ciples in explanation of slope, 236; in explanation of change

in form of figures, 237; the phantom not a perfect plane, but

an arch, 238; explanation of the arching, 238; diagram

illustrating, 239; Prof. Stevens's device, 240; experiments

and diagrams illustrating, 241; another beautiful experi-

ment of Prof. Stevens, 241; explanation of result, 243; cx-

planation incomplete, 244; explanation completed by addi-

tional experiments, 245.

CHAPTER IV.

ON SOME PHENOMENA OF BINOCULAR VISION USUALLY OVER-

LOOKED, AND ON A NEW MODE OF DIAGRAMMATIC REPRE-

SENTATION FOUNDED THEREON

Usual mode of representation untrue, 248; experiments illustrat-

ing, 249; heteronymous shifting of the two fields of view and

experiments illustrating, 251-256; œil cyclopienne, 251-256;

first law or law of heteronymous shifting stated, 258; ho-

monymous rotation of the two fields and experiments illus-

trating, 261-263; second law or law of homonymous rotation

stated, 263; statement of the two laws, 264; determination

of the interocular space, 265; experiments illustrating the

necessity of the new mode, 267-273; application of the new

mode to representation of stereoscopic phenomena, 273-280;

some curious phenomena resulting from the heteronymous

shifting of the fields of view, 280; to trace the outline of a

picture where it is not, 280; to trace the outline of a candle-

flame on an opaque screen, 284; to see through a book or a

deal board, 285.

CHAPTER V.

VISUAL PHENOMENA IN OCULAR DIVERGENCE

1. In drowsiness and probably in sleep, 287; 2. In other modes

of producing divergence, 290; 3. Prevalence of law of cor-

responding points over law of direction, and some curious

results, 293; diagrams illustrating, 294.

CHAPTER VI.

COMPARATIVE PHYSIOLOGY OF BINOCULAR VISION

1. OPTIC CHIASM: Its peculiar relation to the two eyes and thus

to binocular vision, 297; less perfect as we go down, and

disappears in fishes, 298. 2. POSITION OF OPTIC AXES, 298;

parallel in man and apes, 299; more and more divergent as

we go down, until = 180° in fishes, 299; relation of position

of axes to binocular vision, 299; even in mammals easy con-

PAGE

248

287

297

vergence in a point of sight is more or less sacrificed to wide-
ness of view, 300. 3. CENTRAL SPOT, OR FOVEA: Central in
man and apes, 300; absent in most mammals and in reptiles
and fishes, 300; relation of corresponding points to optic
axes in man and mammals, 300; in some birds = two foveæ,
one monocular, the other binocular, 301; former more per-
fect, 301; figure illustrating, 301; general conclusion as to
binocular vision in vertebrates, 302; none in invertebrates,

302.

PAGE

EVOLUTION OF THE EYE

CHAPTER VII.

Extreme interest of the subject. 303.
1. INVERTEBRATE EYE: General sensibility to light, 303; coex-
tensive with life, 303; this not special sense, 303; first be-
ginnings of special sense determined by light itself, 303;
define eye-spot, 304; several stages in development of eye-
spots and diagrams illustrating, 304; may perceive light,
not objects, 305; earliest appearance of eye-(1) pin-hole
image, 305; (2) lens-image, 305; (3) compound lens-image,
305; same order in embryonic development of a squid's eye,
306; thus far no chiasm, 306; no binocular vision and can
not be any blind-spot, 306.

2. VERTEBRATE EYE: Invertebrate versus vertebrate eye, 306;
(1) as to mode of termination of the fibers in the rods, 306;
(2) as to mode of formation of the retina, 307; diagrams illus-
trating formation of optic vesicle and the retina in vertebrates,
307; formation of lens, 307; origin of bacillary layer, 308:
of choroid, 303; explanation of turning back of nerve-fibers
to terminate in rods and cones, and thus of the blind spot,
308; retina epidermal in invertebrates and cerebral in ver-
tebrates, 309: bacillary layer in both epithelial, 309; transi-
tion from invertebrate to vertebrate eye, 309; the difficulty,
309; Beranek's view, 309; the gradual perfecting of the
vertebrate eye, 310; decreasing divergence of optic axes,
310; formation of the chiasm, 310; the use of the two eyes
as one instrument, 311: the addition of fovea is a condition
of thoughtful attention and mental development, 312.

303

SIGHT.

INTRODUCTORY.

THE RELATION OF GENERAL SENSIBILITY TO SPECIAL

SENSE.

SENSORY nerve-fibers are cylindrical threads of microscopic fineness, terminating outwardly in the sensitive surfaces and sense-organs, and inwardly in the nerve-centers, especially the brain. Impressions on their outer extremity are transmitted along the fiber with a velocity of about one hundred feet per second, and determine changes in the nerve-centers, which in turn may determine changes in consciousness which we call sensation. The simplest and most general form of sensation is what is called general sensibility, or common sensation. This is a mere sense of contact, an indefinite response to external impression. It gives knowledge of externality-of the existence of the external world-but not of the properties of matter. The lowest animals possess this, and nothing more. But, as we go up the scale of animals, in order to give that wider and more accurate knowledge of the various properties of matter necessary for the complex relations of the higher animals, sensory nerve-fibers are differentiated into several kinds, so that each may give clear knowledge of a dif

PREFACE TO THE FIRST EDITION.

In writing this treatise I have tried to make a book that would be intelligible and interesting to the thoughtful general reader, and at the same time profitable to even the most advanced specialist in this department. I find justification for the attempt in the fact that there is not, to my knowledge, any work covering the same ground in the English language. Vision has been treated either as a branch of optics or else as a branch of physiology of the nervous system. Helmholtz's great work on "Physiological Optics," of which there exist both a German and a French edition, is doubtless accessible to scientists, but this work is so technical that it is practically closed to all but the specialist. I believe, therefore, that the work which I now offer meets a real want, and fills a real gap in scientific literature.

The form in which the subject is here presented has been developed entirely independently, and as the result of a conscientious endeavor to make it clear to students under my instruction. As evidence of this, I would draw attention to the fact that, out of one hundred and thirty illustrations, only about twelve have