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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.
TABLE OF CONTENTS.
24; chromatism, 25; correction of chromatism, 25 ; aberra-
tion, 29 ; correction of aberration, 30; adjustment for light,
31; adjustment for distance, 35; accommodation of the eye,
35; experiment illustrating, 37; theory of adjustment, 38;
Helmholtz's view, 39.
Section II.—DEFECTS OF THE EYE AS AN INSTRUMENT, 40; THE
SINGLE AND DOUBLE VISION
The two eyes as one instrument, 105; the binocular field, 106 ;
double images, 107; experiments illustrating, 108; analogy
170; experiments illustrating, 170; objection to. 171; Dove's
experiment, 171; the author's view, 174.
235; also figures change in form, 235; experiment illustrat-
ing, 235; principles explaining, 235; application of the prin-