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OINTARlO

COLLEGE OF PHARNIACY

A-A GERRARD ST. E.
TORONTO,

ONTARIO

COLLEGE OF PHARMACY

44 GERRARD ST. E.
TORONTO,

REFRACTION

HOW TO REFRACT

THORINGTON

BY THE SAME AUTHOR.

Retinoscopy (The Shadow Test) in the Determination
of Refraction at One Meter Distance witli the Plane
Mirror. 38 Illustrations, a number of which are in
Colors. Third Edition. Jus^ /^eady. Cloth, net, fi.co

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 he 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."

REFRACTION

AND

HOW TO REFRACT

INCLUDING SECTIONS ON OPTICS, RETINOSCOPY, THE
FITTING OF SPECTACLES AND EYE-GLASSES, ETC.

JAMES THORINGTON, A.M., M.D.,

ADJUNCT PROFESSOR OF OPHTHALMOLOGY IN THE PHILADELPHIA POLYCLINIC AND COLLEGE

FOR GRADUATES IN MEDICINE ; ASSISTANT SURGEON AT WILLS* EYE HOSPITAL ; ASSOCIATE

MEMBER OF THE AMERICAN OPHTHALMOLOGICAL SOCIETY ; FELLOW OF THE

COLLEGE OF PHYSICIANS OP PHILADELPHIA; MEMBER OF THE AMERICAN

MEDICAL ASSOCIATION ; OPHTHALMOLOGIST TO THE ELWYN AND THE VINE-

^ ^k LAND TRAINING SCHOOLS FOR FEEBLE-MINDED CHILDREN ; RESIDENT

.nS

PHYSICIAN AND SURGEON PANAMA RAILROAD CO. AT COLON
(aSPINWALL), ISTHMUS OF PANAMA, 1882-1889, ETC.

TWO HUNDRED ILLUSTRATIONS

THIRTEEN OF WHICH ARE COLORED

ONTAt-iiO

COLLEGE OF PHARMACY

44 GERRARD ST. E.

TORONTO,
PHILADELPHL\
P. BLAKISTON'S SON & CO.

IOI2 WALNUT STREET
1900

WM. F. FELL & CO.,

Electrotvpers and Printers,
1220-24 sansom street, philadelphia.

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 the standard 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

VI 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. H.
Murphy, of Philadelphia, and to Dr. J. Ellis Jennings, of St.
Louis, Mo., for many valuable suggestions.

120 S. i8th St., Philadelphia, Pa.

November, i8gg.

CONTENTS.

CHAPTER I.

PAGE

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, 58

CHAPTER III.
Ophthalmoscope. — Direct and Indirect Methods, 86

CHAPTER IV.
Emmetropia. — Hyperopia. — Myopia, loi

CHAPTER V.
Astigmatism, or Curvature Ametropia. — Tests for Astigma-
tism, 120

CHAPTER VI.
Retinoscopy, 154

CHAPTER VII.
Muscles, 172

CHAPTER VIII.
Cycloplegics. — Cycloplegia. — AsTHENoriA. — Examination of

THE Eyes, 200

vii

Vin CONTENTS.

CHAPTER IX.
How TO Refract, 220

CHAPTER X.
Applied Refraction, 233

CHAPTER XI.
Presbyopia. — Aphakia. — Anisometropia. — Spectacles, .... 260

CHAPTER XII.
Lenses, Spectacles, and Eye-glass Frames. — How to Take
Measurements for Them and How They Should be
Fitted, 285

INDEX, 295

LIST OF ILLUSTRATIONS.

FIG. _ PAGE

1. Reflection, 12

2. Reflection from Plane Minor, 12

3. Lateral Inversion, 13

4. Reflection from Concave Minor, 14

5. Erect Image P'ormed by Concave Mirror, 15

6. Inverted Image Formed by Concave Mirror, 16

7. Image Fonned by Convex Mirror, 17

8. Perpendicular to Plane Surfaces, 18

9. Refraction, 18

10. Critical Angle, 19

II and 12. Angle of Refraction, 20

13. Density, 20

14. Index of Refraction, 21

15. Maximum Deviation, 22

16. Minimum Deviation, 22

17. Angle of Deviation, 23

18. Displacement, 24

19. Centrad, 24

20. Prism Diopter, 24

21. Neutralization of Prisms, 25

22. Correction of Diplopia, 28

23. 24, and 25. Convex Lenses, 29

26, 27, and 28. Concave Lenses, 30

29. Peripheral Refraction Through a Convex Lens, 30

30. Peripheral Refraction Through a Concave Lens, 30

31. Parallel Rays Passing Through a Convex Lens, 31

32. Parallel Rays Passing Through a Concave Lens, 32

^^. Conjugate Foci, 33

34. Ordinary Foci, 34

35. Negative Focus, 35

36. Secondaiy Axes, 35

37. Optic Center, 36

38. Inverted Image Formed by a Convex Lens, . . 37

39. Erect Magnified Image Formed by a Convex Lens, 39

40. Image Formed by a Concave Lens, 39

41 and 42. Cylindric Lenses, 43

43. Cylinder Axis, 43

44. Parallel Rays Passing Through a Convex Cylinder, 43

45. Parallel Rays Passing Through a Concave Cylinder, 44

46. Trial-case, 45

X LIST OF ILLUSTRATIONS.

FIG. PAGE

47 and 48. Trial-frames, 46 and 47

49. Combining Sphere and Cylinder, 49

50, 51, and 52. Finding Optic Center of a Lens, 54

53 and 54. Finding Cylinder-axis, 55

54, 55, and 56. Action of a Cylinder, 55 and 56

57. Standard Eye, 59

58. Angle of View, 60

59 and 60. Size of Retinal Image, 61

61. Minimum Visual Angle, 62

62 and 63. Five-minute Angle, . . 63

64. Retinal Image in the Standard and Ametropic Eyes, 64

65. Crystalline Lens at Rest and Accommodating, 67

66. Accommodation, 68

67. Hyperopic Eye at Rest, 70

68. Myopic Eye at Rest, 71

69. Randall's Test-letters, 73

70. Wallace Test-letters, 74

71. Illiterate Card, 74

72. Gould's Test-letters, 75

73. Gothic Type for Testing Near Point, 79

74. Block Letters for Testing the Near Point, . . 80

75. Meter Angle of Convergence, 82

76. Angle Gamma, 83

77. Positive Angle Gamma, 84

78. Negative Angle Gamma, 85

79. Loring Ophthalmoscope, 87

80. Direct Ophthalmoscopy, 88

81. Emmetropia with the Ophthalmoscope, 94

82. Hyperopia with the Ophthalmoscope, 95

83. Myopia with the Ophthalmoscope, 96

84. Indirect Ophthalmoscopy, 98

85. Condensing Lens, 98

86. Emmetropia, loi

87. Emmetropic and Ametropic Eyes 23 mm. Long, 102

88. Hyperopic Eye at Rest, 105

89. Hyperopic Eye Refracted, 105

90. Parallel Rays Entering a Myopic Eye IIO

91. Myopic Eye at Rest, Ill

92. Myopic Eye Refracted, ill

93. Astigmatic Lens, 121

94. Simple Hyperopic Astigmatism, 124

95. Simple Myopic Astigmatism, 125

96. Compound Hyperopic Astigmatism, 125

97. Compound Myopic Astigmatism, 126

98 and 99. Mixed Astigmatism, 127

ICXD. Symmetric Astigmati.sm, 128

loi. Asymmetric Astigmatism, 128

102. Astigmatism with the Rule, 129

103. Astigmatism Against the Rule, 129

104. Placido's Disc, I33

105. Stenopeic Slit, 133

106. Green's Astigmatic Chart, 135

LIST OF ILLUSTRATIONS.

FIG. _ PAGE

107. Astigmatic Clock-dial, 136

108. Astigmatic Clock-dial in Black, 137

109. Author's Pointed Line Test, 139

1 10. Perforated Disc, 140

111. Pray' s Letters, 140

112. Scheiner's Disc, 141

113. Scheiner's Disc in Hyperopia, 141

114. Scheiner's Disc in Myopia, 142

115 and 116. Cobalt-blue Glass, 143

117. Refrangibility of Cobalt-blue Glass, 144

118 to 129, inclusive. The Diagnosis of the Different Forms of Ametro-
pia with Cobalt-blue Glass, 145

130. Thomson's Ametrometer, 147

131 and 132. Ophthalmometer, 148 and 149

133 and 134. Mires or Targets, 150

135. Indirect Ophthalmoscopy, 153

136. Author's Schematic Eye, 154

137. Point of Reversal, . 155

138 and 139. Author's Mirror with Folding Handle, 156

140. Author's Iris Diaphragm Chimney, 157

141. Position of Light and Mirror, 158

142. High Myopia as Seen with the Concave Minor, 159

143. Hyperopia as Seen with the Concave Mirror, 159

144 and 145. Rate of Movement of Retinal Illumination in Hyperopia

and Myopia, 162 and 163

146. Retinal Illumination in Emmetropia, 164

147. Band of Light, 167

148. Axonometer, 168

149. Scissor Movement, 170

150. Positive Aberration, 171

151. Negative Aberration, 171

152. Homonymous Diplopia, 173

153. Heteronymous Diplopia, 174

154 and 155. Maddox Rods, 182

156. Rotary Prism of Risley, 183

157. Phorometer, 184

158. Strabismometer, I94

159. Angle of Deviation in Strabismus, 195

160. Monocular Blinder, 197

161. Aphakia, 267

162 and 163. Franklin Bifocals, 273

164. Merck's Bifocals, 273

165 to 171, inclusive. Cement Bifocals, 274 and 275

172 and 173. Acromatic Bifocals, 275

174 and 175. Solid Bifocals, 276

176 to 180, inclusive. Half Lenses, 277

181. Toric Lenses, 278

182 to 191. inclusive. Different Sizes and Shaped Lenses, . . 283 and 286

192. Measuring Interpupillary Distance, 289

193 and 194. Fitting of Spectacle Bridge, 290

195. Measurement of Bridge, 291

196. Measurement for Spectacles, 292

XU LIST OF ILLUSTRATIONS.

FIG.

197. Measurement for Eye-glasses, 29

198. Distance Frames

PAGE

293

199. Near Frames, 293

200. Measurement for Guards, 293

REFRACTION

HOW TO REFRACT.

CHAPTER I.

OPTICS.

Optics (from the Greek Zr^ropM, meaning "to see") is
that branch of physical science which treats of the nature
and properties of hght.

Catoptrics (from the Greek xdruirrfjir^, meaning " a mir-
ror") and dioptrics (from the Greek <5£'""/'<'v, meaning
" to see through ") are subdivisions of optics ; the former
treating of incident and reflected rays, and the latter of the
refraction of light passing through different media, such as
air, water, glass, etc., but especially through lenses.

Light. — Light may be defined as that form of energy
W'hich, 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
in all directions and cross one another as they diverge from
their respective points. The intensity of light decreases

10 REFRACTION AND HOW TO REFRACT.

as the square of the distance from the hght 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.

Ray. — Ray (from " radius ") is used in optics in prefer-
ence to wave, and means the smallest subdivision of light
traveling in a straight line. Rays of light are considered
as incident, emergent, reflected, refracted, divergent, par-
allel, and convergent.

Incident Rays. — Rays of light are said to be incident
when they strike the surface of an object.

Emergent Rays. — Rays of light are emergent when
they have passed through a transparent substance.

Reflected Rays. — Rays of light are reflected when they
rebound from a polished surface.

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.

Parallel Rays. — The greater the distance of an)- lumin-
ous point, the more nearly do its rays approach to paral-
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, ra}-s of light which
proceed from a distance of six meters or more are spoken of
as parallel, although this is not an absolute fact, as ra)-s of
light at this distance still maintain a slight anunnit of di\-er-
gcnce. If the pupil of the emmetropic e)'e is represented
by a circular opening four millimeters in diameter, then
rays of light from a luminous point at si.x meters (6000

OPTICS. 1 1

mm.) will have a divergence of -q^\-q to enter such a
pupil.

Convergent Rays. — Convergent rays are the result of
reflection from a concave mirror or refraction through a
convex lens.

A Beam. — This is a collection or series of parallel rays.

A Pencil. — A pencil of light is a collection of conver-
gent or divergent rays. Convergent ra}'s are those which
tend to a common point, whereas divergent rays are those
which proceed from a point and continually separate as they
proceed. This point is called the radiant point.

A Focus. — This is the point of a convergent or diver-
gent pencil ; the center of a circle ; the point to which
converging rays are directed.

A Positive or Real Focus. — This is the point /o ivJiich
rays are directed after passing through a convex lens or
after reflection from a concave mirror.

A Negative or Virtual Focus. — This is the point from
which rays appear to diverge after passing through a con-
cave lens, or after reflection from a convex mirror, or after
refraction through a convex lens w4ien the light or object is
closer to the lens than its principal focus, or after reflection
from a concave mirror when the light or object is closer to
the mirror than its principal focus.

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 ra)'s 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 only transmits that portion of the light's rays whose
combined effect upon the retina is that of red. The relative

REFRACTION AND HOW TO REFRACT.

proportion of absorption and reflection of rays of light
greatly depends upon the quality of the surface — whether
light colored or polished, or dark colored or rough.

Reflection. — From the Latin rcflcctcrc, " 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.
While 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.

Fig. 2.

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 tlic angle of incidence. (2) The reflected and in-
cident rays arc in the same plane with the perpentiicular to
the surface. (See Fig. i.)

If A B represent a polished surface and I the incident
ray, tlien P D I is the angle of incidence ; R being the re-

OPTICS.

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. 2.)

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.
60.) The apparent dis-
tance of the object I from the observer is equal to 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 lateral inversion. 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. 3.) 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

E F

^ 3

LEG

03J

T ON

HOIT

Fig. 3. — Lateral Inversion.

REFRACTION AND HOW TO REFRACT.

moving in the same 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. 4). — 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.F.). The principal axis of a concave mirror is a
straight line drawn from the center of the mirror to the

Fig. 4.

center of curvature (i-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). 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. — To find the
positi(Mi 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., P'igs. 5 and 6);
the other, the secondary axis, from the same point, passing

OPTICS.

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. Unhke the plane
mirror, which produces images at all times and at all dis-
tances, the concave mirror produces either an erect, virtual,
and enlarged image, as 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 imacfe formed.

Fig. 5.

Figure 5 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.F. 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 ;- behind the mirror, forming a magnified image of
A R. If the mirror is withdrawn from the object, the
erect magnified imag-e would increase in size, but at the
principal focus no image would be formed, as the rays
would be reflected parallel.

REFRACTION AND HOW TO REFRACT.

Figure 6 shows a real inverted image of A R at ^ 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. When 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. 6.

object is situated between the center of curvature and the
principal focus. When 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
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 ra)\s
diverge as if they came from the principal focus situated
behind the mirror at a distance equal to one-halt its radius

OPTICS.

of curvature. The principal focus of a convex mirror is
therefore negative. The foci of convex mirrors are virtual.

Images Formed by a Convex Mirror. — These are
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 object does not change position.

In figure 7 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

^.. — t:^''^'

r'"'

c _— — — ^-

-^~-p-p.------J

k-c-

..1^ _:

/ '

Fig. 7.

secondary axes, and 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
image is virtual and erect.

Refraction. — From the Latin rcfrangcrc, meaning "to
bend back " — /. c, 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.

1 8 REFRACTION AND HOW TO REFRACT.

Two laws govern the refraction of rays of light :

1. A ray of Hght 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
densities, so long as it forms right angles with the surface

ICE

FLINT GLASS

CROWN

PLATE

Fig. 8.

or surfaces. Such a ray is spoken of as the normal or
perpendicular ; such surfaces are plane, the surfaces and
perpendicular forming right angles. (See Fig. 8.) In any
case of refraction the incident and refracted ra}^s may be
supposed to change places.

Figure 9 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 1" 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 tlic
ray in tlie glass comes to the second surface at R, and

OPTICS.

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
size of the critical angle depends upon the index of refrac-
tion of different substances. Figure 10 shows an electric
light suspended in water. The ray from this light which
forms an angle of 48° 35' with the surface of the w^atcr

Fig. 10. — 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 of refraction is the
density or refractive power of any substance as compared

REFRACTION AND HOW TO REFRACT.

with a vacuum. According to the first law of refraction, a
ray of Hght 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. In the study of the compara-
tive density of any substance it
will be seen that the angle of re-
fraction is usually smaller the
more dense the substance ; this
is well illustrated in figures ii
and 12.

The greater the density, the
slower the velocity or the mere
effort apparently for the wave or ray to pass through
the substance. This is illustrated in figure 13, 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

rmrf]

Fig. II.

Fig. 12.

Vacuum

Air

Glass Diamond

Fig. 13.

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 inde.x of refraction of any
substance as compared with a x-acuum or air, it is neces-
sary to divide the sine of the angle of incidence by the sine
of the antrle of refraction.

OPTICS.

In figure 14 the angle of incidence I C P is the angle
formed by the incident ray I with the perpendicular, P P.
The angle of refraction R C P 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 then
drawing the sines D
X and B F, perpen-
diculars to the per-
pendicular P P, divide
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

C ]

/ Air \

\ Water

3 2 l\^/

_>\T^

Fig. 14.

1.33 -|-, 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 y^, or 0.75.

Indexes of Refraction.

Air, 1.00029

Water, 1. 333

Cornea, ^Zm

Crown glass, 1-5

Flint glass, i-S^

Crystalline lens, nucleus, 1-43

" " intcrniecliatc layer, I.4I

" " cortical layer, 1-39

REFRACTION AND HOW TO REFRACT.

A prism is any refracting substance bounded by plane
surfaces which intersect each other. The sides of a prism
are the incHned 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

Fig. 15.

Fig. 16,

nose ; and base out means that the thick part of the prism
is toward the temple.

Prismatic Action. — Rays of light passing through a
prism are always refracted toward the base of 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 i 5 the incident ray (I) is perpendicular to the
surface A B, and is not refracted until it comes to the sur-

OPTICS. 23

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
angle with the first surface of the prism, then it will be re-
fracted twice — as it enters and as it leaves the prism.

In figure 16 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
the surface A C, and in the direction of the base of the prism.
If the incident ray (I) so falls upon the surface at A B that
the refracted ray (N E) is parallel to the base (B C), then
the emergent ray is such that the angle of emergence
equals the angle of incidence (I N X) ; as in this instance the
angles of incidence and of emergence
are equal, the deviation, therefore, is
at a minimum, or the least possible.

Angle of Deviation (Fig. 17). —
This is the angle formed between
the directions of the incident and
emergent rays, and measures the total
deviation. In all prisms of ten degrees p,,^, i7_

or less the angle of deviation is equal

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.

Effect of a Prism. — An object viewed througli a prism
has the appearance of being displaced, and in a direction
opposite to the base — /. c, toward the aj^ex.

Ra}'s from the object (X, Fig. 18) strike the prism at C.

REFRACTION AND HOW TO REFRACT.

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 position of X is changed

to X', azuay from the base of the

prism and totvard the apex.

Numbering of Prisms. — Form-
erly, prisms were numbered by their
refracting' anorles ; now, howev^er,
two other methods are in use :
Dennett's method, known as the
centrad ; and Prentice's method,
known as the prism-diopter.

Dennett's Method (Fig. 19). — The unit, or centrad (ab-
breviated V ), is a prism that will deviate a ray of light the
y^Q- 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. 18.

Radius
Fig. 19.

prism, base down, at the center of curvature that will devi-
ate a ray of light downward just yj,y part of the arc of the
radian is a one centrad, and cciuals j-,^, |y of 57--95 degrees,
or 0.57295 of a degree.

OPTICS.

98765432

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. 20). — The unit, or prism-diopter
(abbreviated P.D.,or A), is a prism
that will deflect a ray of light
just I cm. for each meter of dis-
tance— that is, the y^-g- part of
the radius measured on the tan-
gent. The deflection 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-
flected by centrads and prism -
diopters up to 20, in the ratio of
I cm. for each meter of distance,
then to 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 2 1 shows a
series of vertical lines I3 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 y^ of a meter (as
3

Fig. 21.

REFRACTION AND HOW TO REFRACT.

the lines are ^3 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).

Centr

ADS. Prism-diopters.

Refracting Angle.

I°.0O

2.000I

2°. 12

3-0013

3°.i8

4.0028

4°-23

5 -0045

5°. 28

6.0063

6°. 32

7.0115

7°-35

8.0172

8°.38

9.0244

9°-39

10.033

10°. 39

11.044

11°. 37

12.057

12°.34

13-074

i3°.29

14.092

I4°.23

15.114

15°. 16

16.138

16°. 08

17,164

16°. 98

18.196

i7°.85

19.230

i8°.68

20.270

i9°-45

25-55

23°.43

30.934

26°. 81

36.50

29°. 72

42.28

32°. 18

48.30

34°. 20

54-514

35°-94

68.43

38°.3i

S4.22

39°- 73

102.96

40°. 29

126.01

40°. 49

100

155-75

39°- 14

Or a prism ma\' be neutralized by placing another prism

in apposition to it, with their bases opposite, so that in look-

OPTICS. 27

ing through the two prisms at a straight Hne, no matter at
what distance, the straight edge 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 arm.ed (without the 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 muscles. For example : A patient looking at a
point of light focused on the macula (M) of the left eye
(L), and the right eye being turned in toward the nose,
receives the rays upon the nasal retina, 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.
22.)

REFRACTION AND HOW TO REFRACT.

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 2j4
P. D. The strength of the prism placed base in which.

Fig. 22.

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. 185.)
Lenses. — A lens is a portion of transparent substance
(usually of glass) having one or both surfaces curved.
There are two kinds of lenses — spheric and cylindric.

OPTICS. 29

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. 23, 24, 25.) The following are synony-
mous 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. —

1. Planoconvex, meaning one surface flat and the other
convex. It is a section of a

sphere. (See Fig. 23.)

2. Biconvex, also called con-
vexoconvex or bispheric, for the
reason that it is equal to two
planoconvex lenses with their
plane surfaces together. (Fig.

-4-j Fig. 23. Fig. 24. Fig. 25.

3. Concavoconvcx. This lens

has one surface concave and the other convex, the convex
surface having the shortest radius of curvature. (Fig. 25.)
The following are synonymous terms for a concavocon-
vcx lens : (i) Periscopic ; (2) convex meniscus ; (3) con-
verging meniscus (meniscus meaning a small moon). (See
Fig. 25.) 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. 26, 27, 28.) The fol-
lowing 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 ( — ).

REFRACTION AND HOW TO REFRACT.

Varieties or Kinds of Concave Lenses. —

1. Plajioconcave, meaning one surface flat and the other
concave. (Fig. 26.)

2. Biconcave, also called concavoconcave or biconcave

spheric, for the reason that it is
equal to two planoconcave lenses
with their plane surfaces to-
gether. (Fig. 27.)

3. Convcxoco7icave. This lens
has one surface convex and the
other concave, the concave sur-
face having the shortest radius
of curvature. (Fig. 28.) The
following are synonymous terms for a concavoconvex lens :
(i) Concave meniscus ; (2) diverging meniscus ; (3) peri-
scopic.

Fig. 26. Fig. 27. Fig. 28.

Fiu. 29.

Fig. 30.

A spheric lens may be considered as made up of a scries
of prisms which gradually increase in strength from the
center to the periphery, no matter whether tlie lens be con-
cave or convex.

OPTICS.

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. 29, 30.)

Knowing that a prism refracts rays of hght toward its
base, it may be stated as a rule that every lens bends rays
of light more sharply as the periphery is approached — /. 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 3 i passes through the bicon-
vex lens unrefracted, or without any deviation from its
course whatsoever, for at its points of entrance and emer-

FiG. 31.

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 is called the principal axis.
The axis of a planoconvex or planoconcave lens is the line
drawn through the center of curvature perpendicular to the
plane surface.

The ray B in figure 31, 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.F". The rays C, D, and E, also parallel to A and B,
form progressively larger angles with the surface of the lens,

A-^

32 REFRACTION AND HOW TO REFRACT.

and finally meet the axial ray at P.F. It will be seen at
once that the rays all meet at P.F., showing the progres-
sively stronger prismatic action that takes place as the per-
iphery of the lens is approached.

In figure t,2 we have similar rays, A, B, C, D, and E,

passing through a con-
cave lens. The axial ray
A passes through the cen-
ters of curvature unre-
fracted, but the ra3^s B,
C, D, and E are progres-
sively refracted more and
Fig. 32. more as the periphery is

approached. The ray E
in each instance is refracted the most.

T/w action of a cotn'cx lens is similar to that of a concave
mirror, a)id the action of a concave lens is similar to tJiat 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, therefore, has two principal foci, an
anterior and a posterior, according to the direction from
which the parallel rays come, or as to which radium of cur-
vature is referred to. iMgure 31 shows parallel raws, B,
C, D, and E, pas.sing 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

OPTICS. 33

matter what its direction, then if a point of hght 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 33 the rays diverging from A and passing

Fig. ^i.

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-
gence is at twice the distance of the principal focus. The
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 Hght from the fovea of such an

34 REFRACTION AND HOW TO REFRACT.

eye would be projected convergently after passing out of
the eye, and would meet at some point inside of infinity.
In other words, only rays which have diverged from some
point inside of six meters will focus upon the fovea of this
long eye. The fovea of the myopic eye represents a con-
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. 34.

foci. 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. This
point is the principal focus. (See Fig. 34.) 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 di\'ergenll\' ; 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

OPTICS.

35
(See

side of the lens from which they appeared to come.

Fig- 35-)

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

——-P.P.

Fig. 35.

the fovea of such an eye would project outward divergent
rays, and would only be in a position to receive 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-

FlG. 36.

faces, it was found that it underwent lateral displacement
(see Fig. 9), and so in lenses there is a place where ra}-s
undergo lateral displacement. Figure 36 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

36 REFRACTION AND HOW TO REFRACT,

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^ in the lens on the axial ray, and
the emergent ray, if projected backward, would meet the
axial ray at N'^. 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^ and N^ are
spoken of as nodal points. 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. 37.

one nodal point is recognized. It is spoken of as the
optic center.

Optic Center. — This term is used synonymously with
nodal point, and is the point where the secondary rays (s.(7.
in F"ig. 37) cross the axial ray. It is not always the geo-
metric center. Rays of light crossing the optic center in thin
lenses are not considered as undergoing refraction. (See

i^'^S- 37)

Action of Concave Lenses. — Rays of light passing
through a concave lens, no matter from what distance, are
always refracted div^ergently, and its focus is, therefore,
always negative or virtual, and is found by projecting tliese

OPTICS.

divergent rays backward in the direction from which they
came until they meet at a point on the axial ray. The
principal focus and conjugate foci of concave lenses are
found in the same way as in convex lenses. (See Figs. 32,
40.)

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. 38.

To find the position and size of an image it is necessary
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. Only 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 38 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

38 REFRACTION AND HOW TO REFRACT.

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, an object ten millimeters high, three meters
(3000 mm.) from the optic center of a lens, and its image
situated sixty millimeters from the lens : the image will be
-gll^ or Jq- of the size of the object ; the image will be ^L- of
ten millimeters (the height of the object)— namely, 3- of a
millimeter high.

As conjugate foci are interchangeable, then in figure 38
if A' B' was the object, the image A B would be the image
of A' B', and, therefore, smaller than the object.

Three facts should be borne in mind in the study of real
images formed by a convex lens :

1. The object and image are interchangeable.

2. The object and the real image are on opposite sides of
the lens, and,

3. As the rays which pass through the optic center
cross each other at this point, the real image must be in-
verted.

Rays of light from an object situated at the distance of
the principal focus would proceed parallel after refraction,
and no image of the object would be obtained.

If an object is situated just beyond the principal focus,
tlicn the image would be larger than the object, real and
inverted. (See Fig. 38, 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,

OPTICS.

inverted, and at a corresponding distance, as these conju-
gate foci are equal.

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. 39.

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

R
A"

^^^A

^ — -4^^::C"

/____

__________

---'■'''R^^^^^^

r''^^^^-^ — — ^

-^^"^^~~-r'

Fig. 40.

virtual. Such an image is only seen by looking through
the lens ; the lens in this instance being a magnifying
glass. (Fig. 39.)

Images Formed by Concave Lenses. — These images are
always erect, virtual, and smaller than the object. (Sec
Fig. 40.) A concave lens is, therefore, a minifying lens.

40 REFRACTION AND HOW TO REFRACT.

Parallel rays from the extremes of the object A R 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
seen 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 about the distance of one English inch
(25.4 mm.) from its optic center.

As lenses for purposes of refraction were never so 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 i^, or a lens that was one-
sixteenth the strength of the unit was expressed as Jg, 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 — yi^ + ^
_|- _!_; (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.), a 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.

Len.ses in the dioptric system arc numbered according
to their refractive power and not according to their radii of

OPTICS. 41

curvature. The strength or refractive power of a dioptric
lens is, therefore, the inverse of its focal distance. To find
the focal distance of any dioptric lens in inches or centi-
meters, the number of diopters expressed must be divided
into the unit of 40 inches or 100 cm. For example, a
2 D. lens has a focal distance of 40 -^ 2 equals 20 inches ;
or 100 cm. -^ 2 equals 50 cm. A +4 D. has a focal
distance of 40 -^ 4 equals 10 inches or 100 h- 4 equals 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 units : 0.25 D. has a
focal distance of 40 h- 0.25 or 100 -^ 0.25, equaling 160
inches or 400 cm. ; 0.50 D. has a focal length of 40 -^
0.50 or 100 -^ 0.50, equaling 80 inches or 200 cm. ; 0.75
D. has a focal length of 40 -=- 0.75 inches or 100 ^ 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, yV equals f |
equals 4 D.; Jq equals f||- equals 2 D. The following
table, from Landolt, gives the equivalents in the old and
new systems :

REFRACTION AND HOW TO REFRACT,

OLD SYSTEM.

NEW

SYSTEM.

II.

III

IV.

VI.

VII.

VIII.

No.

of the

Focal

of the

Focal

Corres-

Lens,

Distance

Equiva-

Lens,

Distance

ponding

Old

in English

in Milli-

lent in

New

in Milli-

in English

of the Old

System.

Inches.

meters.

Diopters.

System.

meters.

Inches.

System.

67.9

1 7 -'4

0.58

0.25

4000

157 48

166.94

56.6

1437

0.695

0.5

2000

7874

8346

45-3

1 150

0.87

075

1333

52-5

5563

39-6

1005

0.99

lOUO

39 37

41-73

863

I 16

■•25

800

3' 5

33 39

28.3

718

I 39

1-5

666

26.22

2779

24 ■

22.6

574

1.74

1-75

571

22.48

23-83

18.8

477

2.09

500

19.69

20.87

431

2.31

2.25

444

1748

1853

2.6

2-5

400

15-75

16.69

14. 1

358

2.79

333

13 '7

13-9

13.2

335

2.98

3-5

286

II 26

11.94

12.2

312

3.20

250

9.84

10.43

II. 2

287

3-48

4 5

222

8.74

926

10.3

261

382

200

7.87

8-35

9-4

239

4.18

5-5

182

7 16

8.5

216

4-63

166

654

693

7-5

190

5-2.S

143

563

5 97

7 ,

6.6

167

5-96

125

492

5.22

6J^

6.13

155

6.42

111

4-37

4-63

5-6

142

7.0

100

3-94

4 17

5 2

132

7-57

3-58

3«.

4.7

119

8.4

3-27

3-46

^^.

106

9-4

3-03

3.21

3-8

10 4

2.8

2.q6

3-3

11.9

2.64

2.8

3-1

12.7

244

2.59

2.8

14.0

2.32

2.46

23/i

2.6

I5-I

2 17

2.29

2.36

17.7

1.97

2.09

25i

2.1

18.7

1.88

20.94

Cylindric Lenses. — Abbreviated cyl., c, or C. A cylin-
dric lens, usuall}' called a " cylinder," receives its name from
being a segment of a cylinder parallel to its axis. (See
Fig. 41.) Occasionally cylinders are made with both sur-
faces curved, and are then equivalent to two planocylinders
with their j^lane surfaces together. A cyliiidi-r i/iay be
defined as a /ens wJneJi refraets rays of lii^ht opposite to its
axis. This definition should be carefully borne in mind in
contradistinction to a spheric lens, which refracts ra\'s of
light equally in all meridians. A cj'liiulric lens has no one

OPTICS.

common focus or focal point, but a line of foci, which is
parallel to its axis.

Axis of a Cylinder. — That meridian of a cylindric lens
which is parallel to the axis of the original cylinder of

Fig. 41.

Fig. 42.

Fig. 43.

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. 44.

its surface corresponding to the axis ground at the edges,
or it may be marked in both ways. (See Fig. 43.) Cylin-
ders are of two kinds — convex and concave. (Figs. 41, 42.)
Cylinder Action. — A convex cylinder converges parallel

REFRACTION AND HOW TO REFRACT,

F-c:

FiG. 45.

rays of light so that after refraction they are brought into
a straight hne which corresponds to the axis of the cyHn-
der ; 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. 44.)

A concave cylin-
der diverges ra)'s of
light opposite to its
axis, as if they had diverged from a straight line on the
opposite side of the lens. (Fig. 45.)

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 spherocylinder lens is also spoken of as
an astigmatic lens. (See Fig. 93.) A spherocylinder lens
is one which has two focal planes. Spherocylinders have
different curves : the spheric curve may be convex, with the
cylinder surface convex ; or the spheric surface may be
concave, with the cylinder surface concave ; or the spheric
surface may be convex, with the c}'Iinder surface concave ;
or the spheric surfece may be concave, with the c}'linder
surface convex.

The Trial-case (see Fig. 46). — This case contains pairs
of plus and minus spheres and pairs of plus and minus
cylinders; also prisms numbered from 34 or 'i to 20 A.
The spheres are numbered in intervals of 0.12 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.

OPTICS.

the interval is i S. The cylinders have similar intervals,
but seldom go higher than 6 or 8 cyl.

The trial-case also contains a trial-frame, which is used
to place lenses in front of the patient's eyes. (See Fig. 47.)
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. 46.

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 laivcr /la/f, counting around to the
horizontal meridian, which at the right hand is numbered
180; this horizontal meridian is, therefore, spoken of as
horizontal, zero (o), or 180 degrees. The meridian midway
between zero and 180 is spoken of as vertical, or 90 degrees.
In some countries the meridians are differently num-

REFRACTION AND HOW TO REFRACT.

berecl (see Fig. 48) ; 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

Fic;. 47.

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-

OPTICS.

faces together, will equal one lens the value of their sum :
for example, +2 S. 3 -f- 1 S. ^ +3 S. will equal -f 6 S.;
or a — 2 S. :3 ^ — 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, +4 S. 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. 48.

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 number; for instance, +4 S. and — 2
S. will equal a +2 5., the — 2 S. neutralizing 2 S. of the
+ 4 S., leaving +2 S.

Combining Cylindric Lenses. — Any number of c)lin-
dric lenses placed together, with their axes in the Si7;j/c

48 REFRACTION AND HOW TO REFRACT.

meridian, are equal to a cylinder the value of their sum ;
for example : +2 cyl. axis 90 degrees and -f 3 cyl. axis 90
degrees will equal a -f 5 cyl. axis 90 degrees ; or — 2 cyl.
axis 180 degrees and — 3 cyl. axis 180 degrees will equal
a — 5 cyl. axis 180 degrees ; or — 2 cyl. axis 180 degrees
and +1 cyl. axis 180 degrees will equal a — i cyl. axis
180 degrees.

As a cylinder refracts rays of light only in the meridian
opposite to its axis, this opposite meridian or axis can
always be found by the following simple rule :

" Add po ivJicn the given axis is po or less than po, and
stdytract po zvhen the given axis is more than po."

For example : + 3 cyl. axis 90 refracts rays of light in
the 180 degree meridian (90-1-90= 180); or -f 3 cyl.
axis 75 refracts rays of light in the 165 meridian (75 -[-90
= 165). A — 3 cyl. axis 135 refracts ra)'s 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 c)l. axis 90
and +3 cyl. axis 180, placed together, will equal a -|-3 S.
— /. e., the +3 cyl. at axis 90 will converge parallel rays in
the 180 meridian, while the +3 cyl. axis 180 will converge
parallel rays in the 90 meridian, producing a principal
focus ; therefore any sphere is also equal to two c}-linders
of its same strength and same denomination with tlieir
axes at riglit angles to each other.

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. I'or example :

OPTICS.

-\-2 cyl. axis 75 O +3 cyl. axis 165 will equal +2 S. O
-(-I cyl. axis 165. The +2 cyl. axis 75 takes +2 of the
4-3 cyl. axis 165 and makes a +2 S., leaving -f- 1 cyl. axis
at 165 ; the result is then +2 S. O +1 cyl. axis 165.

Or ^3.50 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 165 ; this — i cyl. axis 105 is now
joined to the — 3.50 sphere, making — 3.50 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 : -(- 1
sphere O — i cyl. axis 180
will equal + I cyl. axis 90.
The + I S. equals two -\~ i
cylinders, one at axis 90
and one at axis 180, and
the — I cyl. at axis 180 is
neutralized by the -f i c}'l.
at the same axis, leaving

the + 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 1 80 ; the one at axis 90 is neutralized by
the +3 cyl. at the same axis, leaving — 3 C}-1. axis 180.

Combining a Sphere with a Weaker Cylinder of Dif-
ferent Denomination. — Such a combinati(in should be
5

+ 1.00

Fig. 49.

50 REFRACTION AND HOW TO REFRACT.

changed to its simplest form of expression, and will equal
a sphere of the same denomination, of the v^alue of their
difference, combined with a cylinder of the same strength
as the cylinder given, but of opposite sign and axis. For
example: +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 180 meridian of the plus 4 has
not been altered, but still remains +4, and the result is
there is plus 3 in the vertical meridian and plus 4 in the
horizontal meridian, equaling, therefore, +3 S. O + 1
cyl. axis 90.

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-
tion, no matter what their respective strengths may be :

Rule. — Subtract the less from the greater, and to the result
prefix the sign of the greater ; combine zviih this the same
strength cylinder, using the opposite sign and opposite axis.
Example : +2.25 S. O — 0.75 cyl. axis 75 degrees ; sub-
tracting the less from the greater ( — 0.75 from -)-2.2 5), 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 -I-0.75
cyl. axis 165. Result, +1.50 S. O +0.75 c}-l. 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 rever.se of the rule above given, and will equal
— 8 S. O -f- 6 cyl. axis 90.

OPTICS. 5 I

Combining Two Cylinders of Different Denominations
with Opposite Axes. — Commonly called cross cylinders.
Such combinations can be written in three ways :

1. +Cyl. 3 — cyl. axes opposite.

2. -|- Sphere 3 — cyl. (cylinder stronger than sphere).

3. — Sphere -(-cyl. (cyhnder stronger than sphere).

For exainple : ^i.oo cyl. axis 180 ;3 +2- 50 cyl. axis 90
may be changed to one of the following :

— 1. 00 S. 3 +3- 50 cyl. axis 90; or —
-I-2.50 S. 3 — 3-50 cyl. axis 180.

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
and third 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 (4-2.50 S. O
— 3.50 cyl. axis 180) the — 3.50 cylinder is only equal to
— I.OO cylinder, as it has — 2.50 neutralized by +2.50 of
the sphere.

/;/ any splicrocylindcr 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 — /. r., — 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 180 it equals
+ 2.50 (—1 and +3.50).

Cross cylinders in themselves are seldom ordered in a
prescription, preference being given to a spheroc}'linder
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 :

52 REFRACTION AND HOW TO REFRACT.

-|-0. 50 cyl. axis 90 O — 5.00 cyl. axis 180 equals +0.50
S. O — 5.50 cyl. axis 180 or — 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 -|- 3
cyl. axis 90 degrees, equals — i.oo S. 3: +4 cyl. axis 90,
or +3 S. 3 — 4 cyl. axis 180, and for similar reasons
preference would be given to the nii)ius sphere combination.
Whichever combination makes the thinnest and li<ihtest
weight 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,
Ociilus 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 -f 4 D. or -(-4.00 D. S. or -j-4 S. or -{4 sph.
— Sphere, written — 2 D. or — 2.00 D. S. or — 2 S. or — 2 sph.
-[-Cylinder, written -(-4.00 D. C. or -(-4 C. or -(-4 cyl. (axis as indicated).
. — Cylinder, written — 2.00 D. C. or — 2 C. or — 2 cyl. (axis as indicated).
-[-Sphere and -f cylinder, written -(-2.00 S. 3 -h2.cx3 cyl. axis 90 degrees.
— .Sphere and — cylinder, written — 2.00 S. 3 — 2.00 cyl. axis 180 degrees.
-(-Sphere and — cylinder (cylinder stronger than sphere), -(-2.00 S. 3" — 300

cyl. axis 180 degrees.
— Sphere and -(-cylinder (cylinder stronger than sphere), — 2.00 S. 3 4 3-00

cyl. axis 90 degrees.

A plus c)'lindcr and minus cylinder may be prescribed,
and, if so, their axes mu.st be at right angles to each other.
An occasional exception to this may be found in irrcgul.ir
astigmatism. Or a prism with its base indicated max- be

OPTICS.

added in any one of the foregoing formulas ; for example :
— 2 S. O — 2.00 cyl. axis i8o 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
same axis.

Prescriptions are never written for two cylinders at axes
other 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.

Recognition of Lenses.

A convex sphere is thick at the center and thin at the
edge. It has the power of converging rays of light ; hence,
if strong, it is a burning glass. Objects viewed through a
convex lens as it is moved before the eye, from left to
right and right to left or up and down, appear to move
in an opposite direction to that in which the lens is
moved. The weaker the lens, the slower the object
appears to move ; and the stronger the lens, the faster the
apparent movement of the object. A convex lens being a
magnifier, has the effect of making objects appear larger
and closer w hen it is moved away from the observer's eye ;
or if brought toward the eye, objects already enlarged
appear smaller and more distant.

To Find the Optic Center of a Convex Lens. — Look-
ing at a perpendicular straight line and passing a convex
lens before the eye from left to right has the effect of
displacing toward the right edge of the lens that portion of
the line seen through the lens (see Fig. 50). and as the
lens is slowly moved still further to the right, the displaced

REFRACTION AND HOW TO REFRACT.

portion of the line will finall}' coincide with the original
straight line, making one continuous line through the lens.
(See Fig. 51.) 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
two lines. (See Fig. 52.)

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 lens is moved.

A concave lens being a minifier, makes ob'ects appear

Fig. 50.

Fig. 51.

Fig. 52.

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.

OPTICS.

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. 53), 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 5 I.

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. 5:

Fig. 54.

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 axis, 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. 54.)

A Concave Cylinder. — When a concave cjlinder is
moved in front of the e)e in the direction of its axis, ob-

REFRACTION AND HOW TO REFRACT.

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. 55.) A circle viewed through a strong con-
cave cylinder appears as an oval with its long diameter cor-
responding to its axis. (See Fig. 56.) A circle viewed
through a strong convex cylinder appears as an oval with
its long diameter opposite to its axis. In place of using a

Fig. 55.

Fig. 56.

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. 52) 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. — Ha\ing 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

OPTICS.

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 mi" us 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 cor-
rects one meridian, a cylinder may be combined until the
other meridian is neutralized.

CHAPTER 11.

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, yet 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-
nea 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. 57). — Images formed upon the
retina are the result of refraction by three refracting sur-
faces and three refracting media. The refracting surfaces

CARDINAL POINTS. ^g

are the anterior surface of the cornea and the anterior and
posterior surfaces of the crystalline lens. The refractin*'-
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 — /. c, a
line drawn from the pole of the cornea to a point between
the nerve and fovea.

On the principal axis, therefore, are situated the anterior
and posterior principal foci, the anterior and posterior nodal

Fig. 57.

points, and the anterior and posterior principal points The
anterior principal focus is situated upon the optic axis
13.745+ m"i- ill 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 purposes in the study of the for-
mation of images. The first or anterior principal point is

6o REFRACTION AND HOW TO REFRACT.

situiited 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-I- 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. 58.

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.

58.)

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.

59-)

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
were guided alone by the size of the retinal image, it would

MINIMUM VISUAT. ANGLE.

judge erroneously ; but, by experience, distance and com-
parison of size are brous^ht into consideration.

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. 60.)

The nearer an object to the eye, the larger the visual

Fig. 59.

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 59, where arrow i, to be seen

Fig. 60.

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

62 REFRACTION AND HOW TO REFRACT.

limiting angle of vision. In figure 6i, 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 — /. c, 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. 6i.

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 6i 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 staiulartl or cinmetroj)ic
eye, and thus have some method of recortling numerically
any departure from this standard visual condition.

SIZE OF RETINAL IMAGES.

Fig. 62.

Fig. 63.

The standard acuteness of vision is the power of the eye to
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.
62.)

Figure 63 shows the
letter F drawn in a five-
minute square, and each stroke of the letter, and space
betAveen the strokes, measuring just one minute in width.
As twice the tangent of half the angle of five minutes is ex-
pressed by the decimal .001425, 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
1.425 mm. square (1000 X .001425). At six meters
(6000 X .001425) := 8.5 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 15 mm. in front of the retina; then an
object 8.5 mm. square situated 6000 mm. in front of the

* There are two letters in the alphabet which are exceptions to this nile,
L and O. L can be seen under an angle of two minutes and O can be seen
under an angle of three minutes.

64 REFRACTION AND HOW TO REFRACT.

eye would have a retinal image g^f u of 8.5, 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
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
M retinal images in
such eyes will be
smaller in the former
and larger in the
latter. (See Fig. 64.)
Fig. 64. Accommodation.

— This may be de-
scribed as the power of the eye to focus rays of light upon its
retina for 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 (m- sensiti\'e
plate as the object is brought toward the camera, it is

THE MECHANISM OF ACCOMMODATION. 65

necessary to push the lens forward b}- means of the accor-
dion phiits ; 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 ¥ig. 57.) In section the ciliary bod}' 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 cacli
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 lamellze, parallel
with it, and pass back to join the choroid coat of the eye,
forming what is known as the tensor choroideai, or muscle
of Briicke or Bowman. The radiating fibers are fan-shaped,
few in number, and scattered through the ciliar}' bod\'.
6

66 REFRACTION AND HOW TO REFRACT.

The circular or sphincter fibers — also called annular — are
sometimes referred to as the muscle of Muller, 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 o. 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,
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 bod}% as previously
referred to. The lens dors not increase in volnnie.

FAR POINT. 67

8. The anterior chamber becomes shghtly shallower at
the center and deeper in the peripher)'.

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
contraction 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 65 indicate the changes in the shape of the lens
at the height of accommodation.

At Rest.
Radius of curvature of anterior surface of lens,

" " posterior " "
Distance from anterior surface of cornea to ante-
rior surface of lens,

Anteroposterior diameter, on axis,

Distance from anterior surface of cornea to poste-
rior surface of lens, 7-- " 7-2 "

E(|uatorial diameter, 8.7 " 8.2 "

Far Point. — Latin, piiiictimi rciiiofimi ; abbreviated p. r.
or r. The far point ma\' be defined as the greatest distance
at which an eye has maximum sharpness of sight, or the
most remote jx^int at which the e\'e, in a state of rest, has
maximum acuity of vision. Infinity (sign of infinit}-, y. )
is the far point of an emmetropic e}e.

The standard or emmetropic eye. when looking at distant
objects, recei\'es parallel ra)'s of light at a focus upon its

Height of

ACCOMMODATIO.V

10 mm.

mm.

6 "

5-5

3.6 "

3-2

((

3.6"

68 REFRACTION AND HOW TO REFRACT.

fovea (F"ig. 66), 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, pioictum proxiiiiuiii ; abbre\'iatcd
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 t of accommodation, and may be de-
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 point.
For example, an emmetropic eye has infinity for its far
point, and if lo cm. distance is its near point, then the dif-
ference between the lens adapted for infinity and lo cm. will
be lo D.,as lo cm. represents the focal length, lo D. In

other words, there is no accom-
modation used for infinity, but
there is an accommodation of
lO D. for the near point, which
is the amplitude or power of
accommodation. The emme-
FiG. 66. tropic c}'e in a state of accom-

modation adds on to the ante-
rior surface of its lens what is equivalent to a convex
meniscus, h'igure 66 shows an emmetropic eye at rest
receiving parallel rays of light at a focus upon its retina,

* Range applies to tlie si)ace between tlie far and near points.
I Power applies to the force or .strenglli or dinpters necessary to cliange
the refraction from tlie far to the near point.

AMPLITUDE OF ACCOMMODATION. 69

and it also shows the same eye in its niaxinuim state of
accommodation for a point 10 cm. distant ; the broken
Hne 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 100 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 e}'es 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 is absolutely essential, so that any vari-
ations from the standard condition maybe 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.

MPLITUDE

Year.

Near

Point.

DiOPTKKS.

Year.

Near

Point.

Diopters.

cm.

3-5

5 "

2.5

1-75

8.5

100

133

0.75

5-5

400

0.25

4-5

This table of near points applies only to emmetropic eyes
or those eyes which are made emmetropic by the adjust-

70 REFRACTION AND HOW TO REFRACT.

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 64.

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. 64.) (H}-peropia :
Greek, uTzsp, over ; and ux/', 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. 67.

condition to receive only convergent rays of light at focus
upon its retina. (See Fig. 67.) 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 bej-ond the principal
focus of its dioptric media (M in Fig. 64) ; such an e}-e is
spoken of as a long or myopic eye (Greek, /wet'^, to close ; (ix/',
eye). This eye always emits convergent rays, and is, there-
fore, in a state to receive divergent ra}'s of light at a focus
upon its retina. (See Fig. 68.) Parallel rays woukl not
focus upon the retina of a myopic eye, but in the \itreous
in front of the retina.

The Far Point of a Hyperopic Eye. — This nmst neces-

FAR POINT. 71

sarily be negative (see Fig. 67), 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
10 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. 68.

twenty-five years old has an amplitude of accommodation of
8.5 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 remo\'ed than in
the emmetropic eye for a corresponding age, and that the
near point depends upon the amount of accommodation
that is left after the eye has accommodated for in fin it}'.

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. 68.)

REFRACTION AND HOW TO REFRACT.

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. 68.)
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 + 2 S.
placed in front of it to make rays of light focus upon its
retina from a distance of 50 cm., and ra}'S of light from the
retina of this eye with a +2 S. in front of it would focus
at 50 cm. This eye, then, equals a myopic e}'e 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 m}'opic e)'e in a
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 cni., — /. c,
1.5 D., which would equal 10 D., or a near point of 10 cm.
The following table gives the comparative near points in an
emmetropic eye, a hypcropic eye of 2 D., and a m}'opic eye
of 2 D. :

Age.

Emnietropia, p.p.
2 I). Hyperopia, "
2 D. Myopia, "

8.3

8.3
10

10
125
8.3

12
16
10

M
20
II

28.5

22
40
15-3

66
,8

40
200

55
00

100
33

133
36

400
44

00
SO

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, the determina-
tion (jf \-ision ma)' be defined as the seeing qualit\' of tlu-

TEST-TVPE OR TEST-LETTERS FOR DISTANT VISION. 73

un refracted eye. The refraction of an eye should never be
confounded with the visual quality, as refraction applies to
the refractive media ; for ei-cample, 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
numerous in the forepart of ^m ^m

the eye-ground. V^ tr V^ *

Test-type or Test-let- L 6 A O A L

ters for Distant Vision. —

T^j. ,1 TGYD GYDT

1 o determine the vision we „ „

, , ,1 FAVHU VFHUA

employ cards on which are

,... 1,, DLOECM LE CN D O

engraved test-type or letters c p a r-c e o i, c e o r'l c p a

of various sizes, constructed ^l^LY.MV. :L".7.V/. = !
so that each letter subtends .v;.:;/* ,:•

E C

an angle of five minutes, as Fk;. 69.— Randall's Test-letters. Block
. , 1 o II 1 letters in black on cream-colored

suggested by Snellen, and ^^^^^
described on page 63 .

Figure 69 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
7

REFRACTION AND HOW TO REFRACT.

with Snellen's t\'pe constructed on the an^^le 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 70 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. 71.) As white stimu-
^^ lates the retina and black does not, it

" ^1 will be recognized at once that in one

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
avoid reflection, this card should be
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

like the capital letter " \\," and made to conform to the fivc-
niinute angle. As these letters are warioush' placed, the
patient is asked to tell, or indicate with his finger or fingers,

-000

-M X Y V

■fl a a H M

-T s o o A n

• Y V T 3 S X X

Fic. 70. — Four Min-
ute Letters of Dr.
J. Wallace. This
card is constructed
]5rincipally for re-
flection purposes.

3 LU

E UJ 3

E 3 uj m

ui 3 E m m

ui n E Ul 3 E

Fig. 72.

METHOD OF PROCEDURE.

the direction in which the prongs of the "E" point: up,
down, to the right or left. This ilHterate card (see Fig.
72) is much to be preferred to the German, Hebrew, and
"figure" cards occasionally displayed in clinics.

Selection of Test-cards. — The surgeon should have
several of these in duplicate with the order of the letters

Fig. 71. — Gould's Test-letters. Gothic letters in white on black cards.

changed (Figs. 67, 71), as patients not infrequently and un-
intentionally commit them to memory. Care should be
exercised in the selection of test-cards, to see that each
letter on the card measures 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 so\en
minutes horizontally. It is a matter of choice with the

76 REFRACTION AND HOW TO REFRACT.

surgeon whether to use test-cards with the block or Gothic
letters. It is well to have both.

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.
Sometimes the surgeon's ofifice 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.
70), the card being hung as many meters in front of the
mirror as will make six meters when added to the length
of the ofifice. While a distance of six meters is always to
be preferred, yet if this can not be obtained, the surgeon
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
held 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-
nify meters, though there is no fi.xed rule for this. How-
ever expressed, the numerator indicates the size of the
type which the eye reads, at the distance indicated b\- the
denominator. For examj)le, if at VI meters the eye read
the line of letters marked VI, then tlie record would be
This would be the same if the numerator and (knomi-
nator were expressed in feet, |[]. If the e}'e, at a distance

THE RECORD OK IIIE VISUAL ACUITY. //

of VI meters, read only tlie letters on the XII line, then
the record would be ^j, or |-fi- (feet). If the top letter
was the only one recognized at the distance of six meters,
then the record would be i^- (meters), or -^^^^^ (feet). If
the eye read the VI line, miscalling two letters, then the
record could be made in one of three ways and would
each indicate the same thing. ? ? (one question mark

for each miscalled letter), or " ^j- partly," would indicate
that the eye saw —- , but not each letter correctly. This way
of making the record is not so explicit as that with question
marks. Or, ^— ; h would mean that the eye saw all of

' VHss ' •'

,4^ and some of the letters of -^ ; but this, too, is not so

VHss VI ' '

definite as the first record and the one recommended.

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 cdiWjnst 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 — , etc. While it has
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 — -
would be O. SO, or a vision of rrrrrr, would be 0.25, or a
vision of ^^ would be o. I. Most authorities prefer to make
their records in the form of common fractions.

In some instances the eye may not be able to distinguish
any letter on the card, no matter how close it may be
brought to the eye, and in such a case the vision is tested
by holding the outstretched fingers between the patient's
eye and a bright light (an open window), and a note is
made of the greatest distance at which the eye can count

y8 REFRACTION AND HOW TO REFRACT.

fingers ; if at ten inches, the record would be " fingers
counted at ten inches," or whatever the distance may be.
This abihty to recognize form is spoken of as " quahtative
Hght perception." Eyes that are not able to recognize
form may still be able to distinguish light from darkness,
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 L. P., which means "light per-
ception," or the record may be made L. & S., which means
practically the same thing, " light and shade."

Determining the Near Point. — Having obtained and
recorded the distant vision of an unrefracted eye, it is well
to also find out and note what is the nearest point to the
eye at which small type may 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 arc 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 t)'pe, as
shown in figure 73. Another card in block letters is shown
in figure 74. Above each series of letters is marked the
greatest distance (D) at which the respective letters ma)- be
seen ; the.se distances var\- from 0.25 to 2 meters (25 to

TEST-TYPE FOR NEAR VISION.

z
o

1-
Q

J
o

J
u

DC
X

in J

J
o

0 ^

i 2

O X

in O

q:
o

i ■

ff>

7 X

Q O

*•

REFRACTION' AND IIOW TO REFRACT.

0. h «

O O J

ft: » Pi

o > o

t^ CO o>

> O H

•n X tJ tD

-• H K >

o W O K

^ in (o

o: s ti

o »^ :3

O O H

»4 Eh O

-• N n

»^

•J

Cs.

.1 X

(i1

•H

r-l

CONVERGENCIi:. 8 1

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 may
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
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 the 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
+ 2 S. was employed, then the record might be " near
point equals type 0.50 D. at 12 cm. with -\-2 S.," or " -|-2
S. = type 0.50 D. at 12 cm."

Convergence. — Con, "together," and 7'crgcre, "to
turn " ; literally, turning together. This is the power of the
internal recti muscles (especially) to turn the eyes toward

82 REFRACTION AND HOW TO REFRACT.

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 ( oo) to a near
point is called the angle of convergence, and the angle
which is formed at one meter distance by the visual axis
with the median line is called the meter angle, or the unit
of the angle of convergence. (See i, in Fig. 75.)

If the visual line meets the median plane at J^ of a

Fig. 75.

meter, it has then two-meter angles of convergence ; at ^
of a meter, four-meter angles of convergence, etc. Or
five-meter angles means that the eye is converging to a
point -5^ 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 this distance is about 50 mm.,
whereas in atlults it is, on the average, 60 or 64 mm.

While standard eyes, to see a point at one meter distance
would converge just one meter angle, they would also
accommodate just one diopter ; to sec a point at '3 of a

NEAR POINT. 83

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

Fig. 76.

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 (K cc, in F"ig. 75).
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. 75) ; 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 positi\c ( • ).

The amplitude of convergence is the distance measured
from the far point to the near point of convergence, and is

REFRACTION AND IIOW TO REFRACT.

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 76
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

visual line. The angle
formed by the visual line
with the optic axis at the
nodal point i/un' be con-
sidered as the angle gam-
ma. *

If the fovea centralis at
M was situated on the optic
axis at A, then the visual
line and optic axis would
coincide, and there would
not be any angle gamma.

In hyperopia and emme-
tropic eyes the outer ex-
tremity of the visual line

v.r. »„ hes 15, 7, or, in some in-

r in. 77- -" / >

stances, as much as 10
degrees to the na.sal side of the optic axis (averaging

* 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 ihe
center of fixation. The angle V N O and the angle V R () being so nearly
equal, are, for all intents and purpo.ses, consideretl as the same.

ANGLE ALPHA.

about 5 degrees), 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 angle gamma, the patient is told to
look at the point of a pencil or pen held in the hand of
the surgeon, at about 13 inches distant (A in Figs, jy and
78). If the angle gamma is positive, the eyes will appear
divergent to the observer,
who looks at the position
of the poles of the cornea
or centers of the pupils.
(See Fig. JJ^)

If the angle gamma is
negative, the eyes will
appear convergent — that
is, they appear to con-
verge to a point in front
of the pencil. (See Fig.
78.)

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

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 position of the candle -flame on the arc is noted
in degrees, which is the size of the angle gamma.

Fig. 78.

86 REFRACTION AND HOW TO REFRACT.

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 oipOaXpAx;, "eye," and gvmt.zIv,
" 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 ordinarily intercept any of the rays of light which return
from the eye. Ra>^s 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 e)'e 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. 79.)

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, 21/ mm. in diameter, is not
cut through the glass, but is made by rcmoxing the quick-

«7

REFRACTION AND HOW TO REFRACT.

silver. The glass at the siglit-hole gives additional reflect-
ing surface, and at the same time does away with much
annoying aberration which results wlien the glass is per-
forated.

BACK

Fig. 79.

The small sight-hole is an advantage, also, in looking
into small puj)ils. The mirror, oblong in shape, 18 by 33
mm., is secured at the center of its ends, b}' two elevated
.screws, to a hollow disc 4^^ cn- i'l diameter, in which is a
revolving milled wheel, containing small spheres, each
about 6 mm. in diameter. The series of spheres ranges

OrilTIIALMOSCOPE.

from — I D, to — 8 D., and irom +1 D. to -f 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 I^-, 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
si^jht-hole of the instrument records the strength of lens

Fig. 80.

that may be in use ; minus lenses are usually marked in red
and plus lenses in white.

How to Use the Ophthalmoscope. — There are two wa)'s
or methods by which the ophthalmoscope may be used —
the direct and the indirect.

The Direct Method (see Fig. 80). — Proficicnc}- with
the ophthalmoscope docs not come except from long and
constant practice, and se\'eral important matters should
receive very careful attention before the student attempts to
study the interior of an e}'e.
8

90 REFRACTION AND HOW JX) REFRACT.

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. LLxamining the right e)'e, the observer
should be on the patient's right ; if the left e)'e, then on
the patient's left.

When examining the right eye, the ophthalmosct>j)e is
held in the right hand, before the right eye ; and in the left

OPHTHALMOSCOPE. 9 1

hand, and before the left eye, when examining the left eye.
The surgeon's eye should be a little higher than the patient's.
Patient and observer should keep both eyes open. The
one 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.
The 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 prevdously 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 ]iot 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 — /. c, to find out if there is any interference
with the free entrance and exit of the reflected ra\\s, such
as would be caused by opacities in the cornea, lens, lens

92 REFRACTION AND HOW TO REFRACT.

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
is 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
generally 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,
known as the physiologic cup ; this may be shallow or
deep ; it may have shelving or abrupt edges ; it may even
be funnel-shaped.

OPHTHALMOSCOPE. 93

At the bottom of the cupping is frequently seen a gray
stippHng, 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\, upper nasal vein and artery ; upper temporal
vein and artery ; lower nasal artery and vein ; lower tem-
poral artery and vein.

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
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
healthy 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. Pigment areas between the vessels are also
diagnostic of this tunic. The choroidal circulation is best

REFRACTION AND HOW TO REFRACT.

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 health}^ emmetropic
eyes with the accommodation at rest. Figure 8i 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. 8i.

vergently from the concave mirror, and focusing in the vitre-
ous, they cross and form an area of illumination on the
retina at IF. 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 two very important points which nuist bo

OI'IIIIIALMOSCOI'E.

considered when usinL;' the ophthahiioscope in the direct
method : one is the direction which the rays of hght 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
not accommodate.

Figure 82 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. 82. — T B indicate points at the edge of the disc from which rays pass
out of the eye divergently in the direction T' B^, T'' W, T^ W, and being
received by the observer's eye, are projected backward, forming an erect
magnified when image at T^'' B^^. This image is not so large as that seen
when looking into a myopic eye. (Fig. 83.)

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 83 shows rays of light passing out of an ej^e
convergently, and to have them parallel, so as to focus
upon his own retina (emmetrojjic), it is necessarx^ to turn a
concave lens in front of the sight-hole of the ophthalmo-

REFRACTION AND HOW TO REFRACT.

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

Fig. 83. — T B indicate points at the edge of the disc from which rays pass
out of the eye convergently in the direction T' B', 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 tlie hyj)eropic eye. (Fig. 82.)

himself, and the lens used in the ophthalmoscope is plus
four diopters, then the eye under examination has onl)' two
diopters. It is not unusual for beginners to see the e)'e-
ground (disc) in hyperoj)ic eyes with a strong concave
lens ; this is due to the fact that the}' accommodate. Prac-
tice will overcome this habit, and it should be mastered as
soon as possible. There are .several wa\'S of doing this :
one is to begin the examination at a distance of 30 or 40

OPHTHALMOSCOPE. 97

cm. from the eye, with both eyes open, and to gradual 1)'
approach tlie 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 concav^e 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. 136.)

Size of the Image of the Eye-ground (Figs. 82 and
83). — 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 ha\-e an}'
difficulty in appreciating these facts.

The optic disc of an emmetropic eye, as seen through
the ophthalmoscope, appears to be about 25 mm. in diam-
eter, and about 250 mm. away. The retina of the emme-
tropic eye is about i 5 mm. from its nodal point ; then the
actual size of the emmetropic disc is -J/^ of 25, or -1. 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.
9

REFRACTION AND HOW TO REFRACT.

The Indirect Method (see Fig. 84). — Practising this
method, the observer sees a larger part of the eye-ground

Fig. 84.

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 usual!)- too small and too strong for general use.
The writer prefers his plus 13 I), with metal rim and con\-en-
ient handle, shown in figure 85 (reduced one-third in size).

Fig. 85.

Ol'H THALMOSCOPE. 99

This is held at about three inches in front of the eye
under examination, the observer resting his httle and ring
fingers on the temple of the patient. The hght may be
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
the 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
and 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 tJic direct method gi%<es
an erect, virtual, and enlarged image, the indirect method
prodiices an inverted, real, and small image. The principle
of the direct method is similar to a simple microscope, and
the indirect to a compound microscope.

The size of the image depends upon the refraction of
the eye and the distance of the convex lens from the e}'e
under examination. In 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

lOO REFRACTION AND HOW TO REFRACT,

retina in the eye is i 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 eye -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.

CHAPTER IV.
EMMETROPIA.— HYPEROPIA.— MYOPIA.

Emmetropia.
Emmetropia ('-', "in"; iiirpo'^, "measure"; ('^4',
"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 :

1. The standard or schematic eye: Authorities differ
somewhat in the exact measurements of a schematic eye, but
the one suggested by Helm-

holtz is certainly worthy of
careful consideration. (See
p. 58.)

2. An emmetropic eye is
one which, in a state of rest
(without any effort of accom- p^, 35
modation whate\er), receives

parallel rays of light exactly at a focus upon its fovea. (See
Fig. 86.)

3. An emmetropic e}'e, therefore, is one which, in state
of rest, emits parallel ra}-s of light. (Sec Fig. 86.)

lOI

I02

REFRACTION AND HOW TO REFRACT.

4. An emmetropic e}'e is one
whose fovea is situated exactly
at the principal focus of its re-
fractive system. (See Fig. 86.)

5. An emmetropic eye is one
the vision of which, in a state
of rest, is adapted for infinity. •

6. An emmetropic eye is one
which has its near point consist-
ent with its age. (See p. 69.)

7. An emmetropic eye is one
which does not develop pres-
byopic symptoms until forty-
five or fifty years of age. (See
p. 261.)

8. An emmetropic eye, in
contradistinction to a myopic
eye (see p. 1 1 3), is spoken of
as a healthy eye, or one which
shows the least amount of irri-
tation in its choroid and retina.

Because we refer to Helm-
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. 87) ; for while an

I'^ic. 87. — I. Emmetropia. 2. Mvo]iia
due to a strong lens. 3. Hyperopia
due to a weak lens. 4. Myojiia due
to a short radius of curvature of cornea.
5. Hyperopia due to a long radius of
curvature of cornea. The anteropos-
terior diainelcr of all tlu'sc eyes is just
2 ; Mini.

AMETROPIA. 103

eye may be just 23 mm. in Iciigtli, it may have it.s refractive
system stronger or weaker than is consistent v/ith 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. 86.)

Ametropia.

Ametropia (« priv. ; iiirfxiv, "a measure" ; cl^t?, "sight")
literally means "an eye out of measure." An ametropic
eye is one which, in a state of rest, does not form a distinct
image of distant objects upon its retina. An ametropic
e}'e may be defined as one which, in a state of rest, does
not focus parallel rays of light upon its fovea. An eye
which is not emmetropic is ametropic. There are two
forms 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.
88 and 90.) The refraction is measured on the length of
the anteroposterior axis of the eye ; hence its name, axial
ametropia.

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
cun'ature ametropia may be considered as that condition
in wliicii parallel rays of light entering an e\'c ha\e two
focal planes for two principal meridians at right angles to
each other. Curvature ametropia is commonh" spoken of
as astigmatism. (See Chap, v.)

I04 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
dioptric apparatus than its principal focus (see Fig. 88),
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. 90.)

Hyperopia.

Hyperopia (J^-if^, "over"; w^, "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 hyperopia. 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."

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
a state of rest fall upon its retina or fovea before tliey focus.
(See Fig. 67.)

4. Rays of light from the fovea of a hyperopic eye in a
state of rest pass out divergently (see Fig. 88), and the
condition is equivalent to a convex lens refracting ra}'s of
light which proceed from a point closer to the lens than its
principal focus. (See Fig. 35.)

5. A hyperopic eye is one which, in a state of rest, can
only receive convergent rays of light at a focus uptm its
fox'ca (I'ig. 88) ; therefore, to repeat : the h)-peropic e}'e, in
a state of rest, emits divergent ra)s and receives convergent
rays at a focus upon its fovea.

nvpEKoi'iA. 105

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. 89.)

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. (Seep. 71.)

9. From the description contained in 3, it follows that

Fig. 88. Y\q. 89.

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. 88.)

10. From the description contained in 6, and the descrip-
tion of accommodation on page (>(>, it is natural to find the
retina and choroid of many hyperopic eyes in a state of
irritation.

11. From the description contained in 6 and 7, and on
page 262, it fc^Uows that symptoms of prcsb}-opia manifest
themselves earlier in hyperopic than in an\- other form ot
eyes.

12. From the description contained in 5, — and this may

I06 REFRACTION AND HOW TO REFRACT.

appear like repetition), it follows that a liyperopic eye will
accept a plus glass for distant vision. (See Fig. 89.)

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
the 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 where
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. 161.)

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 hyperopia.

Subdivisions of Hyperopia. — For purposes of study
h}'per()j)ia has been classed as :

I. Facultative hyperopia (abbreviated Ilf) 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

HVPEKOPIA. 107

life, and is voluntary. The patient can see clearly, with or
without a convex glass.

2. Absolute hyperopia (abbreviated Ha.). — This is hy-
peropia that can not be overcome by the accommodative
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, develops every variety of
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 h}'peropia and that lens which an eye
would select if its accommodation was put at rest with a
cycloplegic (atropin). P'or example, an eye accepts a
+ 1.25 S. as its manifest H., and, when atropin is instilled,
w^ould 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 h\-per-
opia.

6. Total hyperopia (abbreviated Ht.) is the full amount
of the h)-peropia ; or is represented by the strongest glass
which an eye will accept, and have clear, distinct vision
when in a state of rest.

I08 REFRACTION AND HOW TO REFRACT.

Symptoms and Signs of Hyperopia. — These are many
and various ; tlic principal one, however, and the one that
generally causes the patient to seek relief, is Juadaclic.
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 headaches,
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 211.

Blepharitis marginalis, styes, and conjunctivitis are
frequently present, and in truth the hyperopic eye on
this account can often be diagno.sed in public outside
of the surgeon's office. A feeling as of sand in the eyes,
ocular pains or postocular discomfort, a dr\'ncss of the
lids, 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
begin reading, etc. A drowsiness or desire to sleej) often
comes on after or (iurin<r forced accommodation.

HYPEROPIA. 109

Congestion of the choroid and retina, as evidenced by the
ophthahiioscope, 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 " h}'per-
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 veiy 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
quickly diagnosed by :

1. The narrow palpebral fissure during the act of reading,
and reading very slowly, as each letter has to be studied.

2. The fact that very few children have myopia.

3. The comparatively good distant vision, as a rule,
w^hich myopes never have, unless the myopia is of ver)^
small amount.

4. The ophthalmoscope.

The beginner in ophthalmology should be on his guard

I lO 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 no.se
(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 m}'opia.

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-

* \ I

trary, hyperopic eyes accommodate for distance, and obtain
this acute vision by effort.

7. The patient gives a history of accommodati\e aslhe-
n(ipia, with or without headaches coming on during or
after the use of the eyes.

8. The distant vision of a li)-peropic eye may remain

MVOPIA. I I I

unchanged or may be improved with the addition of a
convex lens, which latter would be impossible in emme-
tropia and myopia.

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 (11) the
ophthalmoscope, (12) the retinoscope, (13) Scheiner's test,
(14) Thomson's ametrometer, and (15) the cobalt-blue
glass test, commonly spoken of as the chromo-aberration
test. These tests are described in the text.

Myopia.
Myopia {p-uth, "to close"; wip, "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 point. Brachymetropia is another name for the
same kind of eye. Myopia is abbreviated M. About 1.5
per cent, of all eyes have simple myopia. The mj^opic 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 eye. The Fig. 90.
origin of this name is pure-
ly anatomic, the fovea lying beyond the principal focus of
the refracting system. (See Fig. 90.)

2. Parallel raj's of light entering a m}-opic eye focus in

112

REFRACTION AND HOW TO REFRACT.

the vitreous humor before they can reach the fov^ea. (See
Fig. 90.)

3. Rays of hght from the fovea of a myopic eye pass out
of the eye convergently (see Figs. 68 and 91), focusing at

Fig. 91.

some point inside of infinity. The refractive condition of
a myopic eye is similar or equivalent to a convex lens
refracting rays of light which proceed from some point
further away than its principal focus. (See Fig. 33.) 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. 33, and also description
of conjugate foci.)

5. As parallel raj's can not
focus on the fo\ea of a
myopic eye, it is necessary to
give parallel ra\'s entering the
eye a certain amount of di\crgence, so as to i)lace the tocus
at the fovea ; and to accomplish this, a concave lens must
]:)c used. (Sec V\g. 92.) A myopic eye, therefore, is one

Fig. 92.

MVOl'IA. I I 3

which requires a concave lens to improve distant vision.
(See Fi^. 92.)

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
distance.

8. A mj'opic eye having a refracting system stronger
than is consistent with its length, or vice versa, greater
length than is consistent with its dioptric S}'stem, 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. 72.)

9. From the description contained in 3 it follows that the
far point of a myopic eye is positive ( + ).

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.

11. 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 e\'e. 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.

12. Eyes in which the m}-opia is progressive are spoken
of as " sick eyes."

Causes of Myopia. — An\' disease or injur\- which will
so alter the refracting sj'stem of an c)-e that parallel rays
must focus in front of the fovea will produce the form of
eye known as long or mj-opic. This may be brought about

I 14 REFRACTION AND HOW TO REI'RACT.

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 convexity ;
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
to 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.

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 c\es hiter.illy,

MVOPIA. I I 5

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 se congenital. " The inherited con-
genital anomalies of refraction, particularly astigmatism, are
responsible for the m)-opic e}-e, by virtue of the pathologic
changes they occasion in hard-worked eyes rather than any
inherited predisposition to disease." (Risley, "School
Hygiene.")

Symptoms and Signs of Myopia. — \\'hile 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 be\ond 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
e}-e come to a focus. ICvery object situated beyond the far

Il6 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 broad 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 usuall}- have
dark-colored irises, but this is often a fallacy, as is onl)' too
evident in the German student with his blue iris.

The foregoing are but signs of mj'opia, 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 ; inabilit)- to see numbers on the
liouses across the street or on the .same sitle of the street ;
history of passing friends without speaking to them. The
myope enjoys clo.se work and takes little or no interest in

MVOI'lA. I \y

sports. A history, in other words, that is in keeping with
a vision of short range.

7. Good near visibn ; abihty 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. 72.)

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.

10. The 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. Scheiner's method.

14. Thomson's ametrometer.

15. Chromo-aberration test.

Direct Ophthalmoscopy in Axial Ametropia. — Pro-
ficiency in this method only comes 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

Il8 REFRACTION AND HOW JO KKFKACT.

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."

T/iird, 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 +2 S., and does not
wear his glasses, and the ophthalmoscope records the fundus
as seen clearly with -|- 5 S., this would mean that the paticMit
had -I-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 — 4S. ;
or if the ophthalmoscope registered +2 S., then the
patient would be emmetropic, and this +2 S. is the sur-
geon's error.

Rules. — I. When the surgeon and patient are both h)--
peropic or both myopic, the surgeon must subtract his cor-
rection from the lens which shows at the sight-hole in the
ophthalmoscope.

2. Wiien the surgeon's c)^c is h)-peropic or myopic, and
the eye of the patient is the opposite, he nuist aild his- cor-
rection to the lens at the sight-hole in Ihe ophthalmoscoi)e.

With the foregoing details clearly in iniiul ami carefully
executed, the surgeon selects small \es.scls near the macula

M\(>I'1A. I 19

for his observations. If it is impossible to sec these on
account of the small pupil, then he will have to observe the
lart^er vessels at the disc (nerve-head, or papilla).

Whenever the vessels in the macular re<^ion 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.

2D. = o.5 "
3D.=:0.9 "

6D.= i.7S "

9D.= 2.6 "

I2D.= 3.5 "
i8D.= s

Indirect Method. — Sec page 98 for a full description of
this method. Slowl}' 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. 135.) This
is merely a method of diagnosis, and is never used for
definite measurements.

I D.

= 0.3

2D.

= 0.5

3D-

= I

5D.

= 1.5

6D.

= 2

9D.

= 3

12 D.

= 4

CHAPTER V.

ASTIGMATISM, OR CURVATURE AMETRO-
PIA.—TESTS FOR ASTIGMATISM.

Astigmatism (from the Greek, a, priv. ; ariyim, " a
point"). — Optically, astigmatism may be defined as the re-
fractive condition in which rays of light from a point, pass-
ing 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. 93.)

Or astigmatism is that condition of an eye in wliich
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 ; tlie\' 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 e\'e is an
extremely rare condition, ant! plain m\-opic e\-es (long e\-es)
witJiont 'M\y astigmatism arc almost as rare as the emmctrt^pic

120

ASTIGMATISM.

121

condition ; and while plain h}'peropic 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.

Figure 93 shows parallel rays of light passing through
an astigmatic lens where the vertical meridian has the

Fig. 93.

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 liorizontal
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

122 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
lens. Tenotomy of one or more of the extraocular mus-
cles will often change the corneal curvature.

Irregular Lenticular Astigmatism. — This is a normal
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 themselv^es 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 appearance. There is no known glass that will
correct this variety of astigmatism.

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 ej^es. 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. 1 23

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
y^ or 3/^ 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 contrar}-, 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
refrular astigmatism, the student's attention is called to two

124 KKFKACTION AND HOW TO REFRACT.

important facts : (^7) 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.

{b) 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 -f 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 cixuiii 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 arc five
different forms of regular astigmatism :
[a) Simple hyperopic. {c) Compound hyperopic.

{p) Simple myopic. {d) 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 lu'peropic (see Fig. 94) ;
the vertical meridian focuses
parallel rays on the retina, and
the horizontal meridian woiiUi
focus back of it. The retinal image of a point is a line,
usually horizontal. (See 2, in Fig. 93.) The correcting
lens is a plus cylinder with its axis usually at 90 degrees,

ASTIGMATISM.

125

Fig. 95.

or within 45 degrees of 90 degrees. Example, -|-2.oo
cylinder axis 90 degrees.

{/?) Simple Myopic Astigmatism. — Abbreviated As. M.,
or M. As., or Am. This is not a common condition. About
I ^ per cent, of all eyes have this form of astigmatism.
This is a condition where one meridian of the eye is emme-
tropic, and the meridian at right angles to it is myopic (see
Fig. 95) ; 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. 93.) 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.

[c) Compound Hyperopic Astigmatism. — Abbreviated
H. As. Co., or Comp. Has., or H + Ah (hyperopia com-
bined with astigmatism hyperopic). This condition repre-
sents nearly fort)'-four per cent, of all eyes ; it is the most

common of all forms of re-
fractions.

The retinal image of a
point is an oval ; ne\-er a line
and never a circle. (See i,
in Fig. 93.)

The correcting lenses are a
plus sphere and a plus cylin-
^ +3.00 cylinder axis 90 de-
grees. Compound hyperopic astigmatism is a combination
of axial ametropia (short eye) and simple hyperopic astig-

der.

¥u.. 96.
E.xample, -|-2.oo S.

126 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 ra)-s
before they can focus. Figure 96 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.+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.

93-)

The correcting lenses are a
minus sphere and a minus cylin-
der. Example, — i sph. O — 2
cylinder axis 180 degrees. A
combination of axial ametropia
Fit;- 97- (long eye) and simple m}-opic

astigmatism.
Figure 97 shows that parallel rays have two points of
foci in front of the retina — one further front than the
other.

(r) Mixed Astigmatism. — This form of refraction is
found in about 6}4 per cent, of all eyes, and is abbreviated
in three different waj's :

1. Ah + Am. (astigmatism hypcropic with astigmatism
myopic).

2. H+Am. (h)'peroj)ia with astigmatism nu'opic).

3. M+Ah. (myopia with astigmatism hyperopic).

The retinal image of a point is an oval or a circle ; never
a line. (See 3 and 4 in l-'ig. 93.)

ASTIGMATISM.

127

The correcting lenses are one of three combinations, and
spoken of as crossed cyhnders. Examples :

1. -|-l.oo cyl. axis 90 degrees 3 — --^o cyl. axis 180 degrees.

2. -j-l S. 3 — 3 cyl. axis 180 degrees (cylinder always stronger than the
sphere).

3. — 2 S. 3 +3 cyl. axis 90 degrees (cylinder always stronger than the
sphere) .

The condition of mixed astigmatism is one of simple
hyperopic 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. 98.

Fig. 99.

back of the retina. Figures 98 and 99 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. — When the combined
values, in degrees, of the meridians of shortest or longest
radii of curvature in both ej-es 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 a.xis 75 degrees, and in the left eye at 105 de-
grees ; 75 degrees and 105 degrees added together will

128

REFRACTION AND HOW TO REFRACT.

make i8o degrees. (See Fig. loo.) 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 e)'es
hav^e axes 180 degrees, they are symmetric also, one
meridian being considered as zero (o).

4. Asymmetric astigmatism is the reverse of sym-
metric, and is, therefore, the condition where the combined
values, in degress, of the cylinder axes do not make 180
degrees. For instance, if the right eye has a cylinder at
axis 75 degrees and the left at 120 degrees, then these

180 0

75' 90' 105

Fig. 100. — Illustrating Symmetric
Astigmatism.

75 90

Fig. IOI. — Illustrating Asymmetric
Astigmatism.

added together would not make 180 degrees, but more than
180 degrees. (See Fig. loi.) Or, if the astigmatism in
the right eye was at 35 degrees, and the left at 90 degrees,
then 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 as)-mmctric i)h\'siog-
nomy, the center of one pupil being further from the
median line of the face than the other.

ASTIGMATISM.

129

Muscular insufficiency, hereafter to be described, is much
more common, and, in fact, should be looked for or antici-
pated in cases of asynmietric astigmatism.

5 and 6. Astigmatism with the Rule and Astigmatism
Against the Rule. — Astii^matism with the rule and asti<^-
matisni 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
accept a plus cylinder at axis 90 degrees, or within 45

80° 0

Fig. 102. — Illustrating Astigmatism
with the rule.

Fig. 103. — Illustrating Astigmatism
against the Rule.

degrees (inclusive) either side of 90 degrees (see Fig.
102); or a minus cylinder at axis 180 degrees, or within
45 degrees (inclusi\'e) 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-
matism is ivitJi 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 astigmatis))i against the rule.
(See Hg. 103.) A plus or minus cylinder at 45 degrees

130 REFRACTION AND HOW TO REFRACT.

or 135 degrees is recognized as asiigmatisDi xvitJi the
rule.

7. Homonymous astigmatism is the condition in which
the cyHndcr axis in each eye is the same.

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. +2 cyl. axis 90 degrees, and O. S. -f 2.00 cyl. axis 180 degrees.

9. Homologous astigmatism is symmetric astigmatism

with the rule — /. c. :

O. D. -f i.oo cyl. axis 60 degrees O. S. -f i.oo cyl. axis 120 degrees.

10. Heterologous astigmatism is symmetric astigma-
tism against the rule — /. c. :

O. D. 4^1.00 cyl. axis 15 degrees, O. S. -fi-oo 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 fellow, 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. 102.)
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 ha\ing
a complete circle of 360 degrees. Some trial -frames have
the upper, while others have the lower, half immbered ;

ASTIGMATISM. I3I

this makes no difference in the exact numbering ; in the
one case the count is made from the left to the right, and in
the other the count is made from right to left. The foreign
trial-frame, as represented on page 47, 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- :

Corneal reflex.

10. Chromo-aberration or cobalt-blue

Confusion letters.

glass test.

Placido's disc.

II. Thomson's ametrometer.

Stenopeic slit.

12. The ophthalmometer.

5-
6.

Astigmatic chart.
The pointed line test.
Perforated chart or disc.

13. Direct ophthalmoscopy

14. Indirect ophthalmoscopy.

15. Cylinder lenses.

8.
9-

Pray's letters.
Scheiner's Test.

16. Retinoscope.

I. The Corneal Reflex Test. — The cornea and under-
l)'ing aqueous representing a spheric mirror, naturall}- fur-
nish a small image of surrounding objects. If the cornea

132 REFRACTION AND HOW TO REFRACT.

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
image of the window-panes in the corneal 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. 70.) 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. 104). — 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.
On the reverse side is placed a slot to hold a convex lens
for magnif}ing 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 o\er his head. The sur-
geon 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.

133

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 obsolete.

Fig. 104.

Fig. 105.

4. Stenopeic Slit (sec Fig. 105). — 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 ha\e various breadths of
openings, from ^ to 2 mm. ; that with the i mm. opening

134 REFRACTION AND HOW TO REFRACT.

is the one recommended. The purpose of the sHt 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.
To use the stenopeic slit, place it in the trial-frame in front
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
]^, if a -f 1.50 S. is used, and the vision becomes y|, then
1.50 corrects axis 75. Turning the slit to axis 165, pro-
ceeding in the .same way, and +2.50 S. brings the vision
from ^ to ^ ; then +2.50 corrects axis 165, the differ-
ence between the -I-1.50 and +2.50 being i D., and the
formula would be -fi-SO sph. Q -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 ca.ses of
mixed astigmatism, irregular astigmatism, presbyopia, and
aphakia.

ASTIGMATISM.

135

5. Astigmatic Chart. — There is an infinite variety of
these cards (see Fig. 106), and the student is puzzled

Fig. 106. — Astigmatic Cliarts of Dr. John Green.

which one to select. Ordinarily, the " clock-dial " will
answer every purpose. (Fig- 107.) This is a white

136

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

¥iG. 107.

the lines and interspaces \\ill form an anq;le of 5 minutes in
width consistent w ith the distance at which the test is to be
made : if at six meters, 8.5 mm. ; if at four meters, 5.7 mm.
Most charts have the lines subtending an angle much greater
than 5 minutes for the distance at which they are u.sed. and
in tliis way llic true delicacy of the test for sin. ill errors or

*A black card wiili wliite lines is also used. (See Fig. 108.)

ASTIGMATISM.

137

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. 108.

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.

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

138 REFRACTION AND HOW TO REFRACT.

darker than all the others, then it is the horizontal meridian
(o or 180 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 — /. c, 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
180 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.

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 are from II to VIII, then the left eye selects
the dark lines from X to IV, etc.

The clock-dial is the form of cliart in coinnion use,
and as a test for astigmatism is not without considerable
merit.

When the astigmatism is of small amount, it ina\- not be
recognized by means of the clock-dial until after the spheric
correction has been placed before the e)-e or after a c}'clo-
plegic has been instilled.

ASTIGMATISM.

139

6. The writer's pointed line test, as shown in figure 109,
is a series of one-minute black squares, in tliree 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. 109.

7. The Perforated Disc (Fig. 1 10). — 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,

140 REFRACTION AND HOW TO REFRACT.

signifies which scries of perforations appear to coalesce and
form hnes. This test is not commonly known or used. It

J'iG. no.

might be a valuable test if there was any convenient way

of uniformly illuminating it from behind.

8. Pray's Letters (Fig. iii). —
These letters are of the Old English
type, and composed of strokes which
run in different meridians. The pa-
tient, looking at these letters, selects
that letter which appears darker than
all the rest. The direction of the lines
in the letter selected corresponds to the
meridian of greatest ametropia. This
test is very confusing to the patient,
who sees first one letter and then an-
other as darker than its fellows.
Scheiner's Test. — This is an old test for ametropia,

lloilH

iiiliiii

¥1

Fk;. Ill

ASTIGMATISM.

141

good in theory, but rcall}- 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. 1 1 2)
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

Fk;. 112.

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 na.sal 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 13). — The disc is placed
with the red glass (R) above. The patient then sees a red

142

REFRACTION AND HOW TO REFRACT.

114.

and a white lig;ht (\V). The red appears below the white.
Gradual!)' revolvintj 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. 114). — 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 shows the same as in emmetropia, and the meridian
opposite to the emmetropic meridian would show a sepa-
ration of the two lights, as in hyperopia. That plus cylinder
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 In-per-
ojiic astigmatism, but the red light is seen in the direction
of the red glass, and when the disc is rotated to the oppo-

ASTIGMATISM.

143

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 Hyperopia 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.

Fic. 115.

Fig. 116.

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
shows as in simple hyperopic astigmatism, and the meridian
opposite to it shows as in simple myopic astigmatism.
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

144 REFRACTION AND HOW TO REFRACT.

used as a coloring-matter by glass-blowers. Cobalt-blue
glass comes in two forms : one where the glass is colored
throughout, and the other where it is colored only on 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. 1 15 and 1 16.)

The power of cobalt-blue glass to exclude all but blue

Fig. 117.

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 17.)

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 iiijlueuee of a eyeloplei^ie.

2. l^y means of a light-screen, a small rountl area of
steady white light should be looked at from a tlistance of
four or six meters.

3. Each eye is to be tested separately.

ASTIGMATISM,

145

Fig. 118.

Fig. 119.

Fig, 120.

Fig. 121.

Fig. 122.

Fig. 126.

Fig. 127.

Fig. i:

Fig. 129.

ilS. Hi<,'h hyperopia. 119. I Tigh myopia. 120. I. ow simple hvperopic as-
tigmatism. 121. High simple hyperopic astig^iatism. 122. 'lx)w simple
myopic astigmatism. 123. High sim|)le myopic astigmatism. 124. I.ow
compoiiml hypeiopic astigmaliMii. 125. ifigh comixuind hyperopic astig-
matism. 126. Low compouiui myo|)ic astigmatism. 127. High com-
pound myoi)ic astigmati.sm. 128. Low mixed astigmatism. 129. High
mixed astigmatism,

146 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,
and 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. 117.)

Diagnosis of Hyperopia (see H in Fig. 117). — 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 Mm Fig. 117.)

Diagnosis of Astigmatism. — If astigmatic, then he will
describe one of the conditions as shown on page 145. 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. 130). — 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 twcnt}' feet, and
l)rcfcrably under a cycloplegic. The method ot the tost is
to mo\'c one flame along the metal arm until the two lights
appear to fu.se. The scale, as marked on the arm, gives
the approximate strength of lens necessar)' to correct the
ametropia. By raising or lowering the arm an)' meridian

ASTIGMATISM.

may be tested. It is a most ingenious test, but not in
common use.

12. The Ophthalmometer (.see Figs. 131 and 132). —
This name literally means an " e)'e measure," but as tiie in-
strument only measures the different radii of corneal curva-
ture, a much better name would be keratometer, or measure
of the corneal radii. The object of the ophthalmometer is

Fig. 1^0.

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. 132, 133, 134.) The left mire
is stationary, and is made up of two 3 cm. squares, separated

148

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. 131.

frame. At the side of the frame, and attached to it, arc 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 cK'arl\- ; tlun lie
.selects the two central imaiies for further studv and isjnores

ASTIGMATISM.

149

the peripheral imaL^cs. 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. 133) ; and to do the

Fig. 132.

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

ISO

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. 134.)

Each step representing one
diopter of astigmatism, one-
half a step of overlapping
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 ncutrali/X' 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

Fig. 133.

Fig. 134.

ASTIGMATISM. I 5 I

place in testing the refraction in cases of aphakia. The
ophthahnometer 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 -\-4 S., and the horizontal
vessels with a 4-- S., then the amount of the astigmatism
would be +2 D.

In using the ophthalmoscope for refractive estimates, the
student should remember that the vertical vessels are seen
through the horizontal meridian and the horizontal vessels
are seen through the vertical meridian. In other words,
each vessel in the eye is seen through the meridian at right
angles to its course. This is a puzzle to the beginner, but
he must remember that cylinders refract opposite to their
axes. In estimating tiie refraction with the ophthalmo-
scope, the observer looks first at the sJiapc of the disc.
Anatomically, the optic nerve as it enters the eye to form
the papilla (disc) is in many instances round, and, there-
fore, if it appears oval in shape, this would be an evidence
of astigmatism ; secondl}', if the u])[)er and lower edges of
the disc are seen with a different lens from that required to
see the inner and outer edges, this would be a further evi-
dence of the presence of astigmatism ; but the third and

152 REFRACTION AND HOW TO REFRACT.

confirmatory test of the presence of astigmatism should be
the difference in the refraction of the vessels at right angles
to each other, as seen in the neighborhood of the macula.

Examples of estimated refraction by the direct method.

Simple Hyperopic Astigmatism. — Vertical vessels seen
with a -|- I S. and horizontal vessels seen without any lens
would equal + i.oo 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 +3 S.
would equal +3.00 S. O +i.oocyl. axis 90 degrees.

Compound Myopic Astigmatism. — Vertical vessels
seen with — 2 S. and horizontal vessels seen w'ith — 5 S.
would equal — 2.00 S. O — 3. 00 cyl. axis 180 degrees.

Mixed Astigmatism. — Vertical vessels seen with
+ 2 S. 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. 135 and p. 98). — Tliere 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 hyperoj)ic ; or if larger, then that meridian is
myojiic.

2. If the image grows smaller, but more so in one meridian
than the other, it signifies compound hyperopia. If the
image grows larger, but in one meridian more so than the
other, then the condition is one of compound myopia. The

ASTICMATISM.

153

image growing smaller in one meridian, while in the other
it grows larger, indicates mixed astigmatism.

¥lG. 135. — Companion picture to figure 84. Illustrating the indirect method.
Rays from the 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
4-4 S. in the ophthalmoscope magnifies the image T'' B''.

15. The cylinder lens test for astigmatism is described
under Applied Refraction, page 240.

16. Retinoscopy is described in chapter vi.

CHAPTER VI.
RETINOSCOPY.
Retinoscopy, or the Shadow Test. — This may be de-

Fu;. 136. — TIk; Author's Schematic Eye for Stu<lyinp; Ki-tiiK)scoi)y.

fined as tiio method of estimatini; the refraction of an e)-c
by reflecting into it rays of lii^ht Axiin a plane or concave

154

RETINOSCOPV. 155

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. I37-)

I METER.

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. — With an eye otherwise ncirmal except for its
optic error, and being under the influence of a reliable
cycloplegic, there is no more exact objective method of
obtaining its refraction than by retinoscop)-.

156

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. 138. Fig. 139.

Author's Mirror with Folding Handle.
Fig. 138. — 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 j)pening in screen. D is the folding cap handle to pro-
tect B when not in use. A is the metal disc.
Fig. 139. — Shows the light moved to one side as a result of tilling the mirror.

The patient //n/s^ Jiavc his accojiiinodatioii under the in-
fluence of a reliable cyclopleoic ; 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

RETINOSCOPV.

157

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.
138 and 139.)

The concave mirror recommended has a 25 cm. focus
(ten inches) and is 3 ^ 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 in the same way
as that of the plane mirror.

The light should be steady, clear,
and white, 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. 140.)

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 coming 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. 140. — .^utlior's Iris
Diaphragm Chimney.

158

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. 141.) TJic distance bctzvcoi the ligJit and mirror
will not alter the direction of tJie rays of light which conic
from the patient's eye.

Position of the Light and the Concave Mirror (Figs.
141, 142, 143). — As the purpose of the concave mirror in
retinoscopy is to focus rays of light before they enter the

I METER

Fig. 141. — 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 at about 33 cm. in front of the mirror.
Distance of Surgeon from Patient. — With the plane

mirror he may approach w ilhin a few inches of the patient's
eye to fintl the point of reversal, hut with the concaxe minor

RETINOSCOPY.

159

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. 142. — Illustrating High Myopia with a Concave Mirror.
Rays of light from the lamp (L) are reflected by the mirror (w'), and fonn a
conjugate focus at L', and the rays from this focal point illuminate the
retina at L^. Con-esponding 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. 143. — Illustrating Hyperopia with the Concave Minor.
The eye (E) recognizes a virtual image behind the eye under examination, so
that when the mirror {in'') is focusing the rays from the lamp (L) at L',
the upper portion of the retina is illuminated, and vice versa, when the
mirror {m") is focusing the rays at Lj) 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-

l6o 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 corrcsi:)onds to the ])ortion
of the retina which receives the rays of light reflected from
the mirror. The retinal illiiniin.ition is also calleti "the
image," "the light area," etc.

KKTINOSCOl'V. l6l

The Shadow. — This is the non-illuminated portion of
the retina immediately surrounding the illumination. The
illumination and shadow arc, 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 wc 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 moxement 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. Having determined
with the plane mirror at one meter distance that the retinal
illumination moves with the movement of the mirror, and
a +2.50 S. stops all apparent movement, no movement of
the illumination can be seen ; and all shadow having dis-
appeared, the observer knows that his eye is at the point
of reversal. With the concave mirror the retinal illumina-
tion will have just the opposite movement and will stop
with the same lens before the eye. The point at which all
movement of the retinal illumination appears to ha\'e
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 the\' enter the e)c).
14

l62 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
beyond the point of reversal, the retinal illumination will
appear to move opposite to the movement of the mirror
and opposite to the movement of the facial illumination.

Fig. 144.

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. 142); but just as soon as
the observer's eye is within the point of reversal, the retinal
illumination will apj^ear to move against the movement of
the mirror and against the light on the face. (Sec h'ig. 143.)

Rate of Movement of the Retinal Illumination. — This
is influenced by several factors, but practice will teach the
ob.server that when the retinal illumination appears to move
slowly, the refractive error is a high one, and when it mo\-es
fast, the refractive error is a low one.

Figure 144 represents a myopic eye with its far point

RETINOSCOPY. 1 63

(point of reversal) at R', and when rotating the mirror, this
point moves to K" ; but if the eye had its far point at V,
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 145, where 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. — A large, round illumination
may signify emmetropia, hyperopia, or myopia, with or with-
out astigmatism in combination. Astigmatism is recognized

Fig. 145.

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 — /. r., 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 moxement of the retinal
illumination. — With the plane mirror the rays of light are
.reflected as if they came from a point just as far back of

164

REFRACTION AND HOW TO REFRACT.

the mirror as the original source of Hght 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. 146.)

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 /;/ 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. 146.

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^), and the lower portion of the pupillary area be-
comes illuminated. (See also Fig. 142.)

Rule for Neutralizing Lenses with the Plane Mirror.
— When the retinal illumination appears to mo\e 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

KETINOSCOPV. 165

that in which the mirror is tilted, the obsen'er is be)'ond
the point of reversal, and a minus lens must be placed
before the e\'e to stop all apparent movement.

Rule for Neutralizing Lenses with the Concave
Mirror. — When 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 Emmetropia (Fig.
146). — Rays of light proceed parallel from an emmetropic
eye under the influence of a cycloplegic, and if a + i 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
ha\e been obtained at }'^ of a meter if a -f- 3 S. had been
u.sed, or at 4 meters if a -|- 0.2 5 S., or at j4 of a meter
if a +2 S. had been used, etc.

In taking the patient from the dark-room to test his
vision at 6 meters, an allowance must alwa)'s 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, 4 S.; if at
6 meters, nothing, or o. 1 2.

l66 REFRACTION AND HOW TO REFRACT.

Application of Retinoscopy in Hyperopia. — The

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 -{-4 S. 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,
j4 of a. meter, ^ of a meter, or 3^ 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. T^^r example :

Darkroom, o.oo 40.258. -I-0.50S. -[-0.755. -|-i.oo.S. -f 1.25 .S.
Add, . . . — i.ooS. — i.ooS. — i.ooS. — i.ooS. — i.ooS. — i.ooS.

Infinity, . .— i.ool). — 0.75 D. — 0.50D. —0.25 I). 0.00 -f 0.25 I).

Application of Retinoscopy in Astigmatism. — If the

surgeon has mastered retinoscopy in hyperopia antl myopia,
he should not have any difficulty in pursuing exactly the

KETINOSCOPV.

167

same course in cases of astigmatism. As already stated,

the presence of astigmatism is diagnosed by the presence

of a band or ribbon-hke streak of bright illumination which

extends across the pupillary area.

(See Fig. 147.) 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 :

-j-0.75 sph. O +4-50 cyl. axis 105 degrees.
— I.CX3 sph. 3 — 5.00 cyl. axis 165 degrees.

Fig. 147. — Band of Light.
Astigmatism Axis 90 de-
grees.

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. 3 +0.75 cyl. axis 75 degrees.
— 5.00 sph. 3 — 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 tlicii the meridian of greatest ametropia.
Taking the following formula :

-)-2.50 sph. 3 "l-i-OO cyl. axis 90 degrees ;

in the dark-room a +3.50 S. would make all movement
cease in the vertical meridian, at one meter distant ; but
when the mirror is tilted in the horizontal meridian, tiiere
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
the writer advises using spheres, and after each meridian

i68

REFRACTION AND HOW TO REFRACT.

has been refracted, to make the cyHndric correction, and
prove it, if so desired.

Axonometer. — To find the exact axis subtended by the
band of hy;ht 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

Fic. 148.

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 a.xonometer, 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

KKTINOSCOPV. 169

axonomcter slowly until the two heavy white lines accu-
rately coincide, or appear to make one continuous line with
the band of light. (See Fig. 148.)

The degree mark on the trial-frame to which the arrow-
head 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 result (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 hypcropic. 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-
edly 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 kinetoscopic picture obtained by mo\'ing the
mirror so as to describe a circle at the periphery of the
pupillary space is quite diagnostic of the corneal condition.
Whatever result 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 \arict\', and is characterized
by faint striiu in the lens, pointing in toward the center. If
IS

I70

REFRACTION AND HOW TO REFRACT.

the Striae are not very faint, they may be recognized with the
ophthahnoscope, even before any cycloplegic has been used.
Scissor Movement (see Fig. 149). — This is a condi-
tion where there are two bands of Hght present, usually in
the horizontal meridian or inclined a few degrees therefrom.
Tilting the mirror in the vertical meridian, a band of light
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 c}'linders with the axis corresponding to the
axis of the two bands. The resulting prescription should
also be a plus sphere with a minus cylinder, the cylinder
of less strength than the sphere, as the condition is not one
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 obser\'er slioiiid also be on the lookout for a biuul of
light in this central illumination, as most of these cases are
astigmatic.

Fig. 149. — Scissor Move-
ment.

KETINOSCOPV.

171

Spheric Aberration. — This is of two kinds — positive and
negative. (See Figs. 150 and 151.) 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

Fig. 150. — -Positive Aberration.

Fig. 151. — Negative Aberration.

this condition, negative aberration, is seen in conic cornea.
These two varieties of refraction should not worr>^ the
observer, as most of the peripheral aberration is co\'ered
up by the iris when mydriasis passes away, and, therefore,
is not of any great moment, except in conic cornea.

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 both 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 arc 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).

(a) The image of an object formed upon the retina above
the fovea is projected downward — /. c, objects situated
below the horizontal line of vision are recognized b}' that
portion of the retina above the fovea.

{/>) The image of an object formed upon the retina below
the fovea is projected upward — i. c, objects situated above
the horizontal line of vision are recognized b)' 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.

172

MUSCLES.

173

{(I) The imaijc of an object fornietl on the retina to the
temporal side of the fovea is projected toward the nasal side
— /. c, objects to the nasal side have their imat^es formed
upon the temporal portion of the retina.

Homonymous Diplopia (Greek, utiwwiw:; from i,i,.u^,
same, and Zvofia, name). — Mt^ure 152 shows the right

Fk;. 152.

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 fal.se
object to the left of the real object. This condition of the
objects is spoken of as homonymous diplopia.

174

REFRACTION AND HOW TO REFRACT.

Heteronymous Diplopia (Greek, ?r£/«;r, other ; and ovufxa,
name). — Figure 153 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.

Hyperphoria
(Greek, u-ep, over,
above ; il'opa, mo-
tion).— In the con-
sideration of vertical
diplopia, — which is
always a condition
of crossed diplopia,
never homonymous
diplopia, — the eye
which is deviated up-
ward is sj)oken of as
the hyperj:)horic eye,
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 jjrojccted downwartl below the true
object ; and this position of the right e)-e is spoken of as
right h}'perphoria. Or if the right eye fixes an object and

O'M

Fig. 153.

MUSCLES. 175

the left eye sees a false object below, then the position of
the left e)-e is spoken of as left hyperphoria. Unfortunatel)',
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 e}-es, 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 wdiich will just permit the eyes to see one object
and if increased w^ould produce diplopia, represents the
power of the external recti muscles. This is spoken of as
the power of abduction, and is abbreviated Abb. 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 tiiat the
prism is placed base outward. This is spoken of as adduc-

1/6 i<i:fraction and how to rkfract.

tion, and is abbreviated Add. For example, if with 19
centrads, base out, before the eyes two Hghts are seen, and
with iS centrads only one lii^ht, then 18 centrads represent
the power of adduction. In other words, as long as there is
a prism of 1 8 or less than 1 8 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.

Supra- and Infraduction. — The power of the superior
and inferior recti muscles is tested at the same distance as
in testing the lateral muscles. To test the power of the
superior recti muscles, a prism is placed, base down, before
either eye, and to test the power of the inferior recti the
prism is placed base upward before either eye. The power
of the superior recti is spoken of as supraduction ; the
power of the inferior recti is spoken of as infraduction. In
health the superior and inferior recti muscles, as a rule, an-
tagonize each other equally. Usually, the power of infra-
and supraduction is the same — about 2, 2 ^, or 3 centrads.
That is to say, a 3^ centrad prism, base up or down,
before either eye will produce diplopia, whereas a 3 cen-
trad will not.

According to Risley, the relative power of adduction
to abduction is as 3 to i ; that is to sa\', in c}'cs with
normal muscle balance, if adduction is represented b)' 1 8
centrads, then abduction should be 6 ; or if adduction is rep-
resented by 24 centrads, then al)iluction shoukl be 8 cen-
trads. And according to the same authorit}-, the i)ower
of supra- and infraduction is usually 2, 2j,j, or 3 centrads.

MUSCLES. 177

Muscular Imbalance. — Whenever there is any disturb-
ance in the power, strenLjth, 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 simultaneoush- " fix " an
object, any more than it must be supposed that a In-peropic
eye can not see or hav^e .- vision without correcting
glasses.

Just as in h}'pcropia 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 :
Ortliophoria, perfect muscle balance, equipoise, or binocular

equilibrium.
Orthotropia, perfect binocular fixation.

Heterophoria, imperfect binocular balance, or imperfect bin-
ocular equilibrium.
Hctcrotropia, a squint or decided deviation or turning from

parallelism.
HypcrpJioria, a tendency for one eye to deviate upward.
Hvpertropia, a deviation of one eye upward.
Esoplioria, a tendency of the visual axes to de\'iate inward.
Esotropia, a deviation of the visual a.xes inward.
ExopJioria, a tendency of the visual axes to deviate outward.
Exotropia, a deviation of the x'isual axes outward.
HyperesopJioria, a tcndenc}' of the \isual axis of one eye

to deviate upward and inward.
Hyperesotropia, a de\'iation of the \isual a.xis of one eye

upward and inward.

178 REFRACTION AND HOW TO REFRACT.

HypcrcxopJioria, a tendency of the visual axis of one eye

to deviate upward and outward.
Hypcrcxotropia, 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 where 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 ai)pcar double for a moment. Sometimes one of
the eyes feels as if it was turning outward or inward.
There are innumerable reflex .symptoms, dizziness, nau.sea,
vomiting, fainting, and, in some instances, "all becomes
dark for a minute." Such patients often become very
anxious, fearing sudtlen blindness, etc.

MUSCLES. 1 79

Diagnosis or Tests for Insufficiency (Heterophoria). —
Before taking up the iiulividual 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 tlie patient's eyes in the median line, and distant aljout
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 ey^e 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

l8o REFRACTION AND HOW TO REFRACT.

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 tiie 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 ha\e
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

musci.f:s. i8i

base inward which will brin<j one hght directly above the
other.

To Determine Vertical Insufficiency (Hyperphoria). —
Proceed b)' produciii<^ lateral diplopia. Place a ten centrad
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 talked upon the wall.)
If the two lights which the patient sees are on a horizontal
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.

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 j^rism.
placed base outward, before the left eye which will bring
the two dots exactly, on one line. If the upper dot is to

182

REFRACTION AND HOW TO REFRACT.

the left, then there is insufficiency of the interni (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.

These tests for insufficiencies should always be made be-
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, where a point
of light is used as the fixing object, it is customary to place

Fig. 154. — Maddox Rod.

Fig. 155.

a piece of plane dark red gkss before one eye, so that the
red light always corresponds to the eye with the red glass.
Or a Maddox rod (Fig. 1 54), white or red, may be used
for the same purpose. This may be a series of rods (see
Fig. 155) 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
4- cylinder from the trial-ca.se 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.

183

that in which its axis is placed. To expedite the deter-
minations, tlie rotary prism of Cretcs or the revolving
prisms of Rislcy may be employed. This latter apparatus
(see Fig. 1 56) 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
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 ^2 inches from the eyes, and 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. 157.)

Treatment of InsuflBciencies. — 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 studicti 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 tiie

1 84

REFRACTION AND HOW TO RKFRACT.

patient may have to give up any close application of the
eyes for a time and pursue an out-door life. Operative
interference (tenotom)-) 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 permanent
use, which correct insufficiency, except in vertical errors, 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
Trial Frame

Revolving

Rotary Prism

Fig. 157.

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 da}\ The modus operandi
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 xomica is the treatment piv cxcclloicc, and it acts
best in those cases where the nerve edges and the e\-e-
ground in general appear clear and free from irritation.

MUSCLES. 1S5

To use mix vomica it must be i^n'veii iu the form of the
tincture and increased, one drop at eacli dose, until the
patient becomes quite tolerant of it, taking as high as thirty,
forty, or even fifty drops three times a day, and then the
dose is gradually diminished. Nux vomica does not seem
to do well in cases in which tiie bromids are indicated as
above, and vice versa.

Treatment of Insufficiency of the Internal Recti.
— Because the tests for heterophoria at 6 meters show an
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
into consideration in every instance before coming to any
such conclusion.

After the proper correcting glasses have been prescribed
and 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
of systematic exercise, known as ocular gymnastics. That
success shall result from ocular g\'mnastics 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 —

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 exerci.se
and should be practised several times a day and a number
of times at each sitting.

2. Prism Exercises. — The patient is placed, standing,
about a foot or two from a point of steady light, on a level
or slightly below the le\el of the eyes, and told to look at

I<S6 REFRACTION AND llOW TO REFRACT.

it, and at nothing else. In this position a pair of weak
prisms, bases out, 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 again. This exercise is repeated
three or four times a day and 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 e.xcess 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 tlie ex-
ternal recti ; for when the exercises are stopped some of the
strength of adduction will ra])idly disappear.

it has been incidental!}- mentioned that prisms should

MUSCLES. 187

not be prescribed in combination w ith the ainetropic cor-
rection for the treatment of insufficiency, and yet there is
an occasional exception to this statement in cases which
must have prompt, thout^h temporary, rehef 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 may be thrown aside at any time. The full prismatic
correction (except in hyperphoria of presbyopia) is seldom
ordered, — only about two-thirds of it, and this is divided
between the two eyes — base down before one, and base up
before the other.

Treatment of Insufficiency of the Externi (Esophoria).
— As esophoria is a tendency of the visual axes to dc\iate 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-
modates at the same time that he endeavors to maintain equi-
poise (relative hyperopia), he may at times suffer severely.
If the symptoms of muscular astheno})ia persist after pre-
scribing the ametropic correction, then prisms, bases out,
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 gi\es the po.ssessor an\' discomfort
whatever. Prism exerci.ses for esophoria accomplish very
little, if an\', benefit, and often arc a waste of time ; yet they
should be tried thoroughl\- if the case appears to demand it.

1 88 REFRACTION AND HOW TO REP'RACT.

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///// 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 tJic amoiuit 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, lu'steric
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 immediatel)' before, as well
as during and after, the operation. When a simple tenot-
omy is performed, the eye is usual!}- left open (unband-

MUSCLES. ■ 189

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.

Strabismus [<Tzoi<fuj, " to turn aside ") ; 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 one eye is deviated
from the point of fixation. The eye which has the image
of the object on its fov^ea 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 parah'tic.

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 a.xis of one e}'e is deviated inward, the other
fi.xing the object ; or one eye fixing an object, the \isual
axis of the other eye crosses that of the fi.xing e}-e closer
than the object. (See Fig. 152.) This is the most common
form of squint. Both eyes ha\e some form of h)-peropia,
as a rule, the squinting c\'e usually being the most ame-
tropic. The diplopia as a result of this condition is
homonymous.

I go ■ KKFKACTION AND HOW TO REFRACT.

Divergent squint (di, "apart," and vergere, "to in-
cline ") ; also called external squint (strabismus divergens),
exotropia. (See Fig. 153.) 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 only cross
it by being projected backward. The diverging eye is
usually m)'opic.

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
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 man\- and \arious. The
chief causes, howexer, are: (l) Ametropia, uliich nia\- ]iro-

MUSCLIuS. 191

duce a change in the normal rehitionship between accom-
modation and convergence ; (2) anatomic anomaUes ; (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 con\er-
gence, or convergence without accommodation, yet there is
an affinity between the two processes which, if material!)- 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 In-peropia 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 13 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 e)-e deviates
inward and ultimately remains convergent. This act of
convergence explains the presence of convergent squint in
hyperopia, and also shows why the squinting e)'e usually
has the higher refractive error. It must not be supposed
that all hyperopic eyes have a squint, as some of these can

192 REFRACTION AND HOW TO REp-RACT.

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 m)'opic eyes
necessarily have squint, as some of them have room}' 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 mter-
pupillary distance, predisposes to greater con\-ergcnce 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 conx'crgent
stiuint, wliLM-cas strong external recti would dcNclop di\-er-
gent scjuint.

MUSCLKS. 193

4. Amblyopia. — Statistics show tliat 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 — i. c, 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 cau.ses above mentioned, it may be-
come amblyopic from not being used (amblyopia exanop-
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 arter\' coming 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 an}' rcfrac-
ti\'e error, forgetting that the supposed causes the}' 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 ha\'c 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
ha\'e to be placed in front of the fixing eye. The strength
of the prism required to bring the two lights together will
be the prismatic estimate of the deviation. Or the amount
of the squint meiy be roughly determined with the strabis-
17

194

REFRACTION AND HOW TO REFRACT.

mometer. (See Fig. 158.) 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
deviation is supposed to represent
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. 159.) To do this, the patient
is seated with the squinting eye
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, i)laces 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 sciuinting eye. The degree mark on the arc from
which the Hanie was pictured represents tlic amount ^^'i the

MUSCLES.

'95

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 tiie

Fk;. 159.

optic axis and not the visual axis, but for purposes of
approximation they are considered as the same.

Treatment of Strabismus. — .\s ametropia is the dhief
factor in the cause of squint, this cause must be promptly

196 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 e)'cs for near-work
for several days or weeks.

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, final!)' becomes con-
stant. .Such e)X's shouUl be refracted at once, ami before
amblyopia e.\anoi)sia can be estaljlished. It is interesting

MUSCLES. 197

to note that the eyes in many young cliildrcn bes^nn 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 e}-es 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 waj'S.

Fu;. 160.

One is to use drops in the fixing eye, and thus compel the
squinting eye to do the seeing ; or to cover the fixing eye
with a blank over the glass (see Fig. 160), and have the
patient practise in this way for one or two hours each da)-,
using the squinting eye alone.

Cases that are cured by correcting the amctrojiia must
wear their glas.ses constantly. Glasses in such cases can
.seldom be abandonetl. In young children the squint re-
turns almost at the instant the glasses are removed. The
earliest age at which glas.ses can be jirescribed is three )-ears
or thereabouts, as it would be unreasonable in i/iost ca.ses
to expect a child to appreciate the glasses as anything but a
toy before this age.

198 REFRACTION AND HOW TO REFRACT,

The younger the patient when glasses are prescribed, the
more favorable the prognosis and less likelihood of a ten-
otomy. The older the patient wiien glasses are ordered, the
less likelihood that glasses will cure the squint and the
probability of a tenotomy being necessary. This is ex-
plained from the fact that the squint having persisted for
a long time, the muscle which held the eye in the deviated
position has grown strong and the opposing muscle 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, but not
always.

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.

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 which cor-
rect the ametropia have been worn constant!}' for weeks or
months, and perchance for a year or more. 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 anatomical!)' short muscle, then
there need not be an)- great delay in operating after glasses
have been ordered.

Wlicnevcr a tenotoni)- has l^ecn performed, the c\'cs

MUSCLKS. 199

should ai^fain be carcfull)' 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 maybe 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 "
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 an)' 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, /.u/lnq, "a circle," — /. c,
the cihary ring, — and -Ir^yi^, " a stroke ") is a drug which
will temporarily paralyze the action of the ciliary muscle.

A mydriatic (from the Greek, iw^jnad'.q, "enlargement
of the pupil ") is a drug which will temporarily dilate the
pupil.

Atropin will dilate the pupil and also cause a paral}'sis
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 not
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. Cycloi)legics
may be classed as of three kinds : (i) those the effect of
which passes away slowly ; (2) tho.se the effect of which
passes away moderately fast; and (3) th(\se the effect of
which is very brief.

The finst effect of a cycloplegic is its mydriatic (quality,

200

CYCLOPLEGICS. 20I

after which the accomniod;iti\'e effort is suspended. The
paralysis is not permanent. The foUowins^r table, from Jack-
son, shows the leni,^th of time paralysis persists and the
time it takes for the ciliary muscle to fully recover :

Atropin, effect begins to diminish in 4 days ; complete recovery, 15 days.

Daturin, " " " 3 " " " '° "

Hyoscyamin, " " " 3 " " " °

Duboisin, " " " 2 «' " " 8 "

Scopolamin, " " " labours. " " 6 "

Horaatropin, " " " 12 '* " " 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
eye, showing that the drug does not pass through the car-
diac circulation ; otherwise, the pupil and ciliar>^ muscle of
the fellow-eye would be similarly affected.

Some conjunctivas are very sensitive to any of the.se
drugs, and develop an inflammation so severe in individual
instances as to resemble ivy poisoning of the lids. 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 ver)' unpleasant s\-mptoms, such as dizziness, dry
throat, flushed face and bod\' (mistaken for scarlatina), rapid
pulse, a slight rise of temperature, and delirium. To avoid
such an annoyance, which is apt to reflect discredit u[)on
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 u.se of the drug and

202 REFRACTION AXn HOW TO REFRACT,

applying cold compresses will relieve the conjunctivitis, and
if constitutional symptoms manifest themselv^es, a dose of
paregoric, cooling drinks, a darkened room, and stopping
the use of the drug will soon restore the patient.

Form of Prescription.

N^atne, Mr. Brown.

tJ. Atropin. sulphatis, gr. 1

AquiJe dest., f^'j-

M. Ft. sol. Label, poison drops ! !

SiG. — One drop in each eye Uiree times a day, as directed.

R. Dropper. Dr.

Dale, Tuesday, March 14, 1899.

The reason for labeling this prescription " poison drops "
is not to frighten the patient, but to caution him against
lea\'ing the medicine around 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 ej'e
three 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 paral)\sis, and also to give the eyes a physi-
ologic rest. In having the.se " 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) \s placcd{\\o\. dropped) on the sclera at the upper and
outer part. After the drops are placed in the eyes, as far away
from the puncta lachrymalia as possible, the patient holds
the canaliculi closed b}' genth' ])ressing 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-

CVCLOPLEGICS. 2O3

CLition, and the writer has seen a few sucli cHnical subjects
flushed and dcHrious 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 tlie drug is
allowed to flow o\'er 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
there is the least suspicion of glaucoma in one or both
e}'es. 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
as presbyopia, it is seldom necessary to use a C}'cloplegic.
If a cycloplegic is necessary in presbyopia, one of the
weaker drugs is generall)- employed.

In the selection of a c)xloplegic 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 ej-c, making their emplo\-mcnt on this account
very objectionable in many instances. The accommodation
returns sooner after the use of hyoscyamin and duboisin
tlian 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 atroj)in or daturin used in his e\'es, 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, ill man\- instances, have their e)'es refracted in at

204 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
less 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 hype rope.

Whenever a cycloplegic is prescribed, the patient should
be ordered a pair of smoked-glass spectacles to wear dur-
ing the mydriasis. Of the two forms of smoked glasses, —
coquilles and plane, — the latter should always be preferred,
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. Bkown :

R. One pair plane London snioked " D."

Sir,. — F(3r temporary use. Dr.

Marcli 14, 1899.

CYCLOI'LEGICS. 205

The cycloplegics above mentioned for purposes of re-
fraction are ordered in the followinij strenLrths :

Atiopin. sulphatis, . . .
Duboisin. sulphatis, . .
Hyoscyamin. sulphatis, .
Daturin. sulphatis, . . .
Scopolamin. hydiochlor..

■ g*"- J l*^ ^'1- flest., . . f^ij.

• gr-ss " " " • • fS'j.

. gr. ss " " " . . fg iss,

. gr. ss " " " . . f^ij.

. gr. j " " " . . f3J.

All these, except scopolamin, are ordered to be used
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
it is never necessary to prescribe more than one grain for
any one patient. The writer has found the following most
satisfactory, though the strength of the homatropin may be
increased if desired :

For Miss Robinson :

li . Homatropin hydrobromate, gr. j

Aq. dest., TTL xl.

M. Ft. sol. Label, poison drops ! !

SiG. — 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 m}'driasis 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 mydriatic, and only becomes a c\clopIegic under
definite u.sage.

To produce cycloplcgia with homatropin, the patient is

206 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 the next day 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 \s 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 "blood-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, the anatomy of the ciliary muscle shows that
it is possible, with a drug like homatropin, to ha\'e some of
the sphincter-fibers become paralyzed while others ma\'
remain free to act. In this way a spasm of the ciliar)-
muscle may be produced that will give a false astigmatism.

CVCI.OI'I.F.C.ICS. 207

The writer docs not recommend this method of forcinL''

the cihary muscle into repose.

To somewhat obviate the "blood-shot" condition, and
also to assist the action of the "forcing" process, an occa-
sional drop of a two or four per cent, solution of cocain
may be instilled while the homatropin is being used. This
also diminishes the danger of spasm. But cocain is objec-
tionable in that it will, in some cases, "haze" the cornea.
The retinoscope will show this, and the patient will state
that, while 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, and one 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 cycloplegic should be held in reserve
for indi\idual cases, and not used as a routine practice. It
is a good, reliable paralyzcr 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
e)-es can become accustomed to the prescription glasses,
the ciliaiy 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,

208 REFRACTION AND HOW TO REFRACT.

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.

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 -I-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. The 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. — Temporar)- paraK'sis of the ciliarj' muscle ami
iris, as already stated, will result from the external or internal

CVCI.Ol'LKOIA.

209

administration of a cycloplcgic. It is intcrestin<j, in man}-
cases, to find the cause and relieve the patient's anxiety
when the paresis is due to one of the cyclople<^ics. 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 tlie 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 S)'philis, mercury and iodid of potash must be
prescribed. Dark glasses for the photophobia should
ahva\-s be ordered, and lenses for near-work ma)- be worn
as a temporary expedient. The use of eserin locally will
occasionall}' do good work, but is }iot advised for constant
use or for ever}' case. Faradism ma}- be used if the c}-clo-
18

2IO REFRACTION AND HOW TO REFRACT.

plegia is very persistent, but the best results may be ex-
pected from systemic treatment. Tlie use of str}xhnin or
nux vomica are recommended in certain instances.

Cramp of the Ciliary Muscle.

Cramp of the ciHary 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 called " spasm " of the accommodation,
is a permanent condition as compared with the clonic form,
and occurs whenever the eyes are used for distance 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 hyperoj)ia
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
form, and is usually seen among young hyperopic children,
or the " jiseudo-myope " already described. It also occurs
occasionally in hysteric patients or those recovering from
some severe or lontr illness. The writer has seen this k)rm

CRAMP OF THE CILIARY MUSCLE. 211

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
c}'cloplegic 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
for, and frequently prism exercises to develop the strength
of the weak muscles ma}' 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, «. priv. ; trOho-:, " strength ";
w(^', " eye ") means a weakness or fatigue of the eye, appljing
especially to the retina, the ciliary muscle, the extra-ocular
muscles, or a general weakness of any one or two or all

212 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-
strain," 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 usuall)' occurs in females. It is brought about
by overuse of the eyes in too dim or too bright a light,
and ma}' 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 \ibrations, sounds, and lights ; in a patient
whose ner\-ous 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
not good practice to restrict all use of the c\'es. 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 the}' event-
ually become an absolute necessit}' on all occasions. Care-

CRAMP OF TIIK CII.IAKV MUSCLE. 213

ful attention to the ijeneral health is certainl}- indicated ;
tonics, out-door sports, etc., shoukl 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 (exophoria). 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 hypcremic during or after the use
of the eyes, simulating rheumatism of these structures. In
other cases dim \'ision 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 docs 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
ciliary muscle ; it is, therefore, to be expected in hyperopic
eyes. It is caused in various wa)'s : from overuse of the
eyes in too bright or too dim a light, or from using the eyes
for too long a time in an\- kind of a light. The best pair
of e)'es, if overta.xed, may suffer from accommodative
asthenopia, even when wearing the ametropic correction.
Or accommodative asthenopia may result from a weakness

214 REFRACTION AND HOW TO REFRACT.

of the ciliary nuiscle as a part of the L,^eneral condition of the
whole body, and this may come on after or during some long
illness, such as typhoid fever. Accommodative asthenopia
is 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 H}'peropia 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 ; thc\' 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 extern cye-slrain is rcspt)nsible lor

EXAMINATION OF THE EYES. 21 5

these and many other reflexes the writer is not prepared
to say, though every ophthalmologist has certainl)' 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, and in this respect each surgeon may follow
his own desires :

Date,

Name,

Residence,

Occupation,

Age, Sex, Diagnosis,

Accommodation. Astigmatism. Muscles.

O. D. v., p. p.

O. S. v., p. p.

History,

.S'. /'. [status fncsens, *' present condition "). Inspection,

Ophthalmometer, O. D O. S

Ophthalmoscopic examination, O. D O. S

Manifest refraction. Fields. Color sense. Ji .

2l6 REFRACTION AND HOW TO REFRACT.

The above record is filled 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's age and near point
will 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
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
''headache." To get a com{)lete history of the headache,
and be able to differentiate it from headache due to other
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 e}'cs 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 EYES, 2iy

during or after attcndin<^ a place of public amusement or
when shopping ? If a female, is the headache onl)- 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 arc 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
p)or distant vision and good near vision, or vice versa ; this
should be inquired into \'ery careful!}', 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 }'ears of age.

If the patient wears glasses, it is well to inquire whether
they were ordered b)' an ophthalmologist or if the)' are
19

2l8 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 two 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 pnesciis) :

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.

12. Use the cover test at 13 inches for an)' muscular
anomaly.

I<\)]l()wing this record of the histor)- and present condi-
tion, the distant x'i.sion and near point arc taken for i-ach
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. 2I9

momctric measure of corneal curvature may be recorded.
I'inally, and mo.st 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 69 (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
point 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 (m}'opic) 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 jxationts out of
five have hyperopia, or one patient in five has myopia, and
the minus sphere selected almost in\'ariabl\' requires a
C}'lin(kr in coml)ination. Rememht'i-, also, that astigma-
tism is usually with the rule and s\-mmctric, and th.it plus

220

HOW TO REFRACT, 221

cylinders arc 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 180.

The Placing of Trial-lenses. — i. These should always
be placed as close as possible to 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 -' , — a strong lens of 2, t., or 4 D. will often be re-

XL ' LX' '^ ^ ^

quired ; whereas, if the vision is ^7y^ 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 -|-2 sph. has
been placed before the eye and the vision is improved from

— to -^^ : this 4- 2 sph. should not be changed until a
XX vnss ' "- ' I i '^

-|-0. 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 ^y is approximated, then its accuracy
must be determined by placing first a -f O.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 u.sed 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

222 REFRACTION AND HOW TO REFRACT.

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 — , 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 1 80, and revolved in the
same manner. The rule (4) for changing spheres also ap-
plies to cylinders — i. c, to increase or decrease the strength
of the cylinder by placing in front of it a plus or minus
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 cx'linder
improves the vision, then the surgeon may employ a sphere
of the strength of the sphere a)id cylinder combined, and
use a cylinder of the same strength as before, but with
opposite sign and at about the opposite axis, l^'or example,
with +2.25 sph. O I 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. 3 — 0.75 c\-l., when the

HOW TO REFRACT. 223

exact axis (at ri^ht angles) will usually be selected without
any hesitancy or doubt.

9. Proving the Correction. — All tests at the trial-case,
where a cycloplegic is used, should be confirmed with the
retinoscope.

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 cyl. 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.QQ cyl. axis 90 gives a
vision of ^;,t^^, and the crossed cylinder lens is placed in
front of this combination with — 0.25 at axis 180, and the
-(-0.25 at a.xis 90, and the \ision comes down to , it
shows that the vertical meridian was 0.25 too strong, and
the horizontal 0.25 too weak, and the result would be
+ 1.75 sph. O +1.50 cyl. axis 90 degrees. Or, if — 3.00
sjih. has brought the vision to — , and the crossed c}'linder
lens is placed before it and rotated to a.xis 1 5 for the
minus cylinder and axis 105 for the plus cylinder, and the
vision comes to — , the result would be — 2.75 sph.
O — 0.50 cyl. axis 15.

Methods of Estimating Refraction. — To determine
the refraction of an eye it ma)' or may not (as in presby-
opia) be necessary to employ a cycloplegic. When the
refraction is estimated without a cycloplegic, it is spoken of
as manifest or dynamic (Gr., Sovatu^, "power") refraction.
When a cycloplegic is used, the refractive estimate is spoken
of as static (Gr. ffrarUd':, from, IqTd'Mu, " 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 antl then to estimate the strength of the

224 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
two 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 dis-
pensed with. (See Presbyopia.)

Manifest or Dynamic Refraction. — This method is
often pursued with the idea or purpose of obtaining some
approximation of what the character of the refraction may
be, and never for the purpose of prescribing glasses except
in presbyopia, and some i&w exceptional cases as a tempo-
rary expedient. The routine habit of prescribing glasses
from the manifest refraction without any knowledge of the
ophthalmoscopic record is not a method that merits the
attention of the conscientious physician. Such work is
very unscientific, often leading to gross errors, ultimate
dissatisfaction to the patient or injury to the eye.

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 : W'hile the
ciliary muscle is paralyzed with atropin the static refraction
is found to be +1.50 sph. O +2.00 cyl. axis 90 degrees,
which gives a vision of ^r^. The atropin is then stopped
and 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 \ision just c(|uals
-— , as it was when the " drops " were in the eyes. What-
ever this result with the glasses maybe, is ordered. Occa-
sionally the strength of the cylinder as well as its axis is
also ciian"fed.

HOW TO REFRACT. 22 5

Objections to the Postcycloplegic Method. — The pa-
tient is annoyed by the h^ny; delay to whicli he is subjected
before getting his ghisses. 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 fof 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
without any change in the cylinder. The only possible
objection to this method is in cases of hyperopia, where,
after the effect of the cycloplegic passes away, the ciliary
muscle may endeavor to accommodate for distance with
the glasses in position, and the result will be 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 ciliar}' muscle ; the e\'es are
thus placed in an emmetropic condition. If this effect of
the cycloplegic passes away before the patient can receive
the glasses, it will be necessary to use the drops for a day
or so wlien the glasses are received. UnfortunatcK', how-
ever, some hyperopic eyes, in young subjects especial!}',
with vigorous ciliary muscles, will de\elop a spasm of the
accommodation for distant \ision which will make the

226 REFRACTION AND HOW TO REFRACT.

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 (2^ 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.
TJicre is no hai'd and fast ride as to just how much 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 the patient's age and occupation ;
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
very liberal deduction in the strength of the glass and the
patient be allowed to use his relative hyperopia :

Static refraction at 6 meters :

-f 1. 00 spli. or less deduct o. I2 or 0.25.

Yvom -\- 1. 00 spli. up to 3.00 sph. " 0.25 or 0.50 or 0.75.

" 4 3-00 spli. up to 6.00 sjjh. " 0.50 or 0.75 or i.OO or 1.50.

" 4-6.00 spli. up to 8.00 and above " I. or 1. 50 or 2. 00 up to 3.00.

HOW ']•() KKI-KACT. 22/

It is true that glasses ordered in this way do not lea\'e
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 ad-
vantages : first, giving tiie patient his glasses without any
long delay, and giving the eyes an opportunity to become
accustomed 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.

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 ever}- point of view. But as the student must
have s<^me idea as to how to proceed, the writer would sug-
gest the following subdi\'isions :

'. Myopic ej'cs 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

228 REFRACTION AND HOW TO REFRACT.

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 unusualh- 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 stajoh^'loma. 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 he uses
only one eye for near vision. The diverging eye is usually
more or less amblyopic, due to want of use or jxithologic
changes, or to both. Such eyes have lost almost or cpiite
all the power of accommodation. These eyes must be
placed in such a condition that the desire to converge and

HOW TO KKFKACT. 229

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 ncces.sary 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 raj^s 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 an}' 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-
mo\'ed from the eyes.

Caution. — The great danger in an)- refraction at the trial -
case, but especially in myopia, is an overcorrection, and
this is very likel\' to occur if the surgeon is not extraor-
dinarily careful in ha\ing his lenses placed as clo.se to the
eyes as possible.

230 REFRACTION AND HOW TO REFRACT.

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 m\-opia is
produced, since it has been stated that the comlilion is rarely
seen in young children. It is well known that astigmatism

HOW TO REFRACT. 23 1

(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. 3 +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. 3 +0.75 cyl.
axis 90 degrees, mixed astigmatism. In two }'ears more the
refraction is found to be — 0.25 sph. O — 0.75 cyl. axis
180 — /. r., compound myopic astigmatism. From this time
forward these eyes progressively stretch and are subject to
the stretching process unless the progress is staj'ed 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 abundantl)- demonstrated by
statistics.

232

REFRACTION AND HOW TO REFRACT.

The following summary of refracti\'e 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.

Monocular astigmatism, 70 5-0%

Binocular astigmatism, I151 88.5

Total cases, 1300

Binocular symmetric astigmatism, . . . 694 60.2%

Binocular asymmetric astigmatism, . . . 310 26.8

Heteronymous astigmatism, 123 10.6

Homonymous astigmatism, 24 2.1

Total binocular astigmatism, . . .

Symmetric astigmatism :

{/i) According to rule (homologous),
(/') Against rule (heterologous), . .

Total symmetric astigmatism, . .

I151

543 78.2%
151 21.8

694

Asymmetric astigmatism :

{(f) According to rule, 223 71. 8j^

{//) Against rule, 87 28.2

Total as)nnmetric astigmatism, . . . 310

Hospital.

94 7-8%
828 69.0

613 74-4%

158 198

40 5.2

17 1.2

"828

559 97-7%

54 2.3

126 79.1%

32 20.9

"158

Direction of the Meridian of Shortest Radius in all Casf-s of
Symmetric Asticmatism.

Meridian at 90°, S7-0+%

Meridian inclined 15° or less on each side, 19- 7 -|-

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° to 0° on each side, 1 2.0

Meridian inclined 15° or less on each side, 4.0 —

Meridian inclined from 15° to 30° on each side, 2.0

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 moeling of tin- Anurican Medical Association, at I'altimori', .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 -ttt- 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 lOO cases :

January 3, 1899. JoHX S^^TII. Age, twenty. Single. Stenographer.
O. D. - . p. p. = type 0.50 D. at 14 cm.

O. S. -— -. p. p. =r type 0.50 L). at 14 cm.
Add.^ 22 degrees ; abd.^ 6 degrees.

History. — P^rontotemporal headaches almost constantly,
but much worse when using e)"es at near-work. Had
severe headaches when a school-boy. Never liked to
study ; preferred out-of-door sports.

S. P. — Face .s\-mmetric, but narrinv. Blepharitis mar-
ginalis. Irises blue. Pupils rountl, 3 mm. in diam. ICyes
fix under cover.

OplithaluioDU'tcr. — Xegatixe.

OpJitJialmoscopc. — Both e\c.s the same. Media clear.
^ ^11

234 REFRACTION AND HOW TO REFRACT.

Disc small and round, with ph}'siologic cup. All vessels
near the disc are seen clearly with +3 S. Eye-ground
flannel -red and accommodation very active.
Manifest or djniamic refraction :

O. D. +1.258. = ^.
O.S. +i.25S.= ^.

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^j^ = 0. S.
V. ^^r. Cobalt-blue glass shows (each eye separately)
blue center and red halo. (See Fig. i 18.)

Retinoscope, with -|~4 S., developed point of reversal at
I meter for each eye.

With trial-lenses, O. D. and O. S. each select -\-t, S.,
which gives a vision of -y~, and they positively refuse to
see ^ 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. The rule for
Infraction in hyperopia is to employ the strongest lens
ivJiich the eye will accept zvitliout blurring the distant
visio)i.

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 4-4 S-. ^^ 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 +4 S..thcn
the principal focus would be at 13 inches; if j 5 S., then
at 8 inches, etc.

The question is, What glasses shall be ordcretl ? The

AITLIKU RKFRACTION. 235

writer would give the following prescription, and instruct the
patient to stop the drops and wear the glasses constantly :

For Mk. Smith.

li. (). D. +2.75 sph.
O. S. -f 2.75 .sph.

SiG. — For constant use.
January 7, 1899.

January 8th : Glasses from the optician neutralize ; are
centered and accurately adjusted.

January 21st : Add. = 18 degrees. Abd. = 6 degrees.
Near point in each e}'e = 10 cm. No headache or dis-
comfort of any kind.

Considerations. — The static refraction as represented by
-I-3.00 sph. means that rays of light which pass through
this lens and focus at the fovea diverge from six meters'
distance, which heretofore we have considered for purposes
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
the eye may have parallel ra\'s focusing on its retina when
looking beyond a distance of six meters. To have been
mathematically exact, -fo. 12 should have been deducted in
place of +0.25.

The purpose in all cases of refraction is to place the eye
in an emmetropic condition, though this is not ahva)'s advis-
able in e\ery instance. The h}'[)eropic eye naturally accom-
modates for distance, and the emmetropic c\'e 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 ciliar\' muscle is at rest.
The accommodation returns gradually. The eye becomes

236 REFRACTION AND IIOW TO REFRACT.

accustomed to seeing at a distance without the assistance
of the ciHar}' muscle. Atropin produces a ph}'siologic rest,
which the overacting cihary 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, w^here 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, -[-3D., and
the negative far point is therefore at 1 3 inches.

The -I-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. — With the one exception of
emmctropia, it will be found that myopia, plain and simple,
without astigmatism, is one of the 7'arcst conditions of the
eye which the surgeon will meet in careful refractix'c work.
About one and one-half per cent, of all patients, by careful
refraction, are found to have simple m\-o[)ia. Therefore
the condition is not common.

APPLIED REFRACTION'. 23/

January 3, 1899. Miss Rare. Age, twenty-five years. Single.

O. D. V. = -— . p. p. = type 0.50 1). at 9 cm.; p. r. at ^;^ cm.

O. S. V. =: -— —. p. p. = type 0.50 D. at 9 cm.; p. r. at ^;i cm.
Add. 16 degrees. Abd. 6 degrees. E.\ophoria, 3 degrees at 13 inches.

History. — Docs 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 ha\e a front seat to see the fitj-
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 siie drops a pin on the floor, has to get on her knees to
find it. Parents 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.

Oplithalmoinctcr. — Negative.

OpJitlialnioscopc shows each eye the same. Media clear.
Disc large and round. Shallow physiologic cup. Narrow
myopic conus at temporal side of disc. Choroidal vessels
seen throughout periphery of eye-ground and extending
almost to nerve margin. All vessels near nerve-head seen
with — 3. S.

Manifest Rcfractioi. — Mach eye — 3.50 spli. gives vision

Cobalt-blue glass gives red center and dark blue iialo.
(See P^ig. 1 19.)

R. Atropin and dark glasses for refraction.

238 REFRACTION AND HOW TO REFRACT.

January 5, 1899: Patient seated at 6 meters from test-
card. O. D. and O. S. vision equals ^ . 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 777771. 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 iveakcst 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.
R. O. D. —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 21st: 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 ax'oitl the danger of c^rvvcorrecting tlie mx-opic eye,
and, to be on the safe side, the glass is usually ortlered as the
patient selects and the retinoscope confirms it. These lenses
make the eyes, to all intents and purposes, emmetropic.

AI'l'LIKO Ki:FKAC'ri()N. 2y)

Advantages of Atropin. — The choroid and retina are
given a physiologic rest that they could not obtain in an)-
other way. The patient will not select too strong a glass,
as was the case in this very instance when manifested.
Myopic eyes usually have large pupils, and do not suffer,
therefore, from mydriasis to the same extent as hyperopic
eyes. The far point remains unchanged. The power of con-
vergence is somewhat relieved by the glasses, which at
the near working distance are of the nature of prisms,
bases in.

Summary. — Age of patient is twenty-five years. Ampli-
tude of accommodation at this age is 8 D. Near point, 9
cm. = 1 1 U., and far point 33 cm. := 3 D. Difference be-
tween near point and far point in diopters = 8 D., which is
the amplitude of accommodation for the patient's age.

Difference between the near point in diopters (11 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.

Ca.se III. — Simple Hyperopic Astigmatism. — Xot an
uncommon condition. About 5.5 percent, of all ej'es ha\"e
this form of refraction.

April 3, 1899. Miss Robinson. Age, twenty- fmir years. Single. Dress-
maker.

VI
O. D. V. = -jj^ ? ? ?. p. p. = type 0.50 at 18 cm.

VI
O. .S. V. = Y^- ? ? ?. p. p. = type 0.50 at l8 cm.

Add., 23 degrees. Abd., 5 degrees. At 6 meters, esophoria 4 degrees ;

at 13 inches, I degree of esoj)horia.

Astigmatic clock-dial shows darkest lines from X to
IV with O. D.

240 REFRACTION AND HOW TO RKFKACT.

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 aiTd 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.

OpJitlialmoinctcr. — O. D., 3.50 ; axis, 75, with the rule ;
O. S., 3.50; axis 105 degrees, with the rule.

Ophthalmoscope. — O. D., vertically oval nerve axis, 75
degrees. Accommodation very active. Underlying conus
down and out. Vessels at 75 best seen with +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 +2.50, and at axis 15 without any lens.

Manifest Refraction. —

VI
O. D., -f 2. 50 cyl. axis 65 degrees ^ -^ ? ? ?.

O. S., same cylinder with axis 125 degrees.

U. Hyoscyainin and dark glasses for refraction.

April 5th : Six meters from test-card and point of light.

VI
O. D. V. = XX ' ' "

v'l
O. S. V. = XX - '•

Clock-dial shows the same as at first cxaminati(Mi.
Cobalt-blue glass before O. D. gives blue center autl red
on each side at axis 165 degrees. O. S. the .same at axis
15 degrees. (Sec Fig. 121.)

APPLIED REFRACTION. 24 1

Stenopeic Slit. — O. D., axis 75 with +0.25 S., V. = —--^
and at axis 165 with +2.50 S., V. = -^/-. O. S., axis 105
with +0.25 S., V. = -^ ; and at axis 15 with +2.50 S.,
V. = ^^.

Rctinoscopc at i meter sliows : O. D., at axis 75 degrees
-f 1.25 S., and axis 165 degrees, +4.25 S. O. S., at axis
105 degrees, -[-1.25 S., and at axis 15 degrees, +4.25 S.

At Trial-case. —

O. D. -fo.25 sph. O -\2,-^^ cyl. axis 75 degrees, V. = _jlL -\-
O. S. -) 0.25 sph. O -(-3.00 cyl. axis 105 degrees, V. = ^ -|-.

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.oo cyl. axis 105 degrees.
SiG. — 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, 0°. Near point, 12 cm.

Considerations. — Apparently, the static refraction in this
case \\x)iild indicate comp(nnKl lu'peropic astigmatism ; but
when 0.25 is deducted to produce parallel ra)-s, then the
prescription becomes one for simple In-peropic astigmatism.

General Rule for Prescribing Cylinders. — Order the
CN'linder just as found, without an}' change in its axis or
strength.

The \ision in each ej-e at the different visits, before
lenses were placed in front of the eyes, was always uncertain,
21

242 REFRACTION AND HOW TO REFRACT.

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 — /. c\, —-???.

In taking the vision at the first visit, the patient could
read part of r;^^ 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 -j^. 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 h}'oscyamin 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 e)-es 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 I), for distance uncorrected.

As every 6 D. cylinder rei)re.scnts 1 mm. of lengthening
or shortening of the radius of curvature of the cornea,
then this patient, taking a ^ 3 cyl. at axis 75 in the right

APPLIED KEKRACTIOX. 243

eye, has the 165 degree meridian y^ 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 eyes have this
form of refraction.

April 10. Miss Jenks. Age, eighteen years. Single.
O. D. V. = ^ ? ? ?. p. p. 9 cm.

O. S. V. =^,???. 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. 122 and 123.)

VI .
XII '

Stenopeic Slit. — Axis 90 degrees V. =^ rrrr ; axis 1 80 V.

yj}_

VI •

History. — Never had good distant vi.sion. 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 e}'e 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
of 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.

244 REFRACTION AND HOW TO REFRACT.

Manifest Refraction. —

O. D., —2.50 cyl. axis 180 = -^.

O. S., — 2.50 cyl. axis 180 = ^y.
U . Atropin and dark glasses for refraction.

April 1 2th : Six meters from test-card and point of light.

Retinoscopy at one meter. Vertical meridian +I-25 S.
Horizontal meridian — i.oo S.

Stenopeic slit at axis 180 with +0.25 = — ; at axis 90 =

-o^> VI

Cobalt-blnc glass and pointed line test show same results
as at first visit :

O. D. V.=|^???.

as. v. = xv???.
At Trial-case. —

O. D., -fo.25 sph. 3 — 2. 00 cyl. axis 180 degrees = -wj-.

O. S. , +0.25 sph. 3 — 2.00 cyl. axis 180 degrees =r -yj-.

April 1 3th : Same results as yesterday. Add. ^ 20 ;
Abd. = 6. Esophoria, 2 degrees.

For Miss Jenks.

K. O. D., — 2.00 cyl. axis 180
O. S., — 2.00 cyl. axis 180.

April 14th : 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 -fo.25 is deducted
to produce parallel rays, the prescription resoh'cs itself into
one for simple myopic astigmatism.

The general rule for ordering cylinders is the same in

APPLIED RKFK ACTION. 245

myopia as in hyperopia — /. c, no chany;e 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 \'ision 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 yr.gg. When the
Hds were thus drawn together, the myopic vertical. meridian
was excluded in part and the horizontal meridian was util-
ized. The stenopeic slit was of some assistance before
drops were used, as the accommodation could not be
exerted, as in the case of the hyperope.

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 — 2.00 cyl. at axis 180 represents j/3 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.

C.\sE V. — Compound Hyperopic Astigmatism. — The
most common form of all refraction. It is a combination
of simple h)-peropia with simple h)-peropic astigmatism.
About 44 per cent, of all e\-es ha\e this f(M-m of refraction.

April I2lh. Mr. Common. Age, twenty-eight. Married. Bookkeeper.

VI
O. D. V. r- -jr ??. p. p. ^: type 0.75 I). = 22 cm.

VI
O. S. V. =: -^ ? ?. p. J). = type 0.75 I). 22 cm.

Add., 23 degrees. Ahd., 7 degrees. Esophoria, 2 degrees; at 13
inches, o.

246 REFRACTION AND HOW TO REFRACT.

Astigmatic Clock-dial. — O. D. and O. S. each selects
darkest series of lines from IX to III.

Placido's disc shows each corneal image as a horizontal
oval. Scheiner'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 +3.00 and horizontal with -f i.oo.

K . Duboisin and dark glasses for refraction.

April 14th; Six meters from test-card and point of light.
O. D.V.=-^???.

o. s. v. = -^'i???.
Rctinoscopy develops point of reversal at one meter in
each eye; vertical meridian with +2.25 S., and iiorizontal
meridian with -f 4-0O .S.

APPLIED REFRACTION. 24/

StcJiopfic slit axis 90 dcijrccs with -]- 1.25 = ^^ ; at axis
180 degrees with -f-3.00 = ^., .

Cobalt-blue glass, blue center and red all round, more
conspicuous on the right and left sides. (See Figs. 1 24
and 125.)

At Trial-case. —

O. D., -fi.25 sph. 3 -t- 1.75 cyl. axis 90 = ~.
O. S., -f 1.25 sph. O +1.75 cyl. axis 90 = -^.

April I 5th : Same results as April 14th. Add., 22. Abd.,
7. Esophoria, i degree.

For Mr. Common -.

U. O. D., 4 I. GO sph. O -|- 1.75 cyl. axis 90 degrees.

O. S., -f 1. 00 sph. O ^1.75 cyl. axis 90 "
SiG. — For constant use.

April 17th: 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
c)'linder. A cycloplegic as a means of obtaining a prompt
and satisfactory result in such cases can not be dispensed
with.

The decided change in the vision before and with the
cycloplegic is quite diagnostic of compound hyperopic as-
tigmatism. When the c\-linder and sphere are of an\' con-
siderable strength, the patient can often o\-ercome (faculta-
tive h}'peropia) the spheric but not the c)'lindric correction.

Summary. — After recovery from the cycloplegic the

248 REFRACTION AND HOW TO REFRACT.

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 negati\'e —
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
with the sphere.

April 1 2th. Mrs. Usual. Age, thirty years. Manied. Housewife.

VI
O. D. v., -^ ? ? ?, type 0.50 = 7 to 14 cm.

VI
O. S. v., ~j^ ? ? ?, type 0.50 = 7 to 14 cm.

Add., 16 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.). tlas all the
symptoms of near-sightedness. Family history of father
and two sisters wearing glasses for " n^ar 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.

OplitJialnioscopc. — (). D., many fine floating \itreous
opacities. Nerve large and round, with broad underhing
conus down and out. Choroidal vessels seen throughout
eye-ground. Vessels at about axis 1 20 degrees best seen

APPLIKD REFRACTION. 249

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 i 50 degrees.

Indirect method shows a vertically oval nerve, with the
conus to the nr^sal side of tiie 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-bhie 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. 126 and 127.)

Stenopeic slit before O. D. at axes 1 20 V. =: ^' ? ? ? ; at
axis 30 V. ^-^^ ? ?• O. S., the same with axes 60 degrees
and 1 50 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.

OpIitJialDiometcr. — O. D., i D., axis 35 degrees. O. S.,
I D., axis 145 degrees.

Manifest. —

VI
O. D., —2.50 sph. O —0.75, cyl. axis 35 = ^^r^ ? ? ? ?.

O. S., — 2.50 sph. O — 0.75 cyl. axis 145 = -^.^^ ? ? ? ?.

R . Atropin and dark glasses for rest and refraction.

April 13th: At si.x meters from test-card and point of
light :

O. D. V. = ^;j-?. O.S.V.=j^?.

O. D., — 2.00, sph. 3 — 1. 00 cyl. axis 30 := v.j ? ? ?.

O. S., — 2.CX) .sph. O — I.CXD cyl. axis 150 ... ? ? ?.

Retinoscope confirms this trial -case result. Retinoscope
also shows a general cloudiness of the media (vitreous),

250 REFRACTION AND HOW TO REFRACT.

which, of course, will account in part for the vision not
being -— p in each eye with correcting glasses.
April 14th : Same result as on the 13th.

For Mrs. Usual.

R. O. D., — 2.00 sph. O — 1. 00 cyl. axis 30 degrees.

O. S., — 2.00 sph. O — 1. 00 cyl. axis 150 "
SiG. — For distance, as directed.

April 15th:

IJ . Tonics. Rest of eyes. Attention to general health.

April 1 7th : Glasses neutralize ; are centered and ad-
justed.

April 29th: 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.

Case VII. — Mixed Astigmatism. — Not an uncommon
condition. About 6^ per cent, of all eyes have this form

APl'LIia) KEFKACTION. 25 I

of refraction. This is a combination of the simple h)per-
opic and simple myopic astiL,miatisms, with their axes oppo-
site or at right angles to each other, as a rule.

April 8th. Mr. Crook. Age, twenty-one years. Single. Clerk.

VI
O. D. V. = -j^,^. p. p. = 14 cm. with type 0.75 D.

VI
O. S. V. = -^^. p. p. ^ 14 cm. with type 0.75 D.

Add., 20. Abd., 10.

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 ha\'e
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 +2, and at right angles with — 2.
O. S., same conditions found, except that the meridians are
at 75 degrees and 165 degrees.

Oplitliabnomctcr. — O. D., 4 D. axis 100. O. S., 4 D.
axis 75.

Cobalt-bhic Glass. — (). D. violet center, blue abo\e 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.
128 and 129.)

Stenopeic Slit. — O. D. axis 15 with 4 2 sph., \'.= ^.^ ;
at axis 105 with — 2 sph., V. = ^'^ . O. S. axis 165 with
+ 2 sph., V.= ^j^- ; at axis 75 with — 2 sph., V.= ^-.

252 REFRACTION AND HOW TO REFRACT.

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 e}'e
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. = ~^.

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 — 1.50D., and
axis 15 with +3 D. O. S., axis 75 with — 1.50 D., and
axis 165 with +3 D.

Stenopeic Slit. — O. D., axis 15 with +2 sph., V. =

^; axis 105 with — 2.50. sph., V. = j"^ . O. S., axis

165 with +2 sph., V. = Jl^ ; at axis 75 with — 2.50 sph.,

V = ^

IX • ^^

At Trial-case. — O. D, — 2.50 cyl. axis 15 degrees O
+ 2 cyl. axis 105 degrees, V. = —j^g. O. S., — 2.50 cyl.
axis 165 O +2 cyl. axis 75, V. = ^^^.

Or.

O. D., —2.50 sph. O +4-50 cyl. axis 105, V. ^ ynss-

VI
O. S., —2.50 sph. O +4- 50 cyl. axis 75, \ . = vfiSS'

Or,

VI
O. D. -(-2 sph. O — 4.50 cyl. axis 15 degrees, V. = yuss -f-

O. S. -\-2 sph. O — 4.50 cyl. axis 165, V. — yiisg -(-.

AriMJKl) REFRACTION. 253

April nth: Same results as April loth. Add., 20.
Abd., 8.

For Mr. CROdK.

K. O. D. -{-2 sph. O — 4.50 cyl. axis 15 degrees.

O. S. +2 sph. O — 4.50 cyl. axis 165 "
SiG. — 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 w^ere
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 s/it 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 + 2 spli. with the — 4.50 cylinder at axis 15 in the
right eye or at a.xis 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 equally my-
opic 2 D. Combining a — 2 sph. with this +4.50 cylin-
der at axis 105 in the right eye or at axis 75 in the left eye,
the refracti(jn would be corrected.

For a further consideration of combination of lenses see
page 5 I .

The rule for ordering c\-lindcrs is the same in mixctl
astigmatism as in other cj'linder correction.s — without
change.

SuMMARV. — The character of the astigmatism is regu-

2 54 REFRACTION AND HOW TO REFRACT.

lar, symmetric, and with the rule. The near point returns
to the normal for the age. Eyes with such errors do not,
as a rule, obtain a visual acuity of -ttt, for the reason that
changes have taken place in the eye-ground, especially at
the macula.

Case VIII. — Irregular Astigmatism. —

April 2d. Mary Sjules. Age, ten years. Scholar.

O. D. V. := yyy slowly. No p. p. obtained.

VI
O. S. V. = 7-^". 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."

S. 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.

OplitJialmoscopc. — 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 -\-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 hypcropic.

R. Atropin and dark glasses for refraction.

April 22d : Retinoscope at 1 meter shows band of light
at a.xis 35, indicating In-j^eropia. Other meridians very
irregular. O. S., nothing definite made out.

APPLIED REFRACTIOX.

255

Placido's disc shows irregular, distorted circles.

With Pui-holc Disc.—O. D. V. = >'^-. O. S. V. = ^.

Witli Stenopeic Slit. — Axis 45 degrees before O. D. and
with +2.25 S., V. = jT^ ? ?. 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 23d : At trial-case O. D., + 1.75 cyl. axis 35 = —j^.

For Mary Smiles.

R. O. D., — 0.25 sph. = +1.75 cyl. axis 35 degrees.

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. = ^^xv??- p. p. = 9 cm. (?) Add., 24 degrees. Abd., 6

degrees. Esophoria, 4 degrees.
VI
O. S. V. = XXV ''• 1^' ^' '~ 9 *^'"- (') ^° vertical deviation.

History of ha\ing had glasses changed on three different
occasions during the past }'ear. Drops were used each
time, and the three prescriptions were all different. Glasses
were always satisfactory' for the first week, but after this
time she was ahvaj-s able to see better at a distance with-
out them. Has pains in her eyes and all over the heat!
whenever she attempts to use the eyes with or without any
glasses. Headaches nearly set her " wild " if she tries to

256 REFRACTION AND HOW TO REFRACT.

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.

^. P. — No external manifestations of any ocular irregu-
larity.

Manifest Refraction. —

O. D., — 0.50 S. O -|-i.oo cyl. axis 90 degrees = -yj-.

O. S., the same as O. D.

Ophtlialnioscope. — 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 shovjs a red center and broad blue halo.
(Patient is certainly accommodating.)

R . Atiopin and dark glasses for refraction.

Static Ref -action. —

VI
O. D. -I-1.50 S. = -fi -75 cyl. axis 90 degrees = -y-.

O. S. -I-I.50 S. ^ -f 1.75 cyl. axis 90 degrees = A-_

Patient states that her " pains and headaches all ilisap-
peared after using the drops for the third time."

AI'l'l.IKl) KKFKACriON. 2 57

Refraction repeated on three different occasions, and the
followini^ prescription given :

For Mrs. L.

K. (). D., 4-1.25 S. O -f 1.75 cyl. axis 90 degrees.
O. S., -i-1.25 S. O +1.75 cyl. a.\is 90 degrees.

Glasses properly centered, and accurately adjusted.

After ten da)-s jiatient 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.

R . Atropin ^^ of a grain to the ounce.

SiG. — 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. Not to attempt any
near-work. This treatment was continued, off and on, for
six months. Patient was ahvaj's 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 s{)heres as hook-fronts for any
near-work.

Case N. — Exophoria. —

Miss. \'. 1!. I). Age, twenty-two years.

VI
O. D. V. T= -^1 . p. p. = 0.50 I)., type at li cm.

O. S. V. ^ -yj . p. p. = 0.50 D., type at 11 cm.

Add. and abd., 12 degrees. E.xoplioria = 4.

History of seeing double several times a day. Friends
and members of her family have told her she was "squint-
22

258 REFRACTION AND HOW TO REFRACT.

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.

^. P. — P^yes 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 emmetropic ; 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., 4^0.50 S. "~" -i-0.37 cyl. axis 90 degrees ^ —-. And
this is ordered, less 0.25.

With this correction carefully centered, add. = 14 degrees
and abd. = 12 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 discomfort arose at any time. To wear
her glasses constantly.

Case XI. — Anisometropia. —

Mr. Albert S. Age, twenty-nine years. In general business.

O. D. V. = ^-^. p. p. type o. 50 D. = 20 cm.

VI
O. S. V. = XXX ■ !'• P- ^>P^ °-75 I^- = 30 cm.

Add., 10 degrees. Abd., 6 degrees. I^eft Hy., 2 degrees.

Plistory. — 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

AFI'IJICD KKFRACTIOX. 259

frontal headaches, w liich have been ahiiost constant for the
past four years or more. On account of tlie ocuhir 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 this
length of time they become bloodshot and very tender
to the touch. General health of patient is excellent ; has
a good appetite and sleeps well. Does not use tobacco or
liquor of any kind.

5. P. — Face symmetric. Nose very prominent. Inter-
pupillary distance, 62 mm.

OplitJialnioscopc. — O. D., nerve-head over capillar)^ Not
swollen. Accommodation very active. Eye-ground "fluffy."
Refraction is that of compound hyperopia. O. S., same
general conditions, but the nerve is vertically oval and the
refraction is that of hyperopic astigmatism.

R . Atropia and dark glasses for refraction.

Static Refraction. —

VI . 2/\ of

O. D., 4 2.00 S. O -f 1. 00 cyl. axis 75 degrees = -yj- I j^ft

O. S., -I 0.25 S. O -|3.ooc\I. axis 105 degi-ees ^= - "'I "yP^*""

\ 1 • " " -' phoria.

U . O. D., 4 1.75 S. O -(- i.oo cyl. axis 75 degrees O },i [^ base up.

< ). S., 4 300 cyl. axis 105 degrees O )il\ base down.
Sic. — For constant use.

This patient was not made comfortable until he was given
five-grain doses of the bromid of potash three times a day
for four weeks. Is now able to use his eyes without the
least discomfort.

CHAPTER XI.

PRES*BYOPIA.— APHAKIA.— ANISOMETROPIA.
—SPECTACLES.

Presbyopia. — The word presbyopia (from the Greek,
T.pia^iu-:^ "old" ; <i^^', "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 presbyopic. 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 — /. c,

1. (31d 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 pair of glasses) are required, one
for distance and one for near vision.

260

PRESBYOPIA. 261

7. Tlie condition of the c)'c in which one pair of glasses
will not answer for distant and also for near vision.

8. Presbyopia may be described as the condition where na-
ture has instilled a slowly acting but permanent cycloplegic
(the term cycloplegic being used here in a general sense).

Causes of Presbyopia. — i. Aj^'-c-. — 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 e\ent-
ually interfere with its ability to become more con\'ex 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
ciliar}' 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
Agr. Nkar Point. Accommodation.

40 years, 22 cm. 4- 50 diopters.

45 " 28 " 3.50

50 " 40 " 2.50

55 " 55 " 1-75 «>r 2.00 "

60 *' 100 " 1. 00 "

65 " 133 " 0.75

70 " 400 " 0.25 *'

75 " «= " -oo "

262 REFRACTION AND HOW TO REFRACT.

Ordinarily, the average adult holds a newspaper or book
at about ^^ 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 histor\' 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."

I'KEsnvopiA. 263

Correction of Presbyopia. — The presbyopic state repre-
sents a class of patients for whom, as a general rule,
i^lasses may be prescribed by the manifest refraction,
althou<^h tliere are exceptional cases in which a quick cyclo-
plegic will be necessary.

For a working, reading, writing, or sewing distance of
33 cm. (13 inches), the writer makes it a rule to add to the
distance correction at fort)'-fi\e )'ears of age a -f i sphere ;
at fifty years of age, a +2 .sphere ; at fifty-five years of
age, a +2.50 sphere; and for sixty or more years, a -f 3
sphere.

The following table for emmetropic eyes shows these addi-
tions for the different years, and also the near and far points
zvith these additions as well as the range of accommo-
dation or "play " between the near and far points. It will
be obsen'cd that the range of 78 cm. at forty-five years
rapidh' diminishes in the succeeding )'ears, until at sixty
there is only a play of about 3 inches, and at seventy the
range is practically gone.

Years.

Add.

Near Point.

Far Point.

Range.

-f-I.OO

22 cm.

100 cm.

78 cm.

-I-2.00

22 "

50 "

28 "

+ 2.50

23 "

40 "

17 "

-f 3.00

25 "

a •'

8 "

-1 300

27 "

ii "

6 "

-t 300

30 "

Zi "

3 "

4 300

iZ "

33 "

0 "

Because a patient is fifty years of age does not signify
that he will be able to read at 33 cm. with a pair of -i 2
spheres, or because he is sixty years of age that he can use
his eyes at 33 cm. comfortab]\- with a pair of -^ 3 sj^heres ;
on the contrary, this rule that the writer has gi\en only
applies to cases of emmetropia. It often happens that pres-
byopic patients state that they do not want glasses for dis-

264 REFRACTION AND HOW TO REFRACT.

tance ; that they do not need them ; that all they wish is a
pair of glasses to use at near-work, reading, etc. When the
vision is taken in such cases, it may be found to be — or
approximating — ; but the young ophthalmologist must
not be thrown off his guard by this record, as it has already
been stated that a vision of -7^ 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
alivays carefully estimate the amount of the distance ametropia
first, tio matter hozv zveak or ivJiat its form {sphere or cylifider) ;
and to the result thus obtained, add the plus sphere which will
be reqtdred for the working distance or point at which the
patient ivisJies to see clearly.

Illustrative Cases. — Case I. — Accepts +0.50 sph. for
distance. At forty-five years this case would require
+ 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-
sary.

Case II. — Accepts +2 sph. for distance; at forty-fixe
years these eyes would require +3 sph.; at fifty years tiiey
would require +4- 50 sph.; and at sixty or more years they
would require + 5 sph. 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 + I required
for reading. At fifty years, however, the patient would
require a -j- i sphere for near, and at fifty-five a -f 1.50, and
at sixty years a -|-2 sphere. Case II re([uired two correc-
tions, one for distance and one for near ; and the same ma)'
be said about Case III ; but in this latter instance there was
a time at forty-five years when there was no necessity for

PRESBYOPIA. 265

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 sj)h. for readintj ; at fifty years
would require — i sph. for reading ; and at sixty years
can read ivitJioiit any glasses. Such a patient says he has
gotten his " second sight."

Cask V. — .Accepts H 0.50 cylinder a.xis 180 for distance
and requires the usual adtlitional spheres for the increasing
years for his reading distance.

Case VI. — Accepts -(- i.oo sph. O -j- i.oo cyl. axis 180
for distance and requires the spheric additions as the years
increase. Two pairs of glasses should be prescribed.

C.\SE VII. — Accepts — I cyl. axis 90 for distance, and
requires -|- i cyl. axis 1 80 to read with at forty-five years
of age ; at fift}- years he requires -|- 1 sph. O + I cyl.
axis 180 ; and at sixty years requires -\-2 sph. O + I cyl.
axis 180. At fort\'-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.

Cask VIII. — Accepts — i.oo sph. O — 1.50 c)l. axis
1 80 for distance, and at forty-five years will need — 1.50 cyl.
axis 180 for reading ; at fifty years will require -{- i.OO sph.
O — 1.50 C}'1., axis 180 degrees ; at sixty years, -fo. 50
sph. O -f 1.50 c\l. a.xis 90 degrees.

Two pairs of glas.ses should be used throughout life.
At forty-five }'ears this patient has a compound mj-opic
correction for distance and sinijile nu'opic astigmatism for
near ; at fifty years the correction for near is that of crossed
cylinders (mixed astigmatism) ; and at si.xt)' -j-ears the near
correction is that for compound hyperopic astigmatism.
23

266 REFRACTION AND HOW TO REFRACT.

Case IX. — Accepts — i.oo sph. O + 2 cyl. axis 90 for dis-
tance (mixed astigmatism) ; at forty-five years, + 2 cyl. axis
90 is required for reading; at fifty years, -j-i.oo sph. O
+ 2.00 cyl. axis 90. Two pairs of glasses are required. At
forty-five years the distance correction is for mixed astig-
matism and the reading correction is for simjole 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 ^t, 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 fift\'-five years of
age who requires -| 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,
he would require -f 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
180) against the rule. This is presumptive evidence that
the astigmatism is acquiretl, is lenticular, and is due to the
sclerotic changes previously mentioned. The only positixe
way to prove this fact is by the retinoscope, and by the ab-
sence of corneal astigmatism with tlic oi)htlialniomctcr.
If the case has been previously refractetl In' the same sur-
geon, his record will also confirm this extremely interesting-
occurrence. According to able authorities, hyperopic eyes

APHAKIA. 267

become more hypcropic after the age of seventy years,
and emmetropic eyes may become hyperopic, and myopic
e}'es 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 — /. c,
a reduction in the strength of convex glasses.

Aphakia («, priv. ; <fay.6(; "lentil") literally means an
eye " without a lens." (See Fig. 161.) An eye which has
had its lens dislocated
has been erroneously
spoken of as aphakia.
The absence of the lens

means a total absence

of all accommodation,

no matter what the age p,^, jj^,

of the patient may be.

Causes. — .Vphakia may be congenital, but in most cases
is the result of removing the lens by operation.

Diagnosis. — Aphakia maybe diagnosed by inspection —
i. i\, 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 histoiy.

The ametropia of an aphakic e\'e depends in great part

268 REFRACTION AND HOW TO REFRACT.

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 12, 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 invariably
results, and the axis of the correcting c}-linder generally
coincides with the points of puncture and counterpuncture
in the cornea. If a patient had 2 or 3 D. of myopic
astigmatism with the rule, this would be neutralized the
corneal section.

Correction of Aphakia. — As in presbyopia, two correc-
tions are necessary — one for distance and one for near.
Astigmatism must alwa}\s be looked for and carefulK' cor-
rected, especially if the lens has been removed by extrac-
tion.

Case I. —

VI
O. D., 48.00 sjih. ^ -f-3.oo cyl. axis lo degrees. V. = ,y.—

O. D., 4 ii-OO sph. 3 4 3-00 cyl. axis 10 degrees = reading at t^;^ cm.

This patient was presumably myopic before operation.
Anisometropia (/o!T«r, " unequal " ; fjir/xr^, " a measure " ;
wf, " the e}-e ") literally means that the ametropia of

ANISOMETROPIA. 269

the two eyes is not exactly the same. This condition
ma)' be sH<^"ht or one of the most extreme conditions imagin-
able. One eye may be hyperopic +0.25 D. and the
other -(-0.50 D. ; or one eye may be myopic and the other
h)-peropic ; or one eye may be astigmatic and the other
not have any astigmatism ; or one eye may be aphakic with
many diopters of hyperopia and the fellow-eye be myopic.

For purposes of classification o)il)\ the writer would not
class anisometropia as the condition where the strength of the
glass is different in the two eyes, but where the character
of the refraction is diflerent. For instance, if both eyes have
compound hyperopic astigmatism, they are not considered
as anisometropic, even if the sphere and c}-lindcr are of
different strength in the two eyes. Bearing this distinction
in mind, the percentages already given for mj-opia, h)-per-
opia, the different astigmatisms, etc., have been calculated
accordingly, that for anisometropia being about thirteen per
cent.

Causes. — Usualh- the condition is congenital, or it may
be acquired.

Difficulties. — Two difficulties are encountered when
ordering glasses for cases of anisometropia : (i) The lens for
one eye ma}* be conca\e and that for the other ma}- be con-
vex, or both eyes may require a convex or both may re-
quire a concave lens, but one ver^' much stronger than the
otiier ; under these circumstances, when the eyes are
rotated there will be a prismatic result of different amount
in each eye, and this may mean diplo})ia, or at least an
e.xertion on the part of the extraocular muscles to prevent
diplopia which will cause dizziness, nau.sea, headache, 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.

270 REFRACTION AND HOW TO REFRACT.

For purposes of study, the writer would divide cases
of anisometropia 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 anisometropic ;
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 Anisome-
tropia.— Excluding Class I, there is no fixed rule to follow
when ordering glas.ses in decided cases of anisometropia,
and, in fact, such e}-es are a constant stud\- to tlie most
able ophthalmologist. The younger the patient, howe\-er,
the more likelihood of a favorable result from the careful
selection of a glass for each ej'e ; l)ut when the jiaticnt is an
adult, it becomes a xery serious question as to what glass
to prescribe that will give satisfaction. As good results
are to be expected in children, the\- should receive the

AXISOMKTKOFI A. 2/ I

most careful retinoscopic refraction. The child comes under
observation on account of a squint, antl an operation for the
deformity is often demanded ; but the operation must be
refused until the anisometropia has been carefully treated.
Glasses havini^ been prescribed, the squinting eye is put to
work to develop its seeing qualities, which have been per-
mitted 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
trj'ing 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. 3
+ 1. 00 cyl. a.xis 75 degrees.

This child developed the squint on account of the
monocular astigmatism and because it could not accom-
modate sufficiently with the left ex^e. To avoid dijjlopia
at the same time that the e_\-cs were conx'erging, the
left eye naturally turned inward. With correcting glasses,
and practising as above directed, the squint entirely

2/2 REFRACTION AND HOW TO REFRACT.

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 under 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
the distant vision is improved or asthenopic s)-mptoms are
relieved by glasses, it will be sufficient to prescribe the
near correction onl\'. When a distant and near correc-
tion are required, they may be prescribed as two [)airs 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. 162 and 163). —
This form of bifocals consists of an upper and a lower lens,
each with its indixidual center ; the u])pcr lens is for dis-
tance and the lower for near vision. Such lenses must ha\'c
the frame all around the edges, so as to hold them in jiosi-

BIFOCAI3.

273

tioii. Bifocals of this kind arc not in common use. The
field of distant vision is limited by the unnecessarily large
near correction, and where the two lenses come together,

Fig. 162.

Fk;. 163.

there is apt to develop chromatic aberration and a decided

prismatic effect when the vision is directed through this

space.

K. O. D., +2.00 sph.
O. S., -f 2.00 sph.
Sic. — For distance.

B. O. D., -(-4.CX) sph.
O. S., -I-4.00 sph.
SlG. — For near.
Directions to Oi'pician. — Make into Franklin or split bifocals.

2. Morck's Patent or "Perfection" Bifocals (Fig.
164). — 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

l-ii;. 164.
for those who work in a damp

atmosphere and can not wear the cement bifocal. It is,

274

REFRACTION AND HOW TO REFRACT.

however, more expensive than the cement form ; but, like
the Frankhn, it often looks clumsy or heavy on account
of the frame. " Perfection " or " Morck " bifocal must be
signified in writing the prescription.

3. Cement Bifocals (see Figs. 165 and 166). — 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-

FiG. 165.

Fig. 166.

Fig. 167.

Fig. 168.

Fig. 169.

ment of a small pcriscopic sphere on to the lower part of
the distance correction. This pcriscopic sphere or disc or
segment, as it is called, has a prismatic quality (see Fig.
166) suitable to the exigencies of the individual lens to
which it is cemented. The segment may be of an}' shape
desired. Those in common use are shown in figures 167,
168, and 169. It is cemented to the distance correction

BIFOCALS.

with Canada balsam. While this is the usual method of
making a cement bifocal, yet it may be made by cementing

Fig. 170.

Fig. 171.

a concave segment to the upper part of the near correction.
(Figs. 170 and i/i.) This form is not in common use.

R. O. D.,4 2S.
O. S.. +2S.
Cement on the lower part of the above O. D. and O. S., -[-2. 00 S.
SiG. — Make frameless bifocals.

Or,

K. O. D.,+4S.

o. s., 44 s.

Cement on the upper part of O. D. and O. S., — 2.00 S.
SiG. — Make frameless bifocaks.

4. Achromatic Bifocals (Figs. 172 and 173). — This form

Fig. 172.

of bifocal is u.sed principall)- in cases of ajihakia where the
plus sphere is quite thick and corrcspondingl}- heav)'. It

2/6 REFRACTION AND HOW TO REFRACT.

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 173 ; these
three lenses are then cemented together, and when com-
pleted, look like the cement bifocal, as shown in figure 169.
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. 174 and 175). —
Lenses of this character are made in one piece. They

Fic. 174. Fig. 175.

look neat, but are not always comfortable, on account of
the resulting prismatic effect, which is especially apt to
occur when the lens is convex, though this may not be
so troublesome a feature when the lens is moderatel}' con-
cave.

6. Patients who have a very weak distance correction,

BIFOCALS.

277

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-
ring to have the glasses where they can find them ; business
men especially. Other patients prefer to do without a

Fig. 176.

¥ic,. 177.

Fig. 178.

Fig. 179.

distance correction, and will often use a near correc-
tion that has one-third or nearly one-half of its upper
part cut away, so that they can look over the near correc-
tion when they wish to see at a distance. (See Figs. 1 76,
177, 178, 179, and 180.) 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,
they can look under the distance
correction. Fig. 180.

7. Patients who require a dis-
tance correction, and can not get accustomed to cement

278

REFRACTION AND HOW TO REFRACT.

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 every 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 apha-
kia where the lens is very
powerful, a bifocal segment
can sometimes be dis-
pensed with, if the patient
has a long nose, by slid-
ing the lens down from
the eye and then holding the reading matter at the conju-
gate focus. A toric lens (Fig. 181) is very acceptable in
occasional instances, as it reduces somewhat the weight and
thickness of the lens, and also enlarges the field of xision.
A toric {torcinc or toriqin," twisted") lens is one which

Fig. 181.

BIFOCALS. 279

has, combined in one surface, the optic effects of a sphero-
C}'lindric lens, or two cyUnders of different streni^th at ri^^dit
angles to each other. Unfortunately, this kind of a lens is
quite expensive.

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 happ)^) 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,
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 — ma)- 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.

280 REFRACTION AND HOW TO REFRACT.

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 e)'es 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
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 \icw
of the patient's individual welfare and future good of his
eyes, the ne.xt question that arises is what form of glasses
shall be prescribed.

Spectacles. — The child is certainl}- 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 k-ss liable to be broken tlian
in the form of eye-glasses, and for most occupations are to

niFocALs. 281

be preferred. Occasionalh', the shape of the nose will pre-
clude the use of aii}'thint; 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 ma)' be prescribed when the lenses are not too heavy,
or the cylinders too strong or their axes inclined. Eye-
glasses are easily bent, and lose their exact positions before
the e}'es. For the young society girl nothing but the most
delicately made eye-glasses will, as a rule, be accepted.

Bifocals. — These should not, crs 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
luicertain in his gait, or the vision is not brought close to
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 anci 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, othcrwi.se 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 perpendicularl}-
or at an axis of about 5 degrees to the plane of the face,
with the optic centers corresj^onding to the pupillar}- cen-
ters when the ej'cs are directed to a distance. If the lenses
are unusually strong and to be used principal K' at near-
work, then it may be neccs.sary to consider the ad\isabilit\'
of haxing two pairs of glasses, one for distance and one
24

282 REFRACTION AND HOW TO REFRACT.

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 decentered 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
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. 56.)

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 man\' other points for the average
case must receive the careful consideration of the surgeon.

TRIFOCALS. 283

Tinted or Colored Glasses. — Except for the relief of
photophobia follo\vin<^ 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
photophobia instead of correcting it.

Perimetric 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
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. Figures 88 or 89 may
answer the same purpose if properly centered.

Trifocals. — Occasionall}', a patient is not content with
bifocals, but will demand a focal point somewhere between
infinity and his working distance ; this can only be pro-
duced by cementing two segments of different sizes and
strength on the distance correction. Bookkeepers who have
to work at large and lengthy ledgers find great comfort in
this combination, though to be of special service the lenses
mu.st be made large.

Decentering of Lenses. — Instead of writing a prescrip-
tion for a lens and pri.sm, the prismatic effect of the lens
may be obtained by decentering the lens. The rule is

* The writer described this form of lens before the Section in Ophthalmology
of the College of Physicians of Philadelphia, in March, 1897.

284 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, -f 4 sph. O
4 P. D., base out, is about the same as +4 sph. decentered
I cm. outward ; or +4 sph. O 2 P. D.,base in, equals +4
S. decentered 5 mm. inward ; or +2 sph. O +2 cyl. axis
90 degrees O 2 A, base outward, equals +2 sph. O +2
cyl. axis 90, decentered 5 mm. outward. Or to be exact,
decentering a lens 8.7 mm. represents the prismatic
effect of a I A for each diopter of the lens ; for example,
+ 4 sph. O 4 A, base in, equals 1:1^ := 8.7 mm. decen-
tering, or +4 sph. decentered inward 8.7 mm.

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, to 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,
howcx'cr, 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 arc 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. 187, 188, 189,
190, and 191.) Or sizes smaller than O are numbered i, 2.
3. or 4. (See Fig.s. 182, 183, 184, 185). Different shapes
and sizes are lettered A, B, C, D, F, or X. (Sec Figs. i;6.
^77^ 17H. 179. i-'^O, 186.) All these lenses are aLso marked

285

286

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. 182.

Fig. 183.

Fig. 184.

Fig. 185.

Fig. 186.

Fig. 187.

with the facial measurements. The size or " e}-e " 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 V may be ordered for a

LENSES.

287

presbyope who docs not need a distance glass and who
docs not wish to be takingr off the near correction to see at

Fig. igo.

Fig. 191.

a distance ; in other words, such a shaped lens can be
looked over without any difficulty. Or the presbyope who

288 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 liis 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. 196). — 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. 197). — These consist of a
spring and nose-pieces ; the latter are called guards.
Framed and frameless eye-glasses have the nose-pieces or
guards attaclied 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 7'r/y' long, they must be trimmed ;
(3) the lens must be at such an angle that the visual axis
will Idc perpendicular to it.

First Measurement. — TJic IntcrpnpUlary 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.

289

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. 192), 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. 192.

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 onl)', then the
measurement must be for the full interpupillary distance, as
the patient looks into infinity ; but if the glasses are for near-
work onl}% then the distance between the pupils must be
corresponding!}- diminished, and the measurement taken
as the patient looks at a near point. If the glasses are
to be worn for both near atid far vision, for constant use,
25

290

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. 193 and 194), the place where
the nose begins to extend outward after passing down from
the forehead. The points B and D, as shown in figure 195,
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. 193.

Fig. 194.

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 ; tJic bridge itself
sJundd 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 sliow
the plane of the lenses. (See Fig. 195.)

TIIIKD MEASUREMENT FOURTH MEASUREMENT. 29 I

When the wire has been bent into place 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. 193 and 194.)

Another good way to take the foregoing measurements
is to have several ordinary steel frames of different sizes and

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.

292 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.

Patienf s NaJiie,

Forward to,

Fig. 196.

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. 293

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. 198.) They are spoken of as "distance" frames.

Fu;. 198.

Fig. 199.

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. 199.)

Fitting Eye-glasses. — The position of the lenses ap-
plies equally well for eye-glasses. The
principal measurement, therefore, is the
nose-pieces or guards and the arms or
offsets from the guards. (See Fig. 200.)
The width of the patient's nose where
W and D, 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 long,
medium, or short, and whether they are " out " or *' in "

294 REFRACTION AND HOW TO REFRACT.

from the plane of the lenses. The style of spring is usu-
ally that shown in figure 200.

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

Abduction, 175
Aberration, negative, 171

positive, 171
Absorption of light, 1 1
Accommodation, 64

amplitude of, 68, 69, 70

at different ages, 69

binocular, 82

cramp of, 2IO, 21 1

diminution of, 260

in hyperopia, 69, 70, 71

in myopia, "ji, y2

in presbyopia, 261

mechanism of, 65, 66, 67

muscle of, 65

observer's, 96, 97, 156

paralysis of, 208, 209, 210

spasm of, 210, 211
Acuteness of vision, 62, 63

in astigmatism, 76, 77
in emmetropia, 63, 10 1
in hyperopia, 109
in myopia, 115
record of, 76, 77, 78
Adduction, 175, 176
Aerial image, 99
Age, 216
Albino, 91

Alternating strabismus, 189, 190
Amblyopia, 115, 155, 193
Ametrometer, 146, 147
Ametropia, 103, igi

axial, 103, 104

curvature, 103, 151
Angle alpha, 85

critical, 19

gamma, 83, 85

of convergence, 82

of deviation, 23

of five minutes, 63

of incidence, 21

Angle of refraction, 22

of strabismus, 1 94, 1 95

of view, 60
Anisometropia, 258, 259, 260, 268,
269, 270

classification of, 270

correction of, 270, 271, 272
Anterior focal point, 59, 60

focus, 59, 60
Apex of prism, 22
Aphakia, 155, 267, 268

causes of, 267

diagnosis of, 267

treatment of, 268
Aqueous humor, 67
Asthenopia, 21 1

accommodative, 212, 213, 214

muscular, 178, 212, 213

retinal, 212, 213

treatment of, 213
Astigmatic charts, 135, 136, 137, 138,

139
clock-dial, 135, 1 36, 137, 220
lens, 121
Astigmatism, 120, 167, 167, 232
against the rale, 129, 130
asymmetric, 128
causes of, 122

compound hyperopic, 125, 126,
152, 245, 246, 247
myopic, 126, 152,248, 249,
250
corneal, 121
diagnosis of, 131
estimation of, 222, 223. (See

ChaiUcr VI.)
heterologous, 126
heteronymous, 1 26
homologous, 126
homonymous, 130
irregidar, 122, 123, 169, 170,

254. 255

295

296

INDEX.

Astigmatism, lenticular, 122

mixed, 126, 127, 152, 169, 250,
251, 252, 253

physiologic, 12

principal meridians in, I20, 167,
168, 169

regular, 123, 124, 125, 126, 127

shape of disc in, 151

simple hyperopic, 124, 125, 152,
239, 240, 241
myopic, 125, 152, 243

statistics of, 232

symmetric, 127, 128

tests for, 131-151

treatment of, 222, 223

with the i-ule, 129, 130
Atropin, 201, 202, 203, 226
Axiom, 155
Axis of astigmatism, 128, 129

of cylinder, 128, 129, 222, 223

optic, 83, 84

principal, 3 1

secondaiy, 35, 36

visual, 81, 82
Axonometer, 168, 169

Band of light, 167, 168
Base of prism, 22
Beam of light, 11
Biconcave lens, 29, 30
Biconvex lens, 29, 30
Bifocals, 272-282
Binocular accommodation, 82

fixation, 82
Blepharitis, I07
Brachymetropia, 1 10
Briicke, muscle of, 65

Chromo-aberration test, I43, 144,

145, 146
Ciliary body, 65

muscle, 65

anatomy of, 65
Cobalt-blue glass, 143, 144, 145, 146
Cocain, 207

Compound system, 59, 60
Concave lenses, 29, 30

mirror, 14, 15

in retinoscopy. (See Chap-
ter VI.)
Concomitant squint, I90
Condensing lens, 98
Conic cornea, 132, 170, 171
Conjugate foci, 3^, 34
Conjunctiva, 200
Convergence, 81

amplitude of, 83

angle of, 82

insufficiency of, 83

negative, 83

positive, 83

range of, 83
Convergent strabismus, 189
Convex lenses, 29
Coquilles, 204
Cornea, 120
Cover chimney, 77

test, 179
Cramp of accommodation, 210, 211,

255, 256, 257
Cretes' prism, 183
Crossed diplopia, 177, 190
Crystalline lens, 21, 67
Cycloplegia, 208, 209, 210
Cycloplegics, 200-208
Cylindric lenses, 42, 43, 44, 222, 223
action of, 43, 44, 55, 56
axis of, 43
neutralization of, 55, 56, 57

Camera, 64

Capsule, 67

Cardinal points, 59, 60

Cataract, 267

Catoptrics, 9

Center of fixation, 82, 84

of rotation, 82
Centering of lenses, 53, 54
Chalazion, 12
Choroid, 92, 93

Dark glasses, 204

room, 89, 157
Daturin, 201

Decentering lenses, 283, 284
Deviation, angle of, 23

in strabismus, 193, 194, I95
Diopter, 40, 150
Dioptrics, 9

INDEX.

297

Dioptric system, 40, 41
Diplopia, 28

correction of, 28
Direct method, 88, 151, 152
Disc, optic, 91, 151

perforated, 139, 140

pin-hole, 46, 255

riacido's, 132

shape of optic, 151
Distant type, 73, 74, 75
Divergence, 82, 189, 190
Divergent strabismus, 189, 190
Duboisin, 201
Dynamic refraction, 224, 225

Elasticity of lens, 66, 67
Electric-light blindness, 212
Elongation of eyeball, 227, 228, 229
Emergent ray, 10
Emmetropia, loi, I02, 165, 166

description of, loi, 102, 103
Erect image, 94, 95, 96
Esophoria, 177, 180, 1 81

diagnosis of, 180, 1 81

treatment for, 183-187
Esotropia, 177, 189

diagnosis of, 191-195

treatment for, 195-199
Exercises with prisms, 185, 186, 187
Exophoria, 177, 180, 181, 257, 258

diagnosis of, 180, 181

treatment for, 183-187
Exotropia, 177, 190

diagnosis of, 191-195

treatment for, 195- 1 99
Eye, 58

drops, 200

emmetropic, lOl

-glasses, 281

hyperopic, 104, 105

myopic, 110

schematic, loi, 154

standard, 58, loi

-strain, 21 1-215

Face, as\Tnmetry of, 128
Facial illumination, 160
Far point, 67, 68

Finger exercise, 185

Fitting of spectacles. (See Chapter

XII.)
Focal interval, 32

length, 32

points, 121
Focus, 1 1

anterior, 59, 60

conjugate, ^^^ 34

negative, n, 35

ordinary, 34

positive, 1 1

posterior, 60

principal, 32, 59

real, 11

virtual, il
Form of retinal illumination, 163
Formation of images, 37, 38, 39

Glass, crown, 21

flint, 21
Glasses. (See Lenses.)
Glaucoma, 203, 267
Gould, 75
Green, 137

Helmholtz, 58, lOI

Heredity, 105

Heteronymous images, 174

Heterophoria, 177, 179-185

History, 216, 217, 218

Homatropin, 201, 203, 205.206, 207

Homonymous images, 173

How to refract. (See Chapter IX.)

Hyoscyamin, 201

Hyperesophoria, 177

Hyperopia, 104, 155, 166, 233

absolute, 106

acquired, 266, 267

amount of, 119

axial, 104, 232

causes of, 106

description of, 104, 105, 106

diagnt)sis of, 109, IIO

estimation of, 1 19

facultative, 106, 236

latent, 106, 236

length of eyeball in. 1 19

298

INDEX.

Hyperopia, manifest, 106
relative, 106, 236
symptoms of, 107, 232
total, 106, 236
treatment of, 225, 226, 232, 233,

234
Hyperopic astigmatism, 125, 126
Hyperphoria, 174, 175, 177, 181,

188
Hypertropia, 177

Jackson, 201

Keratoscope, 132

Illiterate card, 74

Illiterates, 74, 155

Illuminated area. (See Figs. 81, 82,

and 83.)
Illmnination, 89

facial, 160

retinal, 160
Images, 172

crossed, 174

formation of, 37, 38, 39

formed by mirrors, 13, 14, 15, 16

heteronymous, 1 74

homonymous, 173

in astigmatism, 121-127

in emmetropia, 64, 99, loo

in hyperopia, 64

in myopia, 64

inverted, 98, 1 19

on cornea, 160

on lens, 160

real, 37

retinal, 62, 63

virtual, 37
Imbalance, 177
Inch system, 40, 41, 42
Index of refraction, 19, 20, 21
Indirect method, 98, 119, 152, 153,

252
Infinity, 67
Infraduction, 176
Insufficiencies, 1 78-186
Interval, focal, 121, 122

of Sturm, 121, 122
Inversion, 13
Iris in accommodation, 67

in hyperopia, 1 09

in myopia, 1 15
Irregular astigmatism of the cornea,
122, 123
of the lens, 122, 123

Length of eyeball, 119

in emmetropia, 1 19
in hyperopia, 1 19
in myopia, 1 19
in standard eye, 1 19
Lens, crystalline, 21
Lenses, 28, 285, 286

acromatic, 275, 276

action of, 30, 31, 32

astigmatic, 121

biconcave, 29, 30

biconvex, 29

bifocal, 272-280

collective, 29

combination of, 46-52

cylindric, 42, 43, 44

decentered, 283, 284

dioptric, 40, 41, 42

inch, 40, 41, 42

magnifying, 29

meniscus, 29, 30

minifying, 29, 30

negative, 29

numeration of, 40, 41

perimetric, 283

periscopic, 29

planoconcave, 29, 30

prismatic, 28

spheric, 29

spherocylindric, 44

tinted, 204

toric, 278
Ligamentum pectinatum, 65
Light, 9, 89, 157, 230

and shade, 77

intensity of, 9

-screen, 157

sense, 77

velocity of, 9
Lorgnettes, 278
Lorinir, 86

INDEX.

299

Maddox rod, 182

Malingerer, 27

Manifest refraction, 224, 225

Meniscus, 29, 30

Meter, 40
angle, 82

Metric system, 40

Mires, 147, 150

Mirror, 13

concave, 14, 15, 157
convex, 16, 17
plane, 13, 14, 157
reflection from, 13, 14, 15

Mixed astigmatism, 126, 127

Movements of mirror, 159, 160

Mulatto, 91

Muscles. (See Chapter VII.)
ciliaiy, 65

Mydriatics, 200

Myopia, no, 166, 236
axial, no, 116, 239
causes of, 112, 113, 114
description of, no, ni, n2,

239
diagnosis of, 115, n6, 237, 238
estimation of, 119, 238
image in, 64
length of eyeball in, ng
ophthalmoscopic appearances in,

228, 229
progressive, 227, 228, 229
symptoms of, 114, 115, 238
treatment of, 227, 228, 229, 230,
238, 239

Near point, 68

determining the, 78, 81
Nebula, 193
Negative aberration, 171

angle, 83
Nerve, optic, 92

color of, 92

shape of, in astigmatism, 15 1

size of, in hyjieroi^ia, 95
in myopia, 96
Netlleship, 119

Neutralizing lenses, 40, 41, 164, 165
N()<lal points, 35, 36
Nystagmus, 155

Observer, 89

Occujiation, 216

Ocular gymnastics, 185, 186, 187, 258

Opacities, 132

Ophthalmometer, 147, 148, 149, 150

151
Ophthalmoplegia, 208
Ophthalmoscope, 86-100, 151, 152

how to use, 88
Optic axis, 83

center, 35, 36, 53, 54

disc, 92
Optics, 9
Orbit, 113
Orthophoria, 177
Orthotropia, 177

Paralysis of accommodation, 2 8
causes of, 209
treatment of, 210, 2n
Pencil, converging, n

diverging, 11
Perimeter, 195
Periodic squint, 189, 190
Petit' s canal, 66
Phenomena of light, 1 1
Phorometer, 184
Phorometiy, 175
Pinc-nez, 281
Pin-hole disc, 46, 255
Placido's disc, 132, 133, 255
Pointed line test, 139
Points, cardinal, 58, 59, 60

nodal, 35, 36

of reversal, 161

principal, 58, 59, 60
Pray's letters, 140
Presbyopia, 260

age of, 260, 261

causes of, 261

description of, 260, 261

diagnosis of, 262

glasses for, 263-267

symptoms of, 262
Prescription writing, 52, 53
Principal axis, 31

focus, 32, 59

points, 58, 59, 60
Prism-diopters, 25

300

INDEX.

Prism exercises, 185, 186, 187, 258
rotary, 183
WoUaston, 147
Prisms, 22, 28

action of, 22, 23, 24
centrads, 24

neutralization of, 25, 26, 27
numeration of, 24, 25
uses of, 27, 28, 185
Punctum proximum, 68

determination of the, 78

in emmetropia, 68

in hyperopia, 71

in myopia, J 2
remotum, 67

determination of the, 75, 7^

in emmetropia, 68

in hyperopia, 70, 71

in myopia, 71, 72

negative, 105

positive, 71
Pupil, size of, in emmetropia, lo

in hyperopia, 109

in myopia, 1 15

Randall, 73

Range of accommodation, 68

in emmetropia, 68, 69, 70
in hyperopia, 70, 7 1
in myopia, 71, 72

of convergence, 83
Rays, 10

convergent, 1 1

divergent, lO

emergent, lO

incident, 10

parallel, 10

reflected, 10

refracted, 10
Reflection, 12

by mirrors, 13- 1 7

laws of, 12
Refraction, 17

applied. (See Chapter X.)

by cylinders, 43, 44, 222, 223

by prisms, 23, 24

by spheres, 31, 221, 222

how to refract. (See Chapter
IX.)

index of, 19, 20

laws of, 18

Regular astigmatism. (See Astigma-
tism.)
Retina, 92

Retinal asthenopia. (See Asthen-
opia.)
illumination, 160
image in astigmatism, 121
in emmetropia, 64
in hyperopia, 64
in myopia, 64
Retinoscopy. (See Chapter VI.)
Risley, 176, 183
Rod test, 182
Rods and cones, 73
Room, 89, 157

Scheiner's test, 140-143
Schematic eye, 58, 154
Scissor movement, 170
Scopolamin, 201, 203
Second sight, 265
Shadow test. (See Chapter VI.)
Shadows in retinoscopy, 1 61
Simple hyperopic astigmatism, 124,

myopic astigmatism, 1 25
Snellen, 73
Snow-blindness, 212
Spasm of the accommodation, 2IO,
211
causes, 210
symptoms, 21 1
treatment of, 211, 256, 257
clonic, 210, 211
tonic, 210, 211, 255, 256, 257
Spectacles, 280, 281, 285, 286, 287,
288
for adults, 280, 281
for aphakia, 272
for astigmatism, 281
for children, 280
for hyperopia, 281
for myopia, 281
for presi)yopia, 272
for strabismus, 281
measurements for, 288, 289, 290,
291
Squint, 189. (See Strabismus.)
Standard eye, 58
Static refraction, 224, 225
Stenopeic slit, 46, 133, 134

INDKX.

?OI

Stevens, 177, 184
Stialjismonicter, 194
Strabismus, 189

alternating, 189, 190

amount of, 193, 194, 195

angle of, 194

apparent angle of, 194, 195

concomitant, 189

constant, 189

convergent, 189

divergent, 189, 190

monolateral, 189, 190

paralytic, 189, 190

periodic, 189, 190

treatment of, 195-199

vertical, 190
Sturm, interval of, 121, 122
Supraduction, 1 76
Surfaces of cylinders, 42

of miiTors, 14

of prisms, 22

of spheres, 29
Symptoms of aphakia, 267

of asthenopia, 21 1

of astigmatism, 131

of hyperopia, 107, 108

of myopia, 114, 115

of presbyopia, 262

Tablk of amplitude of accommoda-
tion, 69
of axial length of eyeball, 1 19
of indexes, 21
of near points, 69
of prisms, 26

Targets, 147, 148, 150
Tenotomy, 188, 189, 199
Test for aphakia, 267

for astigmatism, 131

for hyperopia, 69

for malingering, 27

for muscles. (See Chapter VII.)

for myopia, 69

for near point, 68

for vision, 62

-letters, 73, 74

-type, 73> 74, 78, 79, 80
Thomson's ametrometer, 146, 147
Tinted glasses, 283
Trial-case, 44, 45
Trial-frames, 45, 46, 47
Trifocals, 283

Vacuum, 19, 20
Virtual focus, 15, 17

images, 15, 17
Vision, acuteness of, 61

binocular, 172

determination of, 72, 73
Visual acuity, 62, 63
normal, 61

angle, 60

axis, 81, 82

Wallace, 74

/ /
i ■

(.,.

COLLEGE OF PHARMACy

44 GERRAPDST. e
TORONTO

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Full text of "Refraction and how to refract : including sections on optics, retinoscopy, the fitting of spectacles and eye-glasses, etc."

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