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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.


similar objects, 108; experiments illustrating, 108-109; Case

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