Page:Popular Science Monthly Volume 21.djvu/215

This page has been validated.

sounds that coalesce harmoniously. A well-trained ear may pick out some of the overtones without the aid of resonators, and perceive in the background a few duplicated sound-images; but the great majority of them are so faint that their presence can not be perceived apart from each other, or from the fundamental to which they give character. The rich combination of all stands out in strong musical relief, compared with what each voice alone would yield, or with the sweet but thin sound of a tuning-fork that sings forth the same fundamental pitch.

This principle relates to the combination of sensations, whatever may be the cause of dissimilarity among the components of the group. We have not the data from which a binocular image can be graphically expressed as a curve, for the dissimilarity of the components is not due to interference of waves of light. But the facts suggest kinship between the modes of sensation in the two cases. The dissimilar groups of light-images arouse sensations that are simultaneously conveyed to the brain, and the proper interpretation at once comes as the product of past experience. All we can affirm is, that experience has taught us to interpret retinal sensations which are slightly different in the two eyes, as the signs of an external object possessing three dimensions in space, when the images are produced upon parts of the concave surfaces which bear to each other the relations that would be imposed by the presence of such an object if naturally viewed. Such experience has been acquired by each of us individually, and probably with exceeding rapidity in consequence of inherited tendencies. It is therefore not necessary that the localization of what we see in the stereoscope should be limited to cases of optic convergence, or the perception of relief to those in which double images can be distinguished.

Our discussion has led us from the domain of physics to the confines of metaphysics. Explanations are at best only relative, and the psychologist, the physiologist, and the physicist must join hands in working out the problems of binocular vision. The progress made during the last half-century invites the hope that much may yet be accomplished before the next century brings us its morning greeting.


THE Academy celebrates to-day the birth of its royal head and gracious protector. Such a festival is, first of all, devoted to feelings the simplest, purest, and most elevating—love, reverence,

  1. Anniversary Address before the Academy of Sciences at Munich, delivered July 25, 1881. Translated Mr. W. M. Salter.