Page:Popular Science Monthly Volume 81.djvu/73

Rh different lines that structural or functional properties of the eye which, considered from the purely physical point of view, would seem to impair the efficiency of the organ as a registrar and transmitter of objective visual facts, not only do not confuse the recipient sensory nerve centers, but, on the contrary, the psychical apprehension of objective phenomena is distinctly modified by the reaction to such instrumental defects in a manner which leads to the generation in consciousness of a state or an atmosphere of feeling—esthetic feeling. It is as if the stone rejected of the builders were made the chief of the corner.

In pursuance of this line of thought I will offer one additional illustration of the direct dependence of psychic perception upon what may be termed the structural aberration of the visual apparatus.

When external objects are viewed with one eye, held at rest, the image upon the retina is exactly similar to that upon the sensitive plate of the camera, it has length and breadth, but no depth, and it has no power of directly arousing in the mind a perception of the third dimension—projection.

It is inconceivable, indeed, that an anatomical apparatus should be capable of directly presenting to its sensory center an impression of depth. Such a perception is of purely psychological construction from simpler data derived from retinal impulses. By an exceedingly familiar line of evidence it can be shown that the direct visual perception of depth is dependent upon idiosyncrasies of binocular vision. It is a physiological law that an object viewed by the two eyes appears to be single only when the images which it casts upon the retinas fall upon "corresponding points" of the two surfaces. It is obviously of paramount importance to the instrumental efficiency of the eyes that there should be a horopter in which objective and subjective facts must coincide. It is well known that the fixation of objects by means of which their images are retained upon corresponding retinal areas invokes activity of most complex nerve-muscle machinery. Now when a small solid object is viewed with both eyes, it is clear that the right eye must see more of the right side of the object and the left eye more of the left side. Therefore it is certain that the images on the two retinas can not be identical and therefore can not exactly "correspond."

Some extra-mundane theorists summing up these facts would naturally reach the conclusion that distinct binocular vision is in its nature impossible. Nevertheless we know that the mental picture of external objects loses nothing essential in focal sharpness through binocular vision but, on the other hand, the two unlike retinal pictures combine, as it were, in the mind to form a new idea—the concept of depth.

Of all esthetic perceptions that of projection is the highest, the most purely psychic. Through it the universe is instantly converted from a