William Norwood Souter.

The refractive and motor mechanism of the eye online

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Refractive and Motor Mechanism

of the Eye












Vision and Light 13-21

Vision — Light — Theories of the Transmission of Light — Color
—Velocity of Light, Wave Length, Vibratory Period — Lu-
minosity — Wave Front — Pencils and Rays — Superposition of
Waves — Formation of Images.


The Laws of Refraction and Reflection; Refraction and

Reflection at Plane Surfaces 22-35

The Law of Refraction — Passage of Light Through a Medium
Bounded by Two Parallel Surfaces — Prisms — Refraction of
Parallel Rays by a Prism — Minimum Deviation — Refraction
of a Spherical Wave (Divergent Rays) by a Prism — Disper-
sion of Colors — Numeration of Prisms — Combination of
Prisms — Reflection — The Law of Reflection — Reflection at
Plane Surfaces — Total Reflection.


Refraction and Reflection at Spherical Surfaces 36-53

Summary of Collective Refraction — Dispersive Refraction —
Aberration — Algebraic Relation Between Conjugate Foci —
Formation of Images by Refraction — Relative Size of Object
and Image — Cardinal Points and Planes — Spherical Lenses —
Convex Lenses — Concave Lenses — Cardinal Points of Lenses
— Algebraic Relation Between Conjugate Foci in Lens Re-
fraction — Numeration of Lenses — Reflection at Spherical
Surfaces — x\Igebraic Relation Between Conjugate Foci in


Compound Optical Systems 54-63

Cardinal Points of a Compound System — The Eye — Formation
of Tmages by the Eye — Relative Size of Object and Image —

Table of Contents

The Visual Angle — The Schematic Eye — The Reduced Eye —
The Aphakic Eye — Relative Position of the Retina and Pos-
terior Principal Focus of the Eye — Accommodation.


Refraction at Asymmetrical Surfaces 64-72

Principal Meridians — The Toric Surface — Image of a Point
in Asymmetrical Refraction — Image of a Line in Asymmetrical
Refraction — The Toric Lens — The Cylindrical Lens — Notation
of Toric and Cylindrical Lenses — Combination of Cylindrical
Lenses — Asymmetry of Oblique Refraction — Asymmetry of
Prismatic Refraction.


Correction of Optical Defects of the Eye by Lenses 73-86

Use of Spherical Lenses — Far Point of the Eye — Effect of
Changing the Position of the Correcting Lens — Measurement
of Ametropia by the Correcting Lens — Effect of Lenses Upon
the Size of Retinal Images — Enlargement of Images Effected
by Removal of the Crystalline Lens — Length of Axis in Ame-
tropia — Axial Length of the Eye in Relation to the Probable
Refractive Condition After Removal of the Lens — Correction
of Astigmia — Distortion of Images in Astigmia — Determina-
tion of the Axis of a Cylindrical Lens.


Optical Principles of Ophthalmoscopy, Skiascopy and Oph-
thalmometry 87-1 10

Indirect Method of Ophthalmoscopy — Direct Method of Oph-
thalmoscopy — Skiascopy — Point of Reversal — Reversal of
Movement — Variation of Magnification— Movement of the
Shadow Line — Form of the Shadow Line — Illumination of the
Retina — Two Points of Reversal in Astigmia — Rectilinear
Shadow Lines in Astigmia — Summary of Underlying Prin-
ciples of Skiascopy — Ophthalmometry — Keratometry and
Phakometry — Determination of Astigmia by Ophthalmometry.


The Refractive Mechanism 111-142

The External Coat — The Uvea — The Retina — Contents of the
Eyeball — Surfaces and Media of the Eye — Insensitiveness of

Table of Contents

the Periphery of the Retina — Alpha and Gamma— The Iris—
Choroidal and Retinal Pigment — Refraction of the Eye —
Emmetropia — Accommodation — The Ciliary Region — Helm-
holtz's Theory — Experimental Observations — Tscherning's
Theory — Length of Time Required for Accommodation —
Range of Accommodation — Reserve Accommodation.


The Motor Mechanism 143-160.

The Extrinsic Muscles of the Eye — Nerve Supply of the Ex-
trinsic Muscles — Ocular Motions — Center of Rotation — Field
of Fixation — Binocular Fixation — Conjugate Movements —
Listing's Law — Convergence — Measurement of Convergence —
Near Point of Convergence — Relaxation of Convergence —
Far Point of Convergence — Association of Accommodation
and Convergence — Accommodation and Convergence in Side
Vision — The Horopter — Normal Muscular Equilibrium.


Optometry of the Refractive Apparatus 161-195

Subjective Methods — Optometers Based Upon the Action of a
Convex Lens — Optometers Based Upon the Principle of the
Opera Glass or Galileo's Telescope — Optometers Based Upon
Scheiner's Experiment — Optometers Based Upon Chromatic
Aberration — Optometers Based Upon the Measurement of the
Retinal Diffusion Circles — Optometry Based Upon Movement
of the Diffusion Image on the Retina — Optometry Based
Upon Visual Acuteness — Objective Methods — Indirect Method
of Ophthalmoscopy — Skiascopy — Ophthalmometry.


Hyperopia 196-213;

Curvature-Hyperopia — Index-Hyperopia — Axial Hyperopia —
Degree of Hyperopia — Low-grade Hyperopia — Medium Hyper-
opia — High-grade Hyperopia — Latent and Manifest Hyperopia
— Symptoms of Hyperopia — Objective Symptoms — Diagnosis
of Hyperopia — Treatment of Hyperopia — Correction of Low-
grade Hyperopia in Childhood — Correction of Low-grade
Hyperopia in Adult Life — Correction of Medium and High-
grade Hyperopia — Treatment of Muscular Disturbances —
Secondary Effects of Convex Lenses — Enlargement of the-

8 Table of Contents

Retinal Image — Apparent Magnification of Objects — Altera-
tion in the Relation Between Convergence and Accommodation
— Prismatic Action of Convex Lenses — Prescription of Lenses
— Verification and Adjustment of Lenses.


Myopia 214-236

Curvature-Myopia — Index-Myopia — Axial Myopia — The Conus
—Two Theories as to the Origin of Posterior Staphyloma—
Anatomical and Ophthalmoscopic Characteristics — Two ..Types
of Axial Myopia — Statistics of Myopia — Symptoms of Myopia
— Divergent Strabismus in Myopia — Symptoms Arising from
Disturbed Nutrition in Staphyloma — Diagnosis of Myopia —
Differentiation of Mild and Malignant Myopia — Diagnosis of
Curvature-Myopia and Index-Myopia — Treatment of Myopia —
Prophylactic Measures — Use of Lenses in Myopia- — Operative
Treatment of Axial Myopia — Operative Treatment of Conical


ASTIGMIA 237-252

Etiology of Corneal Astigmia — Relation of Astigmia to Cranial
Development — Change in the Form of the Cornea — Etiology
of Lenticular Astigmia — Dynamic Astigmia — Degree of As-
tigmia — Classifications of Astigmia — Classification with Refer-
ence to the Position of the Principal Meridians — Classification
with Reference to the Relative Directions of the Principal
Meridians in the Two Eyes — Classification with Reference to
the Relation Between the Position of the Retina and that of
the Focal Lines — Symptoms of Astigmia — Vision in Astigmia
— Asthenopia — Objective Symptoms — Diagnosis of Astigmia —
Treatment of Astigmia — Prescription of Compound Lenses —
Surgical Treatment of Astigmia.


Anisometropia 253-258

Etiology — Vision in Anisometropia — Anisometropic Asthenopia
— Treatment.


Presbyopia and Anomalies of Accommodation 259-266

Age at which Presbyopia Occurs — Symptoms — Diagnosis —
Treatment — Spasm of Accommodation— Symptoms — Diagnosis

Table of Contents

— Treatment — Paresis and Paralysis of Accommodation —
Diphtheritic Paralysis — Syphilitic Paralysis — Paralysis Caused
by Non-Syphilitic Brain Lesion — Glaucomatous Paralysis — Ac-
ommodative Paralysis Arising from other Diseases — Artificial
Cycloplegia — Symptoms and Diagnosis of Accommodative
Paralysis — Treatment of Accommodative Paralysis — Loss of
Accommodation from Absence or Luxation of the Lens.



Diplopia — Tests Used in Motor Optometry — Cover Test —
Duane's Parallax Test — Colored Glass Test — Graefe's Test —
Maddox Rod Test — Stenopeic Lens Test — Measurement of
Convergence and Divergence — Measurement of Prism Con-
vergence — Graefe's Linear Method — The Perimeter Method —
Priestly Smith's Tape Method — Maddox Tangent Scale —
Measurement of the Field of Fixation — Tests of Binocular


Non-Paralytic Disorders of Equilibrium 291-307

Excess of Convergence — Etiology of Excessive Convergence —
Symptoms of Excessive Convergence — Diagnosis of Excessive
Convergence — Treatment of Excessive Convergence — Defi-
ciency of Convergence — Etiology of Convergence Deficiency —
Symptoms of Convergence Deficiency — Diagnosis of Converg-
ence Deficiency— Treatment of Convergence Deficiency — Ver-
tical Imbalance — Etiology — Symptoms and Diagnosis — Treat-
ment — Cyclophoria and Cyclotropia — Treatment — Anophoria,
Anotropia; Katophoria, Katotropia — Spasmodic Conjugate
Deviation — Nystagmus — Etiology— Symptoms and Diagnosis —
Treatment — Disorders of Motility Caused by Mechanical Im-

r w.vtk Disorders of Motility 308-325

Paralyses of the Ocular Muscles — Etiology — General Symp-
toms — Paralysis of the External Rectus (Sixth Nerve) —
Paralysis of the Internal Rectus — Paralysis of the Superior
Rectus — Paralysis of the Inferior Rectus — Paralysis of the
inferior Oblique — Paralysis of the Superior Oblique — Paralysis
of the Third Nerve — Combined Paralysis of the Ocular

io Table of Contents

Muscles — Diagnosis — Treatment — Paralyses of Associated
Movements — Paralysis of Convergence — Paralysis of Diverg-
ence — Treatment of Paralysis of Associated Movements.

Algebraic Formulae 325-34"

Deviation Effected by a Prism — Relation Between Conjugate
Points in Refraction at a Spherical Surface — Principal Foci
and Focal Distances — Relation Between Conjugate Points in
Refraction by Lenses— The Cardinal Points of the Schematic
Eye— Anterior Principal Focus— Posterior Principal Focus-
First Principal Point— Second Principal Point— First Nodal
Point— Second Nodal Point— Principal Focal Distances— Car-
dinal Points of a Thick Lens— Relation Between Variation
of Curvature and Astigmia — Ophthalmometric Determination
of Astigmia of the Crystalline Lens — Relation Between As-
tigmia Produced at the Anterior Surface of the Crystalline
Lens and the Correcting Lens Placed in Front of the Cornea
— Relation Between Astigmia Produced at the Posterior Sur-
face of the Crystalline Lens and the Correcting Lens Placed
in Front of the Cornea.


Refractive and Motor Mechanism

of the Eye




Vision is the sense which reveals to us the form and color
of objects by the action of light on the retina; in other words,
vision may be defined as the consciousness which results from
the stimulation of the retina by light.

The visual apparatus consists of two distinct parts. The first
of these is the eye, which is analogous to a photograph camera.
The retina, which receives and transforms the light energy
into a nerve impulse, corresponds to the sensitized plate of the
camera. The second part of the visual apparatus consists of the
optic nerve and its brain connections — the conducting and inter-
preting mechanism — by means of which the nerve impulse is
carried to the visual areas of the brain and thence to the centers
of consciousness, where the impulse is manifested as vision.

It falls within our province in this work io deal only with
the former part of this apparatus — to study the eye as an optical
contrivance and to investigate the adaptability and the imper-
fections of its mechanism.

Light, the physical agency by which we see, is a form of
energy. The science of physics teaches that ultimately all energy
is one, but that by the various modifications which it undergoes
different results are manifested from its expenditure.

Light may be produced in various ways, as by mechanical or
chemical action. Although light artificially produced plays an
important part in our life, our chief source of light must always
be the sun.

The study of the laws of light constitutes the science of
optics. In its various branches this is a comprehensive study,
which would lead us far beyond the province of ophthalmology.


14 Principles of Optics

Only a small part of the science of optics can, therefore, be con-
sidered in this work — that part which pertains to the reflection
and, more especially, to the refraction of light.

A substance which has the power of developing light-energy,
or emitting light, is said to be luminous. Thus the sun is a
luminous body. On the other hand, the moon, which does not
originate light, but only transmits it by reflection from the sun,
is a non-luminous body.

Theories of the Transmission of Light. — The question
as to the manner in which light is conveyed from a luminous body
to the eye has given rise to two hypotheses, which are known,
respectively, as the corpuscular or emission theory and the wave

The corpuscular theory naturally presented itself to the
ancients and was universally accepted prior to the development
of the science of optics. In accordance with this theory, it was
believed that light was a substance given off from a luminous
body and that this substance was propelled in all directions in
straight lines. Sir Isaac Newton was an advocate of this, in
opposition to the second hypothesis (which was announced by
Huygens in 1678), because, in the form in which the latter was
then propounded, it failed to explain certain phenomena.

The second hypothesis, or wave theory, as enunciated by
Huygens and as modified by subsequent investigators, satisfac-
torily explains all the observed phenomena of light. In fact,
certain phenomena which follow as a necessary sequence of the
wave theory were discovered through study of this theory, the
mathematical demonstrations which led to such discoveries having
afterward been corroborated by actual experiment.

Ether is the extremely tenuous matter which, it has been
assumed, exists throughout the universe. It is only by the
assumption that such matter exists that we can form a conception
of the transmission of waves through space. There is no other
evidence, except this mental requirement, that such matter
really exists.

A familiar example of a wave is afforded by throwing a
stone into a body of still water. In this case and in sound-waves
traveling in air a vibratory motion of the particles of the con-
ductor takes place in the direction in which the wave is moving.
The earlier advocates of the wave theory of light naturally

Vision and Light 15

supposed that in light-waves the method of vibration was similar
to that of sound-waves, and since certain phenomena could not
be explained under such conditions, the wave theory was aban-
doned for a century and a half, to be again brought into promi-
nence by Fresnal (1815), who introduced the assumption that
the vibratory motion in light-waves was transverse to the direc-
tion of wave motion. With this modification, all the observed
phenomena of light are explainable. But this assumption cannot
be accepted as excluding longitudinal vibrations, for a spherical
wave can advance only in the directions in which vibratory dis-
turbance is taking place. We must conclude, therefore, that light
advances by means of longitudinal disturbances upon which is
superposed a transverse disturbance, and that to the latter are due
certain characteristic phenomena which are explainable only by
means of such vibrations.

The exact nature of the vibratory disturbances which give
rise to light is unknown ; it was formerly supposed that there
was a to-and-fro movement of the particles of the conductor
(ether), just as there is in sound-waves, but our conception of
waves has been greatly broadened by the introduction by Maxwell
of the electro-magnetic theory of wave conduction. In the trans-
mission of electricity, a certain unknown change (polarization)
takes place in the particles of the conductor. These particles
become charged with energy, which they transmit to the ad-
joining particles and so on. Each particle, having transmitted
its energy, returns to its original state and is again charged by
particles behind it, and so the process continues. Since these
changes occur in rhythmical impulses or pulsations, they con-
stitute waves. Doubtless the transmission of light is similar to
that of electricity — in fact, it is practically certain that light
differs from electricity only in the shorter wave-length and more
rapid vibration of the former.

Recent experiments in electricity have led Professor Thomson,
of Cambridge, to return to the propulsion theory in a modified
form. Whatever may be the nature of the corpuscles or electrons
demonstrated by Professor Thomson, their existence is insuffi-
cient evidence for denying the theory of rhythmical impulses
(waves) of electricity and light— a theory which has hitherto
been found indispensable in the explanation of many phenomena.

As with the ear, only waves within certain limits are pro-

16 Principles of Optics

ductive of sound, so also the constitution of the eye is such that
waves within certain limits of periodicity excite vision, while
similar waves, whose oscillatory period is not within these limits,
do not produce this sensation.

Color. — It can be shown with the aid of a prism, which
causes a separation of waves according to their period of oscilla-
tion, that sunlight is composed of a number of waves of varying
periodicity and wave-length (the latter being inversely proportional
to the former), and that other waves also accompany the various
waves of light. Certain waves whose vibratory period is toe
rapid to affect the retina as light manifest themselves by their
power of causing chemical action; while others, whose vibratory
period is too slow to affect the retina as light, are manifested
as heat.

The various colors which we are able to distinguish depend
upon this variation of vibratory period. While a number of
theories have been put forward in explanation of color sensation,
the scheme propounded by the great physicist, Dr. Thomas Young
(1801), and afterward elaborated by Hclmholts is the most
satisfactory. According to this theory the various light-waves
are divided into three groups: (1) Those of least, (2) those of
intermediate and (3) those of greatest rapidity of vibration.
Each of these groups of waves has its distinctive action upon
the retina. Waves comprised in the first group cause the color
red to be seen; those in the second group are productive of green,
and those in the third, or most rapid group, give rise to the
sensation of blue (violet).* These three, red, green and blue, are
the three primary colors — not because there are only three sets
of waves (the division of light into these groups being, of course,
arbitrary), but'because of the limitations of the eye.

So far this hypothesis accords well with the phenomena of
color vision, but in the further endeavor to explain how these
three groups of waves act differently upon the retina serious
difficulties are encountered. It is assumed that there are three
sets of terminal elements, one set for each group of waves, and
that each set contains a characteristic photo-chemical substance
which is affected predominantly by the group of waves to which
it is adapted, while the other waves affect this substance in a

♦Opinions differ as 10 whether blue or violet should be regarded as the primary color.

Vision and Light 17

minor degree only. Many arguments have been advanced against
this assumption, but it remains more plausible than any of the
other hypotheses which have been offered.

Our color perception, however, is not limited to these three
elementary sensations, for by the simultaneous stimulation in vary-
ing proportions of the three sets of elements other color sensations
are afforded. When a screen is placed so as to intercept in a
darkened room a beam of sunlight which has passed through a
prism an observer may count on the screen six colors, clearly
distinct, but merging gradually into the contiguous colors. These
six colors are called the colors of the spectrum. They are red,
orange, yellow, green, blue, and violet. To these Nezvton added
a seventh color, indigo, between blue and violet.

Orange and yellow, lying between red and green, result from
stimulation of the retina, in proper proportion, by the waves which
give rise to the sensation of red together with those which give
rise to green, but with little or no stimulation by the violet-pro-
ducing waves, the latter waves having been eliminated in some
way from the light which enters the eye. On the other hand the
variations of color as seen in the spectrum lying between
green and violet are the result of stimulation of the retina by
those waves which produce green, together with those which
produce violet, but with little or no stimulation by the red-produc-
ing waves. When all three groups of waves simultaneously
stimulate the retina and without predominance of any one group,
xvhiteness results.

One must not infer, however, from the foregoing brief de-
scription of the Y oung-Hehnholtz theory of color perception that
there is a sharp border line between the three groups of waves
which form the visible spectrum. For' instance, those waves of
the second (intermediate) group which approximate in periodicity
the first group (red waves) must act partly as red-producing
waves ; and similarly those waves which approximate the blue-
producing waves must to some extent give rise to the sensation
of blue.

We see, therefore, that the complete spectrum is formed
by a multiplicity of waves, whose rapidity of vibration gradually
increases, beginning with the ultra-red and extending through the
visible spectrum to the chemical waves beyond the violet margin.
In the visible part of the solar spectrum there are seen at various

18 Principles of Optics

intervals gaps or black lines (Fraunhofer lines) which show that
certain waves have been destroyed. The discovery of these lines
has led to the important study of spectrum analysis, for it has been
learned that the absence (absorption) of certain waves is charac-
teristic of the gaseous substances through which light has passed,
and that from the number and position of such lines the chemical
composition of these substances can be determined.

Velocity of Light, Wave Length, Vibratory Period. —
It has been found from astronomical calculations and also from
terrestrial experiments, that light travels through air and through
space at the rate of 300,000,000 meters (approximately) or
186,000 miles a second.

The wave length at various parts of the spectrum has also
been determined by very delicate experiments. The length of
the red wave, near the beginning of the visible spectrum, is about
ttst mm > an d f° r violet, near the terminus, the wave length is
about tj-jVit mm - Hence, the wave length for light is embraced
within these limits. Since light travels through space at the rate
of 300,000,000,000 mm a second, it is apparent that for the first
wave length there must be 390 million-millions of these wave
lengths or vibrations in a second, and for the last, 750 million-
millions of vibrations a second.

Luminosity. — By this term we denote the intensity of the
sensation which results from retinal stimulation. Yellow is the
color of greatest intensity. The sensation of brightness or of light
seems to be in a measure independent of color, for when the
illumination is very feeble, one may be able to detect light and
yet be unable to assign to it any color. Similarly when the
illumination is very powerful no distinction of color can be made.

Online LibraryWilliam Norwood SouterThe refractive and motor mechanism of the eye → online text (page 1 of 29)