., media clear. Discs
vertically oval, eye -grounds "woolly." Accommodation
very active. Shot silk retina. Refraction is compound
hyperopic astigmatism.
Cobalt-blue glass shows a red center and broad blue halo.
(Patient is certainly accommodating.)
& . Atropin and dark glasses for refraction.
Static Refraction.
VI
O. D. +1.50 S. = +1.75 cyl. axis 90 degrees = -y-.
O. S. +1.50 S. = -j-l.75 cyl. axis 90 degrees = -XL.
Patient states that her " pains and headaches all disap-
peared after using the drops for the third time."
268 REFRACTION AND HOW TO REFRACT.
Refraction repeated on three different occasions, and the
following prescription given :
For MRS. L.
R. O. D., +1.25 S. O +1-75 cyl. axis 90 degrees.
O. S., +1.25 S. O +1-75 cyl. axis 90 degrees.
Glasses properly centered, and accurately adjusted.
r After ten days patient returns with the statement that
her pains and aches have recurred as before, and that she
can see better at a distance without her glasses. With
correction, each eye sees ^~, and with both eyes can see
Add., 20 degrees, and abd., 8 degrees. No verti-
cal deviation. Has 3 of esophoria at 6 meters.
Atropin -$ of a grain to the ounce.
SlG. To use one drop in each eye each morning and noon.
To wear a pair of dark glasses with her prescription glasses
when exposed to any bright light. N^LJxL-attempt ..any
near-work. This treatment was continued, off and on, for
six months. Patient was always free from ocular pain and
headache as long as the atropin was being used, but as
soon as the ciliary muscle commenced to contract, then the
pains would return with all their former severity. This
patient eventually recovered by using her distant correc-
tion with a pair of plus 2 spheres as hook -fronts for any
near-work.
CASE X. Exophoria.
Miss. V. B. D. Age, twenty-two years.
VI
O. D. V. = -yj-. p. p. = 0.50 D., type at n cm.
VI
O. S. V. = -yy. p. p. = 0.50 D., type at II cm.
Add. and abd., 12 degrees. Exophoria = 4.
History of seeing double several times a day. Friends
and members of her family have told her she was " squint-
'
APPL1KD K INFRACTION. 269
ing." Always returns home with a severe occipital head-
ache after going shopping or to any place of amusement.
Has headache when using her eyes, but it soon passes
away after resting the eyes.
S. P. Eyes markedly out under cover. Irises react
promptly to light, accommodation, and convergence.
Fixation test shows the right eye divergent.
Ophthalmoscope. O. D. and O. S. No apparent changes,
and refraction almost emmctropic ; some small amount of
hyperopia and astigmatism.
R Atropin and dark glasses for rest and careful refraction.
Static refraction, after several repetitions, O. D. and O.
S., +0.50 S. Cl 4 0.37 cyl. axis 90 degrees = -~. And
this is ordered, less 0.25.
With this correction carefully centered, add. =14 degrees
and abd. = 1 2 degrees, with 3 of exophoria at 6 meters and
7 degrees of exophoria at 1 3 inches. This patient was given
prism exercises for more than two months, and, finally,
after the adduction reached 30 degrees and abduction was
10 degrees and 3 degrees of esophoria were obtained, the
prism exercises were stopped, and patient told to report
promptly if any disconifort arose at any time. To wear . / '^IjJL
her glasses constantly, T&I 1
H
MR. ALBERT S. Age, twenty-nine years. In general business.
VI
O. D. V. = LX". p. p. type 0.50 D. = 20 cm.
VI
O. S. V. = xxx . p. p. type 0.75 D. = 30 cm.
Add., 10 degrees. Abd., 6 degrees. Left Hy., 2 degrees.
History. Has had three pairs of glasses ordered, with
" drops," during the past eighteen months. Has never
had any but the very slightest relief from ocular pains and
2/O REFRACTION AND HOW TO REFRACT.
frontal headaches, which have been almost constant for the
past four years or more. On account of the ocular dis-
comfort and headaches, the patient has given up all at-
tempts to read for more than fifteen minutes at a time
Patient states that if he uses his eyes for more than thu
length of time they become bloodshot and very tendci
to the touch. General health of patient is excellent ; ha. 1
a good appetite and sleeps well. Docs not use tobacco 01
liquor of any kind.
5. P. Face symmetric. Nose very prominent. Inter
pupillary distance, 62 mm.
OphtJialmoscope. O. D., nerve-head over capillary. No
swollen. Accommodation very active. Eye-ground "fluffy.'
Refraction is that of compound hyperopia. O. S., sam<
general conditions, but the nerve is vertically oval and th<
refraction is compound hyperopic astigmatism.
R . Atropin and dark glasses for refraction.
Static Refraction.
O. D., 4-2.00 S. O 4 I -oo cyl. axis 75 degrees ^.j ~\ ^
O. S., +1.25 S. C +3.00 cyl. axis 105 degrees =-^,| -??? j h >l n>r -
K. O. D., -fl-75 S. O +l.oo cyl. axis 75 degrees O ^ A Dase U P-
O. S., -f l-ooS. O +3-OOcyl. axis 105 degrees O ^/^ basedowi
SlG. For constant use.
This patient was rrot made comfortable until he was give
five-grain doses of the bromid of potash three times a da
for four weeks. Is now able to use his eyes without th
least discomfort.
,
x
jf ^
MM.
y
1
I /
4-js-
CHAPTER XL
PRESBYOPIA. APHAKIA. ANISOMETROPIA.
SPECTACLES.
Presbyopia. The word presbyopia (from the Greek,
TT/^'VMT, "old" ; "V', "eye") literally means old sight, and
patients at the age of forty-five or more years are univer-
sally recognized as presbyopes, and the condition of their
eyes as presbyopia There is no exact age limit as to when
presbyopia shall begin, the advent of presbyopia being con-
trolled by the character of the ametropia and physical con-
dition of the eyes themselves. Presbyopia may be described
in several different ways, according to the cause /. e.,
1. Old sight.
2. The condition of the eyes in which the punctum proxi-
mum has receded to such a distance that near vision (close
work) is impossible without the aid of convex lenses.
3. The condition of the eye in which the lens fibers have
become more or less sclerotic, and, as a consequence, the
lens loses some of its inherent quality of becoming more
convex during contraction of the ciliary muscle.
4. The condition of the eye in which the power of the
ciliary muscle has become weakened.
5. The condition of the eye in which the power of ac-
commodation is diminished at the same time that the lens
fibers become sclerotic.
6. The condition of the eye in which two different refrac-
tions (not necessarily two pairs of glasses) are required, one
for distance and one for near vision.
271
272
REFRACTION AND HOW TO REFRACT.
7. The condition of the eye in which one pair of glasses
will not answer for distant and also for near vision.
8. Presbyopia may be described as the condition in which
nature has instilled a slowly acting but permanent cyclo-
plegic (the term cycloplegic is used here in a general sense).
Causes of Presbyopia. I. Age. It is a well-estab-
lished fact that in childhood the center of the lens begins
to harden, becomes sclerotic or sclerosed, to form a nucleus ;
and this process continuing, eventuates in complete sclerosis
at sixty or seventy-five years. The term sclerosis must not
be confounded with opacity.
2. Disease. Ordinarily, presbyopia, as applied to the
lens, should be recognized as a physiologic process, as a
penalty for growing old, though it is a condition which
may be hastened by disease. Any disease, therefore, which
will cause the nutrition of the lens to suffer must event-
ually interfere with its ability to become more convex during
accommodation. The most common ailments that tend to
this result are rheumatism, gout, Bright's disease, diabetes,
lithiasis, la grippe, etc. Any disease which will weaken the
ciliary muscle will produce presbyopic symptoms.
Presbyopic Near Points. The near point and power of
accommodation in a healthy emmetropic eye, or a healthy
eye made emmetropic by the addition of correcting lenses,
is as follows for certain ages :
POWER OF
AGE. H NEAR POINT.
ACCOMMODATION.
40 years, . . j^j . . 22cm. 4
5
diopters.
45 '
. . . u'O . 28
ri
3
50
5 '
. . . / (o. . 40 '
2
50
55 '
...**. 55 '
I
75
r 2.00
60 '
. . . .t? i loo
I
00
65 '
. . . JFp 1 . 133 '
75
70 '
. . . |*0 ( . 400
o
25
7C '
oo '
oo
PRESBYOPIA. 2/3
Ordinarily, the average adult holds a newspaper or book
at about 33 cm. (13 inches) from his eyes when reading ;
and if he is forty years of age and emmetropic, or is made
emmetropic with glasses, he would be using 3 D. of his
normal 4.50 of accommodation, which would leave a reserve
power of 1.50 D.; and in this condition, other things being
equal, he can maintain a reading distance with comfort. In
fact, he could, by using all of his 4.50 D. of accommoda-
tion, see objects as close as 22 cm., but not for any great
length of time, as the ciliary muscle would soon relax.
This same patient at forty-five or forty -six years of age will
have lost i.oo or 1.50 D. of his accommodation, and now
has only about 3 or 3.50 left ; and if he uses all of it at a
working distance of 33 cm., the ciliary muscle soon yields.
In fact, the ciliary muscle can not be held in such a state of
tension without causing all sorts of pains and aches and
reflex disturbances ; and the ciliary effort relaxing suddenly,
the near vision blurs, and the work or reading or sewing
must be put at a greater distance to obtain relief, or else
the effort must be abandoned.
Symptoms of Presbyopia. The principal symptom is
that which indicates a recession of the punctum proximum ;
the patient stating that there is an inability to maintain
the former reading, writing, or sewing distance, and that all
near-work must be held at a greater distance than formerly.
Symptoms of accommodative strain may be present if the
patient endeavors to force the accommodation to its
maximum.
Diagnosis. The age of the patient and the history of
having to hold reading matter at an uncomfortable distance ;
or a history of good distant vision and an inability to retain
clear near vision small objects, to be seen, must be held
far away or " at arm's length."
274
REFRACTION AND HOW TO REFRACT.
^^
ft
Correction of Presbyopia. The presbyopic state repre-
sents a class of patients for whom glasses may be pre-
scribed by the manifest refraction, although there are
exceptional cases in which a quick cycloplegic will be nec-
essary when an amount of astigmatism or cylinder axis is
uncertain.
r a working, reading, writing, or sewing distance of 33
cm. (13 inches), the writer makes it a rule to add to the dis-
tance correction at forty- five years of age a + I sphere ; at
fifty years of age, a -f- 2 sphere ; at fifty-five years of age,
a +2.50 sphere ; and for sixty or more years, a +3 sphere.
The following table for emmetropic eyes shows these addi-
tions for the different years, and also the near and far points
/ /A
feTJ
tt*K/
with these additions as well as the range of accommo-
dation or "play " between the near and far points. It will
be observed that the range of 78 cm. at forty-five years
rapidly diminishes in the succeeding years, until at sixty
there is a play of only about 3 inches, and at seventy the
range is practically gone. ,
Iioo cm.
50
40
33
: / 33
/ 33
/ / 33
'S.-f-f C4*S A+~a** ^
a patient is fifty years of age does not signify
at he will be able to read at 33 cm. with a pair of -\-2
spheres, or because he is sixty years of age that he can use
his eyes at 33 cm. comfortably with a pair of +3 spheres j
on the contrary, this rule that the writer has given applie
only to cases of emmetropia. It often happens that pres-
byopic patients state that they do not want glasses for dis- I ~
.
YEARS.
ADD.
NEAR POINT.
45
+ 1,00
22 cm.
So
^^^+2.00
22 "
55
+ 2.50
23 "
60
-1-3.00
25 "
65
-f3-oo
27 "
fl:LV
> 7o
+3.00
30 "
,. JW
2. I)/)
75
-h3-oo
33 "
IMA* f H rrfa
M<^A^rf *
lA/fl
NT. RANGE
i. 78 en
;. ^,
28
17 '
8 '
6
3 '
o '
/
.0.?
/",
/~~
PRESBYOPIA. 275
tance ; that they do not need them ; that all they wish is a
pair of glasses to use at near-work, reading, ete. When the
vision is taken in such cases, it may be found to be or
vi
VI
approximating .- ; but the young ophthalmologist must
not be thrown off his guard by this record, as it has already
VI
been stated that a vision of VI does not by any manner
of means prove the existence of emmetropia. Let the sur-
geon make it a constant rule in every case of presbyopia to
always carefully estimate the amount of the distance ametropia
^/^ first, no matter how ^veak or what its form (sphere or cylinder) ;
and to the result thus obtained, add the plus sphere which will
be required for the ivorking distance or point at which tJie
patient wishes to see clearly. /&t-*~^-yiS '?
Illustrative Cases. CASK I. Accepts +0.50 sph. for
distance. At forty-five years this case would require
-f 1. 50 sph. for reading at a distance of 33 cm.; at fifty years,
+ 2.50 sph.; at fifty-five years, +3 sph.; and at sixty or
more years, +3.50 sph. Only one pair of glasses is neces-
CASE II. Accepts -(-2 sph. for distance; at forty-five
years these eyes would require +3 sph.; at fifty years they
would require -[-4.50 sph.; and at sixty or more years they
would require -j~5 s P n - Two pairs of glasses would be
indicated in this case.
CASE III. Accepts i.oo sph. for distance ; at forty-
five years this patient could read without any glasses, as
I for distance would be neutralized by the -f I required
for reading. At fifty years, however, the patient would
require a -f I sphere for near, and at fifty -five a -f- 1.50, and
at sixty years a -f-2 sphere. Case II required two correc-
tions, one for distance and one for near ; and the same may
'besaid about Case III ; but in this latter instance there was
a time at forty-five years when there was no necessity for
276
REFRACTION AND HOW TO REFRACT.
glasses for the near-work, as the patient's eyes were in a
suitable condition of refraction to read without them.
CASE IV. Accepts 3 sph. for distance. At forty-five
years would require 2 sph. for reading ; at fifty years
would require I sph. for reading ; and at sixty years
can read without any glasses. Such a patient says he has
gotten his " second sight."
CASE V. Accepts -) 0.50 cylinder axis 1 80 for distance
and requires the usual additional spheres for the increasing
years for his reading distance.
CASE VI. Accepts -fi.oo sph. O -j-i-OO C yl. axis 180
for distance and requires the spheric additions as the years
increase. Two pairs of glasses should be prescribed.
CASE VII. Accepts i cyl. axis 90 for distance, and
requires -(- I cyl. axis 180 to read with at forty-five years
of age ; at fifty years he requires -f- 1 sph. O -f I cyl.
axis 1 80 ; and at sixty years requires +2 sph. O + 1 cyl.
axis 1 80. At forty-five years of age this patient is com-
monly spoken of as having simple myopic astigmatism for
distance (against the rule) and simple hyperopic astigmatism
for near (against the rule also) ; two pairs of glasses are in-
dicated throughout life.
CASE VIII. Accepts i.oo sph. O 1.50 cyl. axis
1 80 for distance, and at forty-five years will need 1.50 cyl.
axis 1 80 for reading ; at fifty years will require -(- i.oo sph.
O 1.50 cyl., axis 180 degrees; at sixty years, +0.50
sph. O -f- 1.50 cyl. axis
Two pairs of glasses
At forty-five years this patient has a compound myopic
correction for distance and simple myopic astigmatism for
near ; at fifty years the correction for near is that of crossed
cylinders (mixed astigmatism) ; and at sixty years the near
correction is that for compound hyperopic astigmatism.
\s *-*.-* / r*
5 QQ degrees. *-fTC -/$"//* ~t~2
rnM$r<~ . *i&u*&i.9T'r*
s should be used rTjrniignmiK lif -^ -
v<
<*V j
PRESBYOPIA. 277
CASE IX. Accepts i.oosph. O -f 2 cyl. axis 90 for dis-
tance (mixed astigmatism) ; at forty-five years, -\-2 cyl. axis
90 is required for reading ; at fifty years, -)-i.oo sph. O
+ 2.00 cyl. axis 90. Tu^cTpairs of glasses are required. At
forty-five years the distance correction is for mixed astig-
matism and the reading correction is for simple hyperopic
astigmatism.
CASE X. Accepts 2.00 cyl. axis 180 for distance ;
at forty-five years requires a mixed astigmatism correction
for near; at fifty years, a simple hyperopic correction *
and a compound hyperopic correction at sixty years. -
In the above illustrative cases the working distance has
been calculated at 33 cm., or 13 inches; but as some
patients use their eyes at a greater or less distance than
this, the additional convex lenses must be calculated accord-
ingly. For instance, the weaver at fifty-five years of
age who requires -f 2 spheres for distance could not see to
weave at 50 cm. ; if -(-2.50 spheres were added to his dis-
tance correction, all he needs is -(-3 for his working dis-
tance. Or the diamond cutter who wishes glasses to see
his work at 8 inches, if he accepted i.oo sph. for distance,
lie would require +2 sph. at forty-five years of age.
In conclusion, there are three facts in the refraction of
presbyopic patients that should receive attention :
i. Many accept a weak plus cylinder ( + 0.50 at axis
1 80) against the rule. This is presumptive evidence that
the astigmatism is acquired, is lenticular, and is due to the
sclerotic changes previously mentioned. The only positive
way to prove this fact is by the retinoscope, and by the ab-
sence of corneal astigmatism with the ophthalmometer.
If the case has been previously refracted by the same sur-
geon, his record will also confirm this extremely interesting
occurrence. According to able authorities, hyperopic eyes
2/8 REFRACTION AND HOW TO REFRACT.
become more hyperopic after the age of seventy years,
and emmetropic eyes may become hyperopic, and myopic
eyes less myopic, from the same sclerotic or shrinking pro-
cess which takes place in all the ocular tissues as a result
of senility. The method of correction by glasses, however,
is just the same, and that is to correct the distant vision
first and then add the near correction.
2. An attack of glaucoma may precipitate presbyopic
symptoms, so that when a presbyopic patient asks for fre-
quent changes in his corrections, this complication should
be borne in mind.
3. The swelling of the lens which occasionally precedes
the formation of some forms of cataract should be remem-
bered when the patient develops symptoms of myopia /. i\,
a reduction in the strength of convex glasses. (_
Aphakia (d, priv. ; <pax6<; " lentil ") literally means an
eye "without a lens." (See Fig. 174.) An eye which has
had its lens dislocated
has been erroneously
spoken of as aphakic.
The absence of the lens
means a total absence
of all accommodation,
FlG I7 no matter what the age
of the patient may be.
Causes. Aphakia may be congenital, but in most cases
is the result of removing the lens by operation.
Diagnosis. Aphakia maybe diagnosed by inspection
i. e., corneal scar, depth of anterior chamber, tremulous
iris, coloboma of the iris, opaque capsule whole or in part,
-> erect corneal image, with absence of lenticular images, and
by the patient's history.
The ametropia of an aphakic eye depends in great part
HETEROMETROPIA.
2/9
upon the previous refractive condition of the eye, and also
upon the kind of operation that was performed for the re-
moval of the lens. It has been calculated that an eye, to
be emmetropic after the removal of its lens, would have to
be myopic at least twelve diopters. If this is always true,
then the correcting lens which is selected by an aphakic eye
is a guide to its former ametropia. An eye which selects a
weak plus sphere would, therefore, have been myopic before
the operation ; and if about a + 12 S., its previous refrac-
tion approximated emmetropia ; if a plus sphere stronger
than 1 2, then the previous refraction was very likely hyper-
opia.
An eye which has had its lens removed by absorption/
(needling) is not likely to be astigmatic ; whereas, when
the lens has been removed by extraction, astigmatism) ,
against the rule of one or more diopters almost invariable/.
results, and the axis of the correcting cylinder generally
coincides with the points of puncture and countcrpuncture
in the cornea. If a patient had 2 or 3 D. of myopic
astigmatism with the rule, this would be neutralized by the
corneal section.
Correction of Aphakia. As in presbyopia, two correc-V^.
tions are necessary one for distance and one for noaxs^
Astigmatism must always be looked for and carefully cor-
rected, especially if the lens has been removed by extrac-
tion.
CASE I.
VI
O. D., 48-00 sph. ^ +3-OO cyl. axis 10 degrees. V. = x
O. D., 4 n.oo sph. C; 4 3- c >'- ax ' s IO degrees = reading at 33 cm.
This patient was presumably myopic before operation.
Heterometropia (",""?, "different"; !^r/mv t "a meas-
ure" ; &<}', "the eye") literally means that the ametropia of
28O REFRACTION AND HOW TO REFRACT.
the two eyes is different in character; examples, O. D. -f- i D.
and O. S. I D., or O. D. +3 cyl. axis 90 and O. S. 3
cyl. axis 180, or O. D. 5 D. and O. S. 5 cyl. axis
1 80, etc.
Anisometropia (W ?, "unequal"; /j-l-pnv > "a measure";
&<}>, "the eye") literally means that the ametropia of the
two eyes is the same in character but of unequal amount.
Examples, O. D. +2 D. and O. S. +6 D., or O. D. 0.50
D. and O. S. 5 D., or O. D. +3 cyl. axis 90 and O. S.
-{-6 cyl. axis 90, etc. This condition may be slight or
one of the most extreme conditions imaginable.
^T'or instance, if both eyes have compound hyperopic
astigmatism, they are not considered as heterometropic, even
if the sphere and cylinder are of different strength in the
two eyes. . Bearing this distinction in mind, the percentages
already given for myopia, hyperopia, the different astigma-
tisms, etc., have been calculated accordingly, that for
j^heterometropia being about thirteen per cent.
Causes. Usually the condition is congenital, or it may
be acquired.
Difficulties. Two difficulties are encountered when or-
dering glasses for cases of anisometropia or heterometropia :
(i) The lens for one eye may be concave and that for the
other may be convex, or both eyes may require a convex
or both may require a concave lens, but one very much
stronger than the other; under these circumstances, when
the eyes are rotated there will be a prismatic result of dif-
ferent amount in each eye, and this may mean diplopia, or
at least an exertion on the part of the extraocular muscles
to prevent diplopia which will cause dizziness, nausea, head-
ache, etc. (2) With lenses as just mentioned, the size of
the two retinal images will not be exactly the same, and this
will mean an interference with clear binocular vision.
ANISOMETKOPIA. 28 1
For purposes of study, the writer would divide cases of
heterometropia and anisomctropia into four different classes.
CLASS I. This class embraces those cases in which the
difference in the ametropia between the two eyes is very
slight or does not exceed two diopters. In fact, there are
very few pairs of eyes that are not slightly anisomctropic ;
such eyes usually receive their exact corrections with com-
fort, regardless of the condition.
CLASS II. Cases that come under this head also accept
their exact correction for each eye, but do not attempt
binocular single vision, and may never suffer the least in-
convenience ; these cases are extremely rare. They do not
complain of diplopia, as they have learned to ignore the
false image. Cases of alternating squint, one eye myopic
and the other eye hyperopic, may be included in this class.
CLASS III. This is a class which will accept the exact
correction before one eye only, and the eye which has the
greatest amount of ametropia will refuse almost any lens
except the very weakest. The eye that has the most ame-
tropia is often quite amblyopic.
CLASS IV. This class includes young children especi-
- , ally ; cases of squint. In children the correction as found
by the static refraction is usually accepted.
The Prescribing of Glasses in Cases of Heterometropia
and Anisometropia. Excluding Class I, there is no fixed
rule to follow when ordering glasses in decided cases of
heterometropia or anisometropia, and, in fact, such eyes are
a constant study to the most able ophthalmologist. The
younger the patient, however, the more likelihood of a favor-
able result from the careful selection of a glass for each eye;
but when the patient is an adult, it becomes a very serious
question as to what glass to prescribe that will give satisfac-
tion. As good results are to be expected in children, they
24
282 REFRACTION AMD HOW TO REFRACT.
should receive the most careful retinoscopic refraction. The
child comes under observation on account of a squint, and an
operation for the deformity is often demanded ; but the opera-
tion must be refused until the ametropia has been carefully
treated. Glasses having been prescribed, the squinting eye is
put to work to develop its seeing qualities, which have been
permitted to lie dormant for want of a proper glass. To do
this, the "good" eye is shielded or blinded with a hand-
kerchief tied over it, or a blinder (see Fig. 1 60) placed over
its correcting lens, for an hour or two each day, and in this
way an attempt is made to bring the vision in the squinting
eye up to that of its fellow.
; Or another way to develop the vision in the squinting
; eye is to use a cycloplegic in the " good " eye, so that the
squinting eye must do most of the work. This is rather
trying to the little patient, and often means the additional
use of dark glasses. As a rule, the "good" eye has the
least amount of ametropia, but occasionally the reverse con-
dition may exist.
In a case like the following, the little girl, five years of
age, was brought on account of convergent squint in O. S.,
which developed or commenced to appear when ten months
of age, and the parents attributed it to the habit of sucking
her thumb at the time of being weaned. Refraction, with
atropin as the cycloplegic, and obtained with the retino-
scope, showed O. D., -(-2.00 sph.; O. S., +4.00 sph. O
+ 1.00 cyl. axis 75 degrees.
This child developed the squint on account of the
monocular astigmatism and because it could not accom-
modate sufficiently with the left eye. To avoid diplopia
at the same time that the eyes were converging, the
left eye naturally turned inward. With correcting glasses,
and practising as above directed, the squint entirely
BIFOCALS. 283
VI
disappeared, and vision one year later was -_.- in each
eye with the correcting glasses. If the glasses are laid
aside for any length of time, the squint returns. This
child must wear the glasses or have " squint."
/To make sure that no injustice is done to an apparently
/amblyopic eye in an adult (Class III, p. 270) where ambly-
opia exanopsia has existed for many years and nothing has
been done to improve its correction, the writer makes it a
rule to prescribe the exact correction for each eye, and at the
time of ordering the glasses explains to the patient what
the purpose and desire is, and that if there is any great
amount of discomfort in any way, he must return and have
any necessary change made in the glass. These patients
should be kept undef observation and the amblyopic eye
given some sort of a correction and improved as much as
possible ; the purpose being not to allow the eye to
degenerate or grow more amblyopic, for if any accident
should befall the " good" eye, then the amblyopic eye will
often be a friend indeed.
Glasses for Presbyopes and Cases of Aphakia. Unless
I the distant vision is improved or asthenopic symptoms are
>
.relieved by glasses, it will be sufficient to prescribe the
i near correction only. When a distant and near correc-
tion are required, they may be prescribed as two pairs of
glasses in separate frames, or two pairs in one frame, known
as bifocals. Bifocals, or what is equivalent to bifocals, are
made in different ways.
i. Franklin* or Split Bifocals (Figs. 175 and 176).
This form of bifocals consists of an upper and a lower lens,
each with its individual center ; the upper lens is for dis-
tance and the lower for near vision. Such lenses must have
the frame all' around the edges, so as to hold them in posi-
* " History of Spectacles," L. Webster Fox, " Med. and Surg. Reporter,"
1890, vol. LXII, 513-519.
284
REFRACTION AND HOW TO REFRACT.
tion. Bifocals of this kind are not in common use. The
field of distant vision is limited by the unnecessarily large
near correction, and where the two lenses come together,
A
FIG. 175.
there is apt to develop chromatic aberration and a decided
prismatic effect when the vision is directed through this
space.
B. O. D., -|-2.oo sph.
O. S., -(-a.oosph.
SlG. For distance.
B- O. D., -f 4.00 sph.
O. S., +4.00 sph.
SlG. For near.
DIRECTIONS TO OPTICIAN. Make into Franklin or split bifocals.
. Morck's Patent or "Perfection" Bifocals (Fig.
). These are a modification of the Franklin or split
bifocals, and in place of having
lenses united in a horizontal line,
the near and distant lenses are
fitted together with correspond-
ing crescent edges. This form
of bifocal gives a larger field for
the distance correction, and, like
the Franklin, is much better
for those who work in a damp
atmosphere and can not wear the cement bifocal. It is,
FIG. 177.
I
BIFOCALS.
28 5
however, more expensive than the cement form ; but, like
the Franklin, it often looks clumsy or heavy on account
of the frame. "Perfection" or "Morck" bifocal must be
j, . "^^ Mg * . f*
signified in writing the prescription. ^j ~ftn_& ' * ' < ^ * f
3. Cement Bifocals-fScTTIgsTi/S and 179*). This is
the most common form of bifocal and the least expensive
in its original cost, as also when making changes in the
near correction. This bifocal is made by cementing a seg-
A
FIG. 181.
FIG. 182.
ment of a small periscopic sphere on to the lower part of
the distance correction. This periscopic sphere or disc or
segment, as it is called, has a prismatic quality (see Fig.
179) suitable to the exigencies of the individual lens to
which it is cemented. The segment may be of any shape
desired. Those in common use are shown in figures 180,
181, and 182. It is cemented to the distance correction
* Described by Dr. Geo. M. Gould, " Med. and Surg. Reporter," Nov. 3,
1888.
286
REFRACTION AND HOW TO REFRACT.
with Canada balsam. While this is the usual method of
making a cement bifocal, yet it may be made by cementing
IB
FIG. 183.
V
FIG. 184.
a concave segment to the upper part of the near correction.
(Figs. 183 and 184.) This form is not in common use.
H. O. D.,-| 2 S.
O. S., +2 S.
Cement on the lower part of the above O. D. and O. S., -\-2.oo
SlG. Make frameless bifocals.
Or,
li. O. D., + 4 S.
O. S., +4 S.
Cement on the upper part of O. D. and O. S., 2.00 S.
SlG. Make frameless bifocals.
4. Achromatic Bifocals (Figs. 185 and 186*). This
FIG. 185.
FIG. 1 86.
form of bifocal is used principally in cases of aphakia where
the plus sphere is quite thick and correspondingly heavy. It
* Borsch patent.
BIFOCALS.
23 7
is made in one of two ways : (i) By grinding out a portion
of the lower part of the distance correction (in crown glass)
and cementing into the concavity a biconvex segment
of flint glass. This form of bifocal is a combination of
the " perfection " and lenticular. (2) In place of grinding
the concavity in one lens, as just described, this achromatic
bifocal is also made by taking two planoconvex spheres
and grinding out a concavity in each, and then inserting a
convex sphere of flint glass, as shown in figure 186 ; these
three lenses are then cemented together, and when com-
pleted, look like the cement bifocal, as shown in figure 182.
It is a matter for very careful calculation as to just how
strong to make the flint glass segment, so that the result
may be just exactly right. The merits of this bifocal are
lightness and the absence of chromatic aberration. These
lenses are very expensive.
5. Solid or Ground Bifocals (Figs. 187 and 188).
Lensts of this character are made in one piece by grinding
<M^*'
FIG. 187.
FIG. 188.
on to the upper part of the near correction the necessary minus
spheric correction for distance. They look neat, but are npt
al \vays comfortable, on account of the resulting prismatic
effect, which is especially apt to occur when the lens is con-
vex, though this may not be so troublesome a feature when
the lens is moderately concave.
288
REFRACTION AND HOW TO REFRACT.
Single Crystal Bifocals. Lenses of this character are
made in one piece, and in this respect are like the solid
bifocal, but~they differ from the solid bifocal in two ways :
the near correction is made by grinding on to the lower
part of the distance correction, the necessary plus sphere.
The solid bifocal gives a decided prismatic effect where the
FIG. 189.
FIG. 190.
FIG. 191.
FIG. 192.
FIG. 193
two corrections meet, whereas this defect is said not to
exist in the crystal bifocals. These bifocals are expensive.
6. Patients who have a very weak distance correction,
and could do without it, sometimes accept it for the con-
venience of wearing bifocals ; they do not wish to be an-
noyed by taking off or putting on a near correction, prefer-
BIFOCALS.
289
ring to have the glasses where they can find them ; business
men especially. Other patients prefer to do without a dis-
tance correction, and will often use a near correction that
has one-third or nearly one-half of its upper part cut away,
so that they can look over the near correction when they
wish to see at a distance. (See Figs. 189, 190, 191, 192,
and 193.) Myopes who do not need a near correction will
wear their distance correction with its lower portion cut
away, so that when they wish to see near at hand, tiiey can
look under the distance correction. ^^ Ji^/L^J
7. Patients who require a distance correction, and can
not get accustomed to cement segments, and" at the same
time do not wish to change the distance correction, but
prefer to keep it on all the time, can put on their addition
for near vision in the form of hook or "grab" fronts of the
same size as the distance lenses or reduced one-half in the
vertical diameter. This is not always a good combination,
as in even- instance the lenses do not lie in contact with
each other.
8. Lorgnettes may be used as a distance correction or as
a substitute for hook fronts. Some myopic women who
wear their near corrections constantly often carry lor-
gnettes, which they hold up in front of the near correction
to improve distant vision for a few minutes, or, wearing the
distance correction, can use a plus lens in the lorgnettes for
near vision.
9. Cases of monocular aphakia where the vision in the
fellow-eye is very defective can wear reversible frames, one
lens for distance and the other for near, that is to say, a
frame which has a free joint at the temples, and in this way
they avoid bifocals, and can change the distance for the
near correction, by turning the temple-pieces.
In some cases of aphakia where the lens is very powerful,
25
290
- 3,
REFRACTION AND HOW TO REFRACT.
a bifocal segment can sometimes be dispensed with, if the
patient has a long nose, by sliding the lens down from the
eye and then heading the reading matter at the conjugate
focus. A toric lens (Fig. 194) is very acceptable in occa-
sional instances, as it reduces somewhat the weight and
thickness of the lens, and also enlarges the field of vision.
A toric (torcine or toriqiic, "twisted") lens is one which
Copyright, 1886, by Chas. F. Prentice.
FlG. 194.
has, combined in one surface, the optic effects of a sphero-
cylindric lens, or two cylinders of different strength at right
angles tp each other. Unfortunately, this form of a lens is
quite^xpensive.
General Considerations. Before prescribing any pair
of glasses, the patient should have the opportunity to wear
the correction in the office for a short time, that he may
study its effect; this is especially necessary (i) when the
glasses are strong ones ; (2) when there is monocular as-
tigmatism ; (3) when one lens is much stronger than the
other (anisometropia) ; (4) when the astigmatism is asym-
metric ; or (5) when there is a strabismus, etc. The patient
loses confidence (and the surgeon is not made happy) when
the patient returns with his glasses in his hands and states
that he can not wear them that they make him " dizzy " or
" tipsy " ; that the glasses make the pavement, houses, trees,
BIFOCALS. 291
people, pictures on the wall, chairs, tables, etc., all appear
as if they were going to fall to one side. The surgeon
should have anticipated all this, and assured the patient
beforehand that, after a little perseverance and practice, this
distortion (parallax) will disappear ; and if not, then a
change will have to be made in the glasses. Very often
the whole difficulty is due to a want of proper centering of
the lenses, presuming, of course, that the glasses ordered
are perfectly correct.
Patients who require weak lenses spherocylinders or
cylinders alone may at some time be informed that " the
correction is but window-glass," and thus the surgeon may
be put in disgrace as having prescribed for mercenary
reasons, when in truth the glasses have already cured an
old blepharitis or asthenopia. In ordering weak correc-
tions, therefore, the character and purpose of the glasses!
should be imparted to the patient.
It is interesting to notice that strong glasses are usually
ordered to improve the vision, and not always for the relief
of asthenopia, whereas weak corrections are prescribed for
the relief of headaches, etc., without any decided improve-
ment in the vision which the patient can appreciate when
looking at a. distance, and many such patients will say they
can see just as well without their glasses. When strong
plus spheres are prescribed for a child, it will do no harm
to inform the parents of the character of the glasses, so
that when a presbyope tries the child's glasses, the sur-
geon may not be accused of ruining the child's eyes by
having ordered a pair of glasses strong enough for a grand-
mother to read with, and the child hurried off to a rival
confrere to have the " outrage " rectified.
A patient who has fought against the inevitable, using
headache powders, liver pills, etc., in the vain hope of not
2Q2 REFRACTION AND HOW TO RKFRACT.
having to put on glasses, may still object to their use for
various reasons. It may be that glasses will not add to
the personal appearance, or the parents may dislike the
idea, fearing that " the oculist puts glasses on every
patient," or that " the eyes will never be the same again,"
or that " the habit of wearing glasses, once established, can
never be stopped." These and many other statements will
serve to enliven the daily routine of ophthalmic practice.
These objections having been met from the point of view
of the patient's individual welfare and future good of his
eyes, the next question that arises is what form of glasses
shall be prescribed.
Spectacles. The child is certainly a candidate for spec-
tacles. The frames must be very durable, and preferably
of 14-carat gold. Spectacle frames keep the lenses in posi-
tion, and the lenses are then less liable to be broken than
in the form of eye-glasses, and for most occupations are to
be preferred. Occasionally, the shape of the nose will pre-
clude the use of anything else but spectacles. When one
lens is very heavy or both have considerable weight, or
when one or both lenses are cylindric, with axes inclined,
spectacles are certainly indicated.
Eye-glasses, also called " pinc-nez," are for the adult,
and may be prescribed when the lenses are not too heavy,
or the cylinders too strong or their axes inclined. Kve-
glasses are easily bent, and lose their exact positions before
the eyes. For the young society girl nothing but the most
delicately made eye-glasses will, as a rule, be accepted.
Bifocals. These should not, as a rule, be prescribed if
the lenses are very strong or the correction a complicated
one, or the patient advanced in years and has never at-
tempted them before, or if the patient is very portly or
uncertain in his gait, or the vision is not brought close to
BIFOCALS. 293
the normal. Two separate pairs of glasses are to be recom-
mended under these circumstances. When ordering any
pair of bifocals, the patient should be cautioned and in-
structed that when looking downward, going up or down
stairs, getting into or out of a conveyance, he is to look to
one side or over the segment of the bifocal and not
through it, otherwise he will be liable to make a false step
or misjudge the distance, which might mean serious bodily
injury, for which the surgeon does not wish to hold himself
responsible.
Glasses for constant use should be placed perpendicularly
or at an axis of about 5 degrees to the plane of the face,
with the optic centers corresponding to the pupillary cen-
ters when the eyes are directed to a distance. If the lenses
are unusually strong and to be used principally at near-
work, then it may be necessary to consider the advisability
of having two pairs of glasses, one for distance and one
for near, each with the centers to answer for the object in
view. If only one pair of glasses has been ordered, and
they happen to be very strong, then a pair of prisms in
hook fronts may have to be used at the near-work, so as to
counteract the prismatic effect of looking through the dis-
tance glasses during convergence. Glasses for near-work
only should be put into a frame made especially for the
purpose, so that the lenses may have an inclination in
keeping with the downward turn of the eyes, and thus be
perpendicular to the axis of the eyes, and the lenses should
be decenterecl inward to equal the convergence. The one
serious objection to bifocals in certain instances is that the
glasses can not be made with the inclination suitable for
both distance and near vision, and very often there must be
a compromise between the two.
The surgeon should make it a point to carefully inspect
294 REFRACTION AND HOW TO REFRACT.
every pair of glasses which he orders, as his painstaking
efforts and best endeavors may be completely frustrated by
poorly fitting lenses.
1. The lenses should neutralize. (See p. 58.)
2. The optic centers should be at the points indicated.
3. The cylinder axes must be exact.
4. The lenses must be perpendicular or inclined to the
front of the eye, as necessary.
5. The distance of the lenses from the eyes should
always be sufficient to clear the lashes ; and if these are
very long, they may have to be trimmed.
6. The most convex or the least concave surface of the
lens should be placed away from the eyes. Or the most
concave surface toward the eye.
7. The lenses should be of the correct size for the indi-
vidual face. These and many other points for the average
case must receive the careful consideration of the surgeon.
Tinted or Colored Glasses. Except for the relief of
photophobia following cataract extraction, mydriasis, or
inflammatory diseases, the surgeon does not order colored
glasses. Colored lenses are to be deprecated except in the
cases just mentioned, as they only increase the tendency to
jiotophobia instead of correcting it.
[metric Lenses. These are made to conform in out-
line to the normal field of vision as recorded by the per-
imeter, hence the name.*
The usefulness of the perimetric lens is limited to those
cases in which the correction contains a plus cylinder and
the lens is of moderate strength. It is not a lens that can
* The writer described this form of lens before the Section in Ophthalmology
of the College of Physicians of Philadelphia, in March, 1897.
TRIFOCALS.
295
be prescribed in myopia or aphakia. The purpose of the
perimetric lens is to give a normal field and have the edge
of the lens sufficiently removed that the patient may not be
disturbed by seeing it. It certainly enlarges the field of
vision, and in this way is a great advantage in certain occu-
pations, playing the piano, etc. Figure 203 or 204 may
answer the same purpose if properly centered.
Trifocals. Occasionally, a patient is not content with
bifocals, but will demand a focal point somewhere between
infinity and his working distance; this can only be produced
by cementing two segments of different sizes and strength
Fie. 195.
FIG. 196.
on the intermediate correction. (Figs. 195 and 196.) Book-
keepers who have to work at large and lengthy ledgers find
great comfort in this combination, though to be of special
service the lenses must be made large. Example: +2.00
equals working distance at I meter. Minus I diopter
added above equals infinity vision. -+-2.00 added below
gives near vision at 1 3 inches.
Decentering of Lenses. Instead of writing a prescrip-
tion for a lens and prism, the prismatic effect of the lens
may be obtained by decentering the lens. The rule is
296 REFRACTION AND HOW TO REFRACT.
that for every centimeter of decentering there will result
just as many prism-diopters as there are diopters in the
meridian of the correcting lens. For example, +4 s ph- O
4 P. D., base out, is the same as +4 sph. decentered I cm.
outward ; or -(-4 sph. O 2 P. D., base in, equals -f 4 S.
decentered 5 mm. inward ; or -f-2 sph. O +2 cyl. axis
90 degrees O 2 A, base outward, equals -\-2 sph. O -f 2
cyl. axis 90, decentered 5 mm. outward.
While it is well for the student to know how to decenter
lenses, yet the writer does not recommend such lenses,
preferring, when necessary, to order a prismatic combina-
tion, and have the optician fill the prescription, starting
direct from the prism.
CHAPTER XII.
LENSES, SPECTACLES, AND EYE-GLASS
FRAMES. HOW TO TAKE MEASURE-
MENTS FOR THEM AND HOW THEY
SHOULD BE FITTED.
The selection of the size and shape of lenses, the char-
acter of the spectacle and eye-glass frames and their adjust-
ment, is the work of the optician. It occasionally happens,
however, that the surgeon may not have an optician in his
town, and will, therefore, have to take the necessary meas-
urements himself and send them, with his prescription, to
an optician in a neighboring city. This chapter is there-
fore added for the benefit of such surgeons. It is hardly
necessary to state that the frames should be very carefully
adjusted and the lenses centered to the patient's eyes. A
lens improperly adjusted may utterly destroy the good
effect of the most skilfully selected correction, giving dis-
comfort to the patient and reflecting seriously upon the
surgeon's ability. In fact, it is always well for the surgeon
to personally inspect every pair of glasses which he may
order.
Lenses. These are spoken of as "eyes," and come in
various sizes and shapes. They are spoken of as O, double
O (OO), triple O (OOO), etc. (See Figs. 202, 203, 204,
205, and 206.) Or sizes smaller than O are numbered I, 2,
3, or 4. (See Figs. 197, 198, 199, 200.) Different shapes
and sizes are lettered A, B, C, D, F, or X. (See Figs. 189,
190, 191, 192, 193, 201.) All these lenses are also marked
297
298
REFRACTION AND HOW TO REFRACT.
in millimeters of breadth and length. The lenses for indi-
vidual patients are selected according to the purpose for
which they are intended, and particularly to be in keeping
FIG. 197.
FIG. 199.
FIG. 201.
FIG. 198.
FIG. 200.
FIG. 202.
with the facial measurements. The size or " eye " O (39 X 30
mm.) is the usual size for the average adult, and number
2, 3, or 4 is for a child. C, D, or F may be ordered for a
LENSES.
299
presbyope who does not need a distance glass and who
does not wish to be taking off the near correction to see at
FIG. 205.
FIG. 206.
a distance ; in other words, such a shaped lens can be
looked over without any difficulty. Or the presbyope who
3OO REFRACTION AND HOW TO REFRACT.
requires a 2 for distance and can see to read without
any near correction, being about fifty years of age, could
have his minus lenses made in the shape of A, B, C, or D
inverted, and, wearing this for distance, would look under
it when he wished to see near at hand. As a rule, the
patient with a narrow face and short interpupillary distance
will require a small "eye," whereas the patient with a
broad face and long interpupillary distance will require a
large "eye."
Spectacle Frames (Fig. 211). These consist of a nose-
piece (called the bridge) and temples (called sides). These
are attached to the lenses ("eyes") by screws passing
through holes which have been drilled through them, mak-
ing what is known as the frameless spectacles ; or a wire is
fitted around the lenses, to which the bridge and sides are
attached with solder, forming the "framed" spectacles.
Eye-glass Frames (Fig. 212). These consist of a
spring and nose-pieces ; the latter are called guards.
Framed and frameless eye-glasses have the nose-pieces or
guards attached to the lenses as in the spectacles.
How to Take Measurements. There are three points
that require particular attention : (i) The center of the
lens should correspond with the center of the pupil ; (2)
the lens must be just far enough from the eyes to avoid the
lashes, and if these are very long, they must be trimmed ;
(3) the lens must be at such an angle that the visual axis
will be perpendicular to it.
First Measurement. The Interpupillary Distance. To
accurately measure the distance from the center of one
pupil to the center of the other is not always an easy thing
to do, especially if the pupils are dilated ; hence, it is good
practice to measure this distance from the inner side or edge
of one pupil to the outer edge of the other. This measure-
FIRST MEASUREMENT.
301
ment can be made with an ordinary rule divided to six-
teenths of an inch or in millimeters, or with a special instru-
ment for the purpose, called a pupilometer. The patient
is told to look directly to the front, at an object across the
room, and the surgeon, in front, with his head nearly in the
line of sight, holds the rule across the patient's face, as
close as the bridge of the nose or eyelashes will permit.
With his thumb-nail as a marker, the surgeon gages the
distance as indicated (see Fig. 207), which illustrates the
conditions. In taking this measurement the surgeon should
be at an arm's length from the eyes, for the reason that his
FIG. 207.
own eye forms the apex of a triangle of which the eyes of
the patient form the base, and the measurement is apt to
be two or three or four millimeters short if he gets too
close.
If the glasses are to be worn for distance only, then the
measurement must be for the full interpupillary distance, as
the patient looks into infinity ; but if the glasses are for near-
work only, then the distance between the pupils must be
correspondingly diminished, and the measurement taken
as the patient looks at a near point. If the glasses are
to be worn for both near and far vision, for constant use,
3O2 REFRACTION AND HOW TO REFRACT.
then the center of the lenses must be placed intermediate
between the distance and near measurements.
Second Measurement. The Bridge. The regulation
spectacle bridge is known as the saddle-bridge, and should
conform to the exact shape of the patient's nose. It is in-
tended to remain in just one place, and that is at the bridge
of the nose (see B in Figs. 208 and 209), the place where
the nose begins to extend outward after passing down from
the forehead. The points B and D, as shown in figure 210,
represent the widest part or base of the bridge. A and R
are the arms, which extend upward or outward and are
fastened to the lenses. The length of the arms controls in
FIG. 208. FIG. 209.
great part the distance of the lenses from the eyes. To
raise or lower the position of the lenses in front of the eyes,
the posts or arms alone should be bent ; the bridge itself
should never be tilted, as its edge will cut into the skin of
the nose ; this is a most important consideration for the
patient's comfort.
The Shape and Size of the Bridge. To take this meas-
urement, the surgeon should have a piece of lead-wire or
thin, pliable copper-wire ; the lead-wire is best. This wire
is accurately molded to the bridge of the patient's nose,
the arms (A and R) are bent to the proper angle, and then
the ends of the wire are curved or bent outward to show
the plane of the lenses. (See Fig. 210.)
THIRD MEASUKKMKNT FOURTH MKAMKKM KNT. 303
When the wire has been 'Dent into plaee and the eyelashes
do not touch at L and L, it is removed and placed on the
under surface of a piece of paper, when an impression and
lead-pencil tracing is made of it. If the measurement is
not taken in this way, then the surgeon, with a pair of
moderately blunt-pointed compasses, measures the breadth
of the nose from B to D, and also the height of the bridge
from F to E. The height of the bridge is spoken of as
"out " or " in " ; the former when F extends beyond the
plane, and "in" when F is behind the plane of the lenses.
(See Figs. 208 and 209.)
Another good way to take the foregoing measurements
is to have several ordinary steel frames of different sizes and
FIG. 210.
shapes, using whichever one of these seems to fit the best,
and then making any additional alterations in the measure-
ments that may be required.
Third Measurement. This is the length of the sides or
temples. This measurement is taken from the top of the
ear to the plane of the lens, or a horizontal line extending
out from the eyelashes.
Fourth Measurement. The Size of the Lenses. This
will depend upon the breadth of the face, the amount of
space taken up by the bridge, its arms and attachments, as
also the space occupied by the hinge and attachment of the
temples. Ordinarily, as stated before, the adult will select
size O and the child No. 2.
304
REFRACTION AND HOW TO REFRACT.
The following blank is a good guide, as covering all
the necessary measurements as referred to in this descrip-
tion for ordinary glasses.
STYLE OF BLANK FOR THE SURGEON TO FOLLOW WHEN
ORDERING GLASSES FOR HIS PATIENT.
Patienfs Name,
Forward to,
FIG. 211.
FIG. 212.
R.
O. D.
O. S.
Distance or Near Frames.
Frames of .
MEASUREMENTS.
Spectacles. Eye-glasses.
Interpupillary distance, Interpupillary distance, . . .
Height of bridge, Length of guard, W to T, . .
Base of bridge, Width at base, W to D, . . .
Shape of bridge (see drawing), . . Width at top, T to P, . . . .
Bridge, " in" or "out," Length of arm of guards, . .
Length of temples, Shape of spring (see drawing),
Size of "eye," Size of "eye,"
Additional notes,
Date,
,M.D.
STYLE OF FRAMES.
305
Style of Frames. If the glasses are to be worn con-
stantly, they should be perpendicular or inclined about 5
degrees from the perpendicular to the front of the eyes.
(See Fig. 213.) They are spoken of as " distance " frames.
FIG. 213.
FIG. 214.
If the glasses are to be worn only at near-work, then the
lenses should be tilted downward ; this is known as the
"near" frame. (See Fig. 214.)
Fitting Eye-glasses. The position of the lenses ap-
plies equally well to eye-glasses. The
principal measurement is for the nose-
pieces or guards and the arms or off-
sets from the guards. (See Fig. 215.)
The width of the patient's nose where
\Y and I), and also T and P, will press,
depends, of course, upon the length of
the guard itself usually about 14 mm.
It is also necessary to measure the posi-
tion of the guards relative to the plane
of the lenses ; that is, whether the arms should be lon^,
medium, or short, and whether they are "out" or "in"
26
306 REFRACTION AND HOW TO REFRACT.
from the plane of the lenses. The style of spring is usu-
ally that shown in figure 215.
Bifocals. The measurements for bifocals are the same
as for the spectacle or eye-glass, except the size and shape
of the segment, and this should never extend above the
median line of the lens, and seldom to it.
Quality of Frame. These are made of silver, steel,
aluminium, or gold ; the latter are always to be preferred,
as more durable in every way. Silver and aluminium bend
easily, and steel frames rust and break. Every surgeon
who does his own fitting should possess a small screw-
driver, two pairs of delicate and yet strong pliers (one with
round and the other with flat ends), and also a small rule.
INDEX.
A.
ABDUCTION, 180
Aberration, negative, 175
positive, 173
Absorption of light, 1 2
Accommodation, 65, 66
amplitude of, 69, 70
at different ages, 70
at rest, 68, 69, 245
binocular, 84
cramp of, 266, 267
diminution of, 70
in emmetropia, 70, 71
in hyperopia, 71, 72
in myopia, 72, 73
in presbyopia, 70
mechanism of, 66, 67
muscle of, 66, 67
observer's, 06, Q7
paralysis of, 216, 217, 218
patient's, 96, 158
range of, 69, 70
relaxed, 70, 97
proof of, 245
spasm of, 266, 267
Acuteness in astigmatism, 78
in emmetropia, 103
in hyperopia, 106
in myopia, 115
of vision, 63, 64, 78, 79
record of, 78, 79
Adduction, 180, 181
Aerial image, 100
Age, 70, 224, 228
Albino, 93
Alternating strabismus, 196
Amblyopia, 157
Amblyoscope, 203, 204
Ametrometer, 148, 149
Amrtropia, 105
axial, 105, 119, 1 20, 121
curvature, 105, 122
Angle alpha, 87
critical, 20
gamma, 85
limiting, 20, 62, 63
meter, 83, 84
of convergence, 83
of deviation, 24
of five minutes, 64
of incidence, 22
of refraction, 21, 22, 23
of strabismus, 199
of view, 6 1
Anisometropia, 280, 281
classification of, 280, 281
correction of, 281, 282
Anterior focal point, 60
focus, 60
Apex of prism, 23
Aphakia, 157, 278
causes of, 278
diagnosis of, 278, 279
treatment of, 279
Aqueous humor, 60
Asthenopia, 219, 220
accommodative, 221, 222
muscular, 183, 184, 221
retinal, 220, 221
treatment of, 220, 221, 222, 223
Astigmatic charts, 137
clock-dial, 138, 139
lens, 123
Astigmatism. (See Chapter V.)
against the rule, 131
asymmetric, 130
causes of, 124
compound hyperopic, 127, 128,
2S 6 2 5 7. 2 5 8
myopic, 128, 259, 260, 261
corneal, 123
diagnosis of, 133
estimation of, 124. (See Chap-
ter VI.)
307
308
INDEX.
Astigmatism, helerologous, 132
heteronymous, 132
homologous, 132
homonymous, 132
irregular, 124, 265, 266
lenticular, 124
mixed, 128, 129, 261, 262, 263,
264
physiologic, 124, 125
principal meridians of, 126
regular, 125
shape of disc in, 153
simple hyperopic, 126, 250, 251,
252, 253
myopic, 127, 254, 255, 256
statistics of, 243
symmetric, 129, 130
symptoms of, 133
tests for, 133
treatment of, 250, 251, 252, 253
with the rule, 131
Astigmia, 122
Astigmic, 122
Atropin, 213
Axiom, 157
Axis of astigmatism, 126
of cylinder, 44
optic, 60, 85
principal, 15, 32
secondary, 15, 36
visual, 85, 86
Axonometer, 170
B.
BAND of light, 169.
Base of prism, 23
Beam of light, 1 1
Biconcave lens, 31, 55
Biconvex lens, 30, 54
Bifocals, 283, 284, 285, 286, 287
Binocular accommodation, 84
fixation, 84
Blepharitis, no
Borsch, 286
Brachymetropia, 113
Briicke, muscle of, 66
Burnett, 122
c.
CAMERA, 65
Capsule, 68
Cardinal points, 59, 60
Cards, 74, 75, 76, 77
Cataract, 276
Catoptrics, 9
Center of fixation, 86
of rotation, 86
Centering of lenses, 294
Chalazion, 124
Choroid, 94, 95
Chromo-aberration test, 145, 146,
147, 148
Ciliary body, 66
muscle, 66
anatomy of, 66
Cobalt-blue glass, 113, 145, 146,
147, 148
Cocain, 91, 215
Compound system, 60
Concave lenses, 31, 37
mirror, 15, 16, 17
in retinoscopy, 159
Concomitant squint, 196
Condensing lens, 99
Confusion letters, 134
Conic cornea, 172
Conjugate foci, 34
Conjunctiva, no
Convergence, 83, 84
amplitude of, 85
angle of, 84
insufficiency of, 183, 184
negative, 87
positive, 86
range of, 84, 85, 86
Convergent strabismus, 195
Convex lenses, 30
Coquilles, 212
Corneal reflex test, 133
Cover chimney, 159
test, 184
Cramp of accommodation, 218, 266,
267
Cretes' prism. 189
Crossed diplopia, 179
Crystalline lens, 68
Cycloplegia, 216, 217, 218
Cycloplegics, 158, 208, 209, 210, 211,
212, 213, 215, 216
Cylinder lenses, 43
action of, 43, 44
axis of, 43, 230
combination of, 49
crossed, 52, 231
neutralization of, 56, 57
INDEX.
309
D.
DARK glasses, 212
room, 91
Daturin, 213
Decentering lenses, 295, 296
Deorsumduction, 181
DeSchweinitz, 191
De/.eng, 102, 175
Deviation, angle of, 24
estimating size of, 199, 200, 201
in strabismus, 199
Diopter, 41,42
Dioptrics, 9
Dioptric system, 60
Diplopia, 29, 178, 179
correction of, 29
Direct method, 90, 91, 153, 154, 155
Disc, optic, 93
perforated, 141, 142
pin-hole, 47, 266
Placido's, 134, 135
shape of optic, 93
Distant type, 81, 82
Divergence, 83, 196, 197
Divergent strabismus, 196
Duboisin, 213
Dynamic refraction, 234, 235
E.
ELASTICITY of lens, 68
Electric light, 102, 175, 220
blindness from, 220
Elongation of eyeball, 242
Epilepsy, 222
Emergent rays, 10
Emmetropia, 103
description of, 103, 104, 105
Erect image, 94, 95, 96
Esophoria, 184
diagnosis of, 185
treatment of, 189, 193, 194
Ksotropia, iS, n;5
Exophoria, 184, 268, 269
diagnosis of, 185
treatment of, 189
Exotropia, 196
Eye (frontispiece), 59, 60
Eye drops, 208
emmetropic, 60
glasses, 292
hyperopic, 71
Eye, myopic, 113
schematic, 156
section of (frontispiece),
standard, 59, 60
-strain, 219, 220, 221, 222, 223
F.
FACE, asymmetric, 130
broad, 300
narrow, 300
Facial illumination, 162
Far point, 68, 69
Finger exercise, 191
Fitting of spectacles. (See Chan-
ter XII.)
Focal interval, 123
length, 33
points, 59, 60
Focus, 12, 15
anterior, 33
conjugate, 34
negative, 12, 35
ordinary, 35
positive, 12, 15
posterior, 33
principal, 15, 33
real, 12, 33
virtual, 12, 35
Fogging method^ 232, 233, 234
Form of illumination, 165
Formation of images, 38, 39, 40
Fox, 283
Fusion tubes, 203, 204
G.
GLASS, crown, 22
flint, 22
Glasses. (See Lenses.)
Glaucoma, 211
Gould, 77, 191
Green, 137
H.
HKI.MIIOLTZ, 103
Heredity, 108, 1 16
Heterometropia, 279, 280
Hcteronymous images, 179
Heterophoria, 182
3io
INDEX.
History, 224, 225
Homatropin, 91, 213, 214, 215, 216
Homonymous images, 178
How to refract. (See Chapter IX.)
Hyoscyamin, 213
Hyperesophoria, 182
Hyperexophoria, 182
Hypermetropia, 36, 106
Hyperopia, 36, 71, 106, 244, 245,
246, 247
absolute, 109
acquired, 278
amount of, 72, 121
axial, 105
causes of, 108
description of, 106, 107, 108
diagnosis of, 112, 113
estimation of, 247
facultative, 108
latent, 109
length of eyeball in, 106, 247
manifest, 109
relative, 109
symptoms of, no
total, 109
treatment of, 237, 245
Hyperopic astigmatism, 126, 127,
128
Hyper phoria, 179, 186, 194
Hypertropia, 182, 196
I.
ILLITERATE card, 75, 76
Illiterates, 75, 76
Illuminated area. (See Figs. 86,
87, 88.)
Illumination, 162
facial, 162
retinal, 162
Images, 177
catroptric, 9
crossed, 179
formation of, 38, 39, 40
formed by mirrors, 14, 15, 16
heteronymous, 179
homonymous, 178
in astigmatism, 126 to 129
in emmetropia, 65, 100, 101
in hyperopia, 65
in myopia, 65
in retinoscopy, 164, 165
Images, inverted, 17, 99, TOO
on cornea, 162
on lens, 162
real, 16, 38
retinal, 61, 98
size of, 61, 62, 64
virtual, 16, 18, 38
Imbalance, 182
Inch system, 41, 42, 43
Index of refraction, 20, 22
Indirect method, 99, 100, 155
Infinity, 35, 68
Infraduction, 181
Insufficiencies, 183, 184
Intensity of lids, 9, 10
Interval, focal, 123
of Sturm, 123
Inversion, 14
Iris in accommodation, 68
in hyperopia, 112
in myopia, 118
Irregular astigmatism of the cornea,
125
of the lens, 124
JACKSON, 209
K.
KERATOMETER, 134, 152
Koratoscope, 134
Kindergarten card, 76
L.
LENGTH of eyeball, 59
in emmetropia, 50
in hyperopia, 106
in myopia, 106
in standard eye, 105
Lens, crystalline, 68
Lenses, 29, 30, 31, 297, 298, 299
acromatic, 286
action of, 31, 32, 33, 35, 36, 37,
38, 39, 40, 41
astigmatic, 123
biconcave, 31, 55
biconvex, 30, 54
bifocal, 283, 284. 285, 286, 287
INDEX.
Lenses, collective, 30
combination, 47, 48. 49, 50, 51,
5 2 . 53. 54
crystal, 288
cylindric, 29, 43, 56, 57
decentered, 295, 296
dioptric, 41, 42, 43
inch, 41, 43
magnifying, 30
meniscus, 30
minifying, 30, 31
negative, 30, 31
numeration of, 41, 42, 43
perimctric, 394
periscopic, 30
planoconcave, 31
planoconvex, 30
prismatic, 53, 54
spheric, 29, 30
spherocylindric, 45
tinted, 294
tone, 290
trifgcal, 295
Ligamentum pectinatum, 66
Light, 9, 91, 241
and shade, 79, 163
intensity of, 9, 10
-screen, 159
sense, 79
velocity of, 9
Lorgnettes, 289
Loring, 84, 88
M.
MACULA, 59, 85
Maddox rod, 188
Malingerer, 28
Manifest refraction, 234, 235
Meniscus, 30
Meridians, 132
Meter, 41, 42
angle, 83, 84
Metric system, 41, 42, 43
Mires, 152, 153
Mirror, 14
concave, 15, 16, 17
convex, 15, 17, 18
plane, 14, 158
reflection from, 14, 15, 16, 17
Mixed astigmatism, 128, 129
Movements of mirror, 161, 162
Mulatto, 162
Muscles. (See Chapter VII.)
ciliary, 66, 67
Miiller's, 67
picture of (frontispiece).
Mydriatics, 208
Myopia, 113
axial, 113
causes of, 115, 116, 117
description of, 113, 114, 115.
248
diagnosis of, nS, 119
estimation of, 249
image in, 65
length of eyeball in, 121
ophthalmoscopic appearances
in, 248
progressive, 242
symptoms of, 117, 118
treatment of, 238, 239, 240, 249
N.
NEAR point, So, 228
determination of, 80
Nebula, 265
Negative aberration, 173
angle, 87
Xerve, optic, 93
color of, 93
shape of, in astigmatism,
153
size of, in hyperopia, 96
in myopia, 97
Nettleship, 121
Neutralising lenses, 54, 55, 56, 57, 58
Nodal points, 36, 37
Nystagmus, 157
O.
OBSERVER, 91, 92
Occupation, 224
Ocular gymnastics, 203
Opacities, 125, 265
Ophthalmometer, 125, 149, 150, 151
Ophthalmoplegia, 216
Ophthalmoscope, 88, 113, 119
how to use, 90
luminous, 101, 102
Optic axis, 60, 85
(enter, 37, 54, 55
disc, 93
3 I2
INDEX.
Optics, 9
Orbit, 116
Orthophoria, 177
P.
PARALYSIS of accommodation, 216
causes of, 217
treatment of, 218
Pencil, converging, n, 12
diverging, 12
Perforated disc, 141
Perimeter, 201
Periodic squint, 196
Phenomena of light, 12
Phorometer, 190
Phorometry, 180
Pinc-nez, 292
Pin-hole disc, 47
Placido's disc, 134, 135
Pointed line test, 141
Points, cardinal, 59, 60
nodal, 37
of reversal, 163
principal, 60
Postcycloplegic, 235, 236
Pray's letters, 142
Presbyopia, 271, 272
age of, 272
causes of, 272
description of, 272, 273
diagnosis of, 273
glasses for, 274, 275, 276, 277
symptoms of, 273
Prescription writing, 53, 54
Principal axis, 15, 32
focus, 15, 32
points, 59, 60
Prism-diopters, 25, 26
exercises, 191, 192
rotary, 189
Wollaston, 149
Prisms, 23
action of, 23, 24
centrads, 25
neutralization of, 26
numeration of, 25
uses of, 28, 29
Punctum proximum, 69
determination of, 79, So, 81
in emmetropia, 69, 70
in hyperopia, 72
Punctum proximum in myopia, 73
remotum, 68, 69
determination of, 72
in emmetropia, 69
in hyperopia, 71, 72
in myopia, 72, 73
negative, 72
positive, 73
Pupil, size of, in emmetropia, 1 1
in hyperopia, 244
in myopia, 248
R.
RANDALL, 74
Range of accommodation, 69, 70
in emmetropia, 70, 71
in hyperopia, 71, 72
in myopia, 72, 73
of convergence, 85
Rays, 10, 32
convergent, n
divergent, 10, 95
emergent, 10
incident, 10
parallel, 10
reflected, 10
refracted, 10
Reflection, 13
by mirrors, 13
laws of, 13
Refraction, 18
applied. (See Chapter X.)
by cylinders, 43, 44
by prisms, 23, 24
by spheres, 31 to 41 inclusive
how to refract. (See Chapter
IX.)
index of, 22
laws of, 19
Regular astigmatism. (See Astig-
matism.)
Reisner, 173, 174
Retina, 69, 94
Retinal asthenopia. (See Astheno-
pia.)
illumination, 162
image in astigmatism, 123
in emmetropia, 65
in hyperopia, 65
in myopia, 65
reflex, 92
INDEX.
3'3
Retinoscopy. (See Chapter VI.)
Risley, 117,1 89
Rods and cones, 74
Rod test, 1 88
Room, 91
S.
SCHEINER'S test, 113, 119, 143, 144
Schematic eye, 156
Scissor movement, 172
Scopolamin, 213
Second sight, 276
Shadow test. (See Chapter VI.)
Shadows in retinoscopy, 163
Simple hyperopic astigmatism, 126
myopic astigmatism, i .-7
Snellen, 75
SnoW blindness, 220
Spasm of accommodation, 218
causes, 218, 219
symptoms, 219
treatment of, 219
clonic, 218
tonic, 218
Spectacles. (Sec Chapter XII.)
for adults, 298
for aphakia, 278, 279
for astigmatism, 292
for children, 298
for hyperopia, 295
for myopia, 300
for presbyopia, 295, 2oS, 2<)<;,
300
for strabismus, 290
measurements for, 300, 301
302, 303, 304, 305, 306
Spheres. (See Lenses.)
Squint. (See Strabismus.)
Standard eye, 59
Static refraction, 236, 237
Stenopeic slit, 135, 136
Stevens, 182, 189
Strabismometer, 200
Strabismus, 195
alternating, 196
amount of, 28, 199, 200, 201
angle of, 199
apparent angle of, 199
causes of, 196, 197, 198, K/;
concomitant, 196
Strabismus, constant, 196
convergent, 195
divergent, 196
monolaleral, 196
paralytic, 196
periodic, 196
treatment of, 28, 201, 202, 203,
204, 205, 206, 207
Sturm, interval of, 123
Supraduction, 181
Surfaces of cylinders, 43
of mirrors, 14, 15, 16, 17, 18
of prisms, 23
of spheres, 54, 55
sursumduction, 181
Symptoms of aphakia, 278, 279
of asthenopia, 219, 220
of astigmatism, 124
of hyperopia, 1 10
of myopia, 117, 118
of presbyopia, 273
T.
TABLE of amplitude of accommo-
dation, 70
of axial length of eyeball, 121
of indexes, 22
of lenses, 43
of near points, 272
of prisms, 27
Targets, 152, 153
Teno'omy, 124, 194, 195, 206, 207
Test lor aphakia, 278, 279
for astigmatism, 133
for hyperopia, 112, 113
for malingering, 28
for muscles. (See Chapter
VII.)
for myopia, 118, 119
for near point, Si, 82
for vision, 78, 79
-letters, 74, 75, 76, 77
-type, 81, 82
Thomson's ametrometer, 113, 119,
148, 149
Tinted glasses, 294
Toric, 290
Trial-case, 45, 46
Trial-frame, 47, 48
Trifocals, 295
314 INDEX.
V. Visual axis, 60
VACUUM, 9, 21 normal, 63, 64
Virtual focus, 12, 35
images, 12, 35
Vision, acutencss of, 62, 63, 78, 79 -^r
binocular, 177
determination of, 73, 74, 77, 78 WALLACE, 75
Visual acuity, 62, 63, 77, 78, 79 Wollaston, 149
angle, 61, 62 Worth, 203, 204
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