4 o THE POPULAR SCIENCE MONTHLY. 



In an essay published many years ago, Carlyle dwelt, in a manner 

 characteristically his own, upon the unconsciousness that is a mark of 

 health in the human body. The dyspeptic man knows full well that 

 he has a stomach, but the eupeptic child has no conception of the ex- 

 istence of such an organ, however vivid may be its ideas of fairies, ogres, 

 and dragons. In like manner the retina is an abstraction for him who 

 has good binocular vision and but little book-lore. "With a single eye 

 he sees many objects at the same time, and judges their different posi- 

 tions ; the only idea aroused is about the objects themselves, and not 

 about the retinal impressions from them. If both eyes be directed to 

 the same distant point, there is still the same consciousness of a single 

 external thing, and not of two eyes. By slowly crossing the two visual 

 lines for the purpose of comprehensively scanning the root of one's own 

 nose, which is the nearest object that can be regarded with entire con- 

 venience, if both eyes are of equal power, the visual impression is found 

 to be that two noses are approaching each other, and closing up the 

 brightest part of the field of view in front. Between them is left a 

 narrow heart-shaped window, with dimly transparent nasal shutters. 

 The outlines of these are most easily discerned by momentarily clos- 

 ing each eye alternately, while the convergence of visual lines is vig- 

 orously retained, and then opening both and depending on indirect 

 vision. If there is any consciousness of an eye at all, it is referred 

 to the sensation of strain in the muscles that seem to be pulling the 

 shutters together, and not to any retina receiving pictures of them. 

 There is, indeed, the consciousness of looking out of the window 

 from a single stand-point, but not from two eyes. The subjective im- 

 pression is that the two points of view are identified into a single 

 eye, whose position is central and constitutes the point of origin from 

 which all our estimates of direction and distance are made. Keeping 

 the nasal window as small as possible by cross-vision, and endeavor- 

 ing to test the real singleness of the double-phantom nose by gently 

 putting the finger upon it from in front, it is easy additionally to con- 

 vince one's self that 



"... things are not what they seem." 



Two fingers will be seen approaching from different directions. If it 

 should occur to the indignant observer that these may be utilized in 

 putting an end to his nasal redundancy by closing up the window, they 

 will steadily converge and strike together upon the root of the nose, 

 almost exactly where he had been supposing his point of view to be. 

 The window at the next moment, instead of being closed, will be 

 opened wide, and, on resting the tired muscles of his eyes, he will find 

 that the phantom-noses have leaped to the two sides, the position of 

 each being indicated by the faithful ghosts of the fingei. The experi- 

 ment is a little surprising at first, and the specters are very shadowy, 

 but a literally close search will be quite sure to reveal them by indirect 

 vision. 



