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 existence 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 positions; 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 convenience, 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 closing each eye alternately, while the convergence of visual lines is vigorously 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 impression 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 endeavoring 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 convince 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 finger. The experiment 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.