38 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 neces- 

 sities 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 physi- 

 ologist, it will not be found sufficient to rest contented with his ulti- 

 matum. 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 read- 

 ily he could examine three sides of it at once, how much more defi- 

 nitely he could judge its distance, in a word how much more compre- 

 hensive 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 real- 

 ities 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 un- 

 consciously 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 physiolo- 

 gist 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 pro- 

 duce 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 tem- 

 porary loss of 'muscular control of the eyes, it is only necessary to 



