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THERE are two classes of students of science to whom some clear and compendious exposition of the phenomena of vision would seem to be especially helpful—viz., ophthalmologists and psychologists. To the former its importance need not be urged, since it is obvious that physiology is the basis of pathology, and therefore of practice, in this as in every other department of medicine. To the latter its importance is not so generally recognized. But it is evident that the physiology of the senses, and especially of the sense of sight, forms the only sure basis of a rational psychology. Now, both these classes of students are rapidly increasing in this country, and the methods in both are becoming more and more scientific.

As an introduction to psychology, I know nothing equal to the study of the phenomena of vision, and especially of binocular vision. IIere pure sense perception passes by insensible gradations into simplest judgments, and these latter into the more complex judgments. The simplest psychological phenomena are therefore found here. I am quite sure that if any

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one will repeat the experiments contained in this little book, whether to verify or to refute the results, he will have acquired an amount of culture in scientific method which will both surprise and delight him.

But the subject is important not only to these special students, but in an eminent degree to every intelligent person, and must be intensely interesting once the field is fairly entered. But the field of binocular phenomena is an almost closed world to most, even intelligent, people—the phenomena have almost completely dropped out of consciousness. And yet on these very phenomena are based our judgments of size, distance, and shape, every day of our lives. Is it not strange that intelligent persons should go through life without analyzing their visual impressions, without even being conscious of phenomena on which are based judgments which are necessary for the safe conduct of physical life? We believe that this reproach is being removed, and it is to help its removal that this work is written.

In justification of my right to teach others on this subject, I would say that from early childhood I have amused myself by practicing binocular experiments, until I have acquired a facility in voluntary movements of the eyes and in analyzing the visual results which I am sure is quite exceptional. On this account some of the experiments, especially in Part III, may at first (but only at first) be found difficult to most persons.

In this second edition I have found little to correct. The changes are mainly in the form of alditions. The principal of these are the following: In Part I (1) a fuller explanation of the cause of astigmatism ; (2) a clearer statement of the nature of space perception and of the law of direction ; (3) a new mode of locating in space the visual representative of the blind spot; (4) a brief account of that curious substance, visual purple, and its probable function ; and (5) a much fuller exposition of color perception and color-blindness, making it now a separate section.

I have made very little change in Part II.

Part III is the part in which I differ most fundamentally from some noted authorities. I have therefore gone over this part again carefully and verified every point, so that I feel more than ever confident of its substantial truth. I have also added in this part a chapter on the form of phantom planes under certain conditions. This chapter is an admirable illustration of some principles previously set forth. I have rewritten and greatly enlarged the chapter on the comparative physiology of binocular vision, and added also a final chapter on the evolution of the eye.



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

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