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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 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.

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CHAPTER III.

EXPLANATION OF THE PHENOMENA OF MONOCULAR. Vision :

SECTION 1.-STRUCTURE OF THE RETINA, 50 ; optic nerve, 51; re-

lations to the eye, 51 ; layers of the retina, 52; bacillary

layer, 53; rods and cones, 54; central spot, 55; blind spot,

57 ; visual purple, 57; its functions, 58.

Section II.—SPACE PERCEPTION, 58; First Law of Vision, 58;

the law of external projection of retinal impressions, 58; com-

parison with other senses, 59 ; illustrations of this law, 61;

phosphenes, 61 ; muscæ volitantes, 62; Purkinje's figures,

62; ocular spectra, 63.

Second LAW OF VISION, 67; the law of direction, 67; corre-

sponding points, retinal and spatial, 68; explanation of cer-

tain phenomena--(1) erect vision, 69; theories of erect vision,

69; comparison with other senses, 71; explained by law of

direction, 72; illustrations of this law, 73; (2) properties of
CENTRAL SPOT, 76; function of central spot, 77; minimum
visibile, 80; minimum tactile, 81; (3) blind spot, 81; pecul-

iar to vertebrates, 82; experiments illustrating, 82; why no

visible representative in field of view, 85; place of its repre-

sentative in the field of darkness, 87; experiment illustrat-

ing, 87.

SECTION III.-Color PERCEPTION, 89.; brightness versus color,

89; primary and secondary colors, 90; view of Brewster,

90; of Young and Helmholtz, 90; of Hering, 91; theories of

color perception, 92; general account of, 92; Young-Helm-
holtz theory, 94; Hering's theory, 94; Mrs. Franklin's the-
ory, 96; color-blindness, what it is, 98; explanation of color-

blindness, 99; Helmholtz's theory, 100; objections to, 100;

Hering's theory, 100; objections to, 101 ; Mrs. Franklin's the-
ory, 101 ; what the color-blind see, 102; tests for color-blind-

ness, 104.

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