Page:Popular Science Monthly Volume 21.djvu/47

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39
THE STEREOSCOPE: ITS HISTORY.

gaze at any clearly defined object for a few moments, and press upon the eyeball near the outer corner of the opening between the lids—double vision is instantly attained. The condition thus induced is unnatural, and the effect is unnatural vision. Our modes of interpreting nerve-impressions, like our modes of mental and bodily action in other respects, are the results partly of individual experience and partly of inheritance through countless generations. If a blow is received upon the right cheek, whether it be from a solid body or from a wave in the medium in which we are immersed, experience at once suggests the direction from which it came. If through many generations every individual were continually receiving gentle blows on the center of the right cheek through all the moments of waking existence, and the accurate perception of these were conducive to his welfare, then on physiological principles it seems in the highest degree probable that the judgment of direction by the cheek might become as habitual and unerring as is our judgment of direction by the eye. By a liberal construction of language we might be said to that extent to see with the cheek; and the man who is blessed with the healthiest, best-trained,

PSM V21 D047 Left and right projection of a cone.jpg
Fig. 1.—Left and Right Projections of Glass Cone.

and most vigorous cheek, aside from other qualities, would be most apt to win in the struggle for existence. If, then, in normal vision each of two eyes receives impressions due to wave-impulses from the same external object, the position of this is referred to the same external place in accordance with the association that experience, individual and ancestral, instantly arouses in response to the sensation felt on each separate retina. If, therefore, every person were blessed with such optic luxuriance as is attributed to the fabled Argus, there is no reason to suppose that, if each eye be healthy and ordinarily controllable, he would have anything else than single vision, unless the necessities imposed in the struggle for success made it advantageous to acquire the power of dissociating the action of the muscles of these eyes, and thus making use of voluntary multiple vision.