PREFACE. 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 been taken from other writers. On those points in which I differ, not only in form but in matter, from other writers, I am willing to abide the judgment of those best qualified to decide. I have devoted a large, perhaps some may think a too large, space to the discussion of binocular vision. I have done so, partly because I have devoted special attention to this department, partly because it is so very imperfectly presented by other writers, but chiefly because it seemed to me by far the most fascinating portion of the whole subject of vision. As a means of scientific culture, the study of vision seems to me almost exceptional. It makes use of, and thus connects together, the sciences of Physics, Physiology, and even Psychology. It makes the cultivation of the habit of observation and experiment possible to all; for the greatest variety of experiments may be made without expensive apparatus, or, indeed, apparatus of any kind. And, above all, it compels one to analyze the complex phenomena of Sense in his own person, and is thus a truly admirable preparation for the more difficult task of analysis of those still higher and more complex phenomena which are embraced in the science of Psychology. BERKELEY, CALIFORNIA, May 20, 1880. ANALYTICAL TABLE OF CONTENTS. THE RELATION OF GENERAL SENSIBILITY TO SPECIAL SENSE Law of differentiation, 10; gradation among the senses, 11; in kind of contact, 13; in distance of perception, 13; in refine- GENERAL STRUCTURE OF THE HUMAN EYE, AND THE FOR- SECTION I.-GENERAL STRUCTURE: General form and setting, 17; illustrations, 18; the muscles, 18; illustrations of their action, 19; the eyeball, 20; sclerotic, 20; cornea, 21; iris, 21; lin- ings, 22; choroid, 22; ciliary muscle, 22; retina, 22; con- tents of ball, 23; lens, 23; humors, 24. SECTION II.-FORMATION OF THE IMAGE, 24; conditions of per- fect image, 25; experiment, 27; illustrations, 27; property of a lens, 27; proofs of a retinal image, 29; nodal point, 29. DEFECTS OF THE EYE AS AN INSTRUMENT Emmetropy, or normal sightedness, 46; myopy, or ncar-sighted- EXPLANATION OF PHENOMENA OF MONOCULAR VISION SECTION I-STRUCTURE OF RETINA, 53; optic nerve, 54; relations to the eye, 54; layers of retina, 55; bacillary layer, 55; cen- tral spot, 57; blind spot, 59; perception of color, 59; primary colors, 60; view of Brewster, 60; of Young, 60; of Hering, 60; theory of color-perception, 61; theory of Young, 61; of Hall, 61; color-blindness, 62; theory of, 63. SECTION II.-FUNCTIONS OF THE RETINA: Law of outward pro- jection of retinal impressions, 64; compared with other senses, 65; illustrations of this property, 66; phosphenes, 67; muscæ volitantes, 67; Purkinje's figures, 68; ocular spectra, 69; corresponding points, retinal and spatial, 72; properties of the central spot, 73; function of the central spot, 74; mini- mum visible, 76; minimum tactile, 77; blind spot, 78; ex- periments illustrating, 78-81; why there is no visible repre- sentative of this spot in field of view, 82; erect vision, 83;, comparison with other senses, 84; explained by law of dircc- The two eyes as one instrument, 90; the binocular field, 91; double images, 92; experiments illustrating, 92-94; analogy with sense of touch, 95; single vision, 95; corresponding points of the two retinæ, 96; law of corresponding points, 97; conditions of single vision, 99; horopter, 101; optic chi- asm, and its relation to the law of corresponding points, 101; theories of the origin of property of corresponding points, 102; nativistic theory, 103; empiristic theory, 103; conscn- |