THE STEREOSCOPE : ITS HISTORY. 



39 



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 un- 

 natural, 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, 



Fig. 1. Left and Kight Projections uf Glass Coae. 



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 ex- 

 ternal object, the position of this is referred to the same external place 

 in accordance with the association that experience, individual and an- 

 cestral, instantly arouses in response to the sensation felt on each sep- 

 arate 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. 



