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Page:Popular Science Monthly Volume 21.djvu/46

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THE POPULAR SCIENCE MONTHLY.

he is never willing to keep it thus inactive any longer than necessary; and, if such a hint is gently suggested, he is prompt to answer it by some prosaic contrast between the artist's clever illusion and the necessities of life in a wide-awake world. Lord Bacon says: "We see more exquisitely with one eye than with both, because the vital spirits thus unite themselves the more, and become the stronger; for we may find by looking in a glass whilst we shut one eye that the pupil of the other dilates." But even the cogent logic of Lord Bacon would scarcely reconcile many of us to the adoption of strictly Cyclopean customs in the enjoyment of vision.

In response to the question, "What is the use of having two eyes?" the answer has been given, "To have one left if the other is hurt." Much as we may admire the sagacious foresight of this youthful physiologist, it will not be found sufficient to rest contented with his ultimatum. He had evidently not tried his skill to find how unexpectedly he would miss the inkstand while endeavoring to dip his pen into it at arm's length, with one eye closed. He had not thought of holding his finger a few inches in front of his face to find what part of the wall it would hide from each eye in succession, or how differently it would look when regarded from those two points of view separately, how much thicker it would appear when both eyes were open, how readily he could examine three sides of it at once, how much more definitely he could judge its distance, in a word how much more comprehensive was the information given by two eyes if used at the same moment. Assuming that he knows exactly how to account for the inversion of the retinal image and the erect appearance of the object there pictured, how our visual perceptions are only signs of what we momentarily feel on the retina, signs that generally represent the realities with a fair degree of accuracy, but may sometimes represent almost anything else on demand, how, if the eyes be healthy, we have no consciousness of possessing any retina at all, but instantly and unconsciously refer every retinal sensation to some external body whose existence we are obliged to assume, unless there be special arguments to convince us to the contrary—granting all this, our young physiologist has not thought of inquiring how it is that, although two retinal images are produced, we see but a single object, and this despite the fact that, like photographs of the same body simultaneously taken from different stand-points, these two images are necessarily dissimilar.

This question, and especially its latter part, is much more easily asked than answered with fullness, clearness, and certainty. There is no antecedent reason why two separate retinal images should not produce the impression of two separate bodies. That they may do so must have come within the experience of every one. A few glasses of champagne are often enough to convince the most skeptical. Without resorting, however, to agencies that produce involuntary though temporary loss of muscular control of the eyes, it is only necessary to