SENSE OF VISION. 



887 



only when at an equal distance from both eyes, so that their pictures upon the 

 retina are of the same size; which will only happen when they are directly in 

 front of the median line of the face. Again, if pictures of dissimilar objects be 

 simultaneously presented to the two eyes, the consequence will be similar 

 to that which is experienced when the rays come to the eye through two 

 differently colored media; the two images do not coalesce, nor do they appear 

 permanently superposed upon one another; but at one time one image pre- 

 dominates to the exclusion of the other, and then the other is seen alone; 

 and it is only at the moment of change, that the two seem to be intermingled. 

 It does not appear to be in the power of the will, Prof. Wheatstone remarks, to 

 determine the appearance of either ; but, if one picture be more illuminated 

 than the other, it will be seen during a larger portion of the time. Many other 

 curious experiments with this simple instrument are related by Prof. Wheatstone; 

 and they all go to confirm the general conclusion, that the combination of the 

 dissimilar images furnished by the two eyes is a mental act, resulting from an 

 inherent law of our psychical constitution; and that our perceptions of the 

 solidity and projection of objects, near enough to be seen in different perspec- 

 tive with the two eyes, result from this cause. In regard to distant objects, 

 however, the difference in the images formed by the two eyes is so slight that 



Fig. 215. 



it cannot aid in the determination ; and hence it is that, whilst we have no 

 difficulty in distinguishing a picture, however well painted, from a solid object, 

 when placed near our eyes (since the idea which might be suggested by the 

 image formed on one eye, will then be corrected by the other), we are very 

 liable to be misled by a delineation in which the perspective, light, and shade, 

 &c., are faithfully depicted, if we are placed at a distance from it, and are pre- 

 vented from perceiving that it is but a picture. In this case, however, a slight 

 movement of the head is sufficient to undeceive us ; since by this movement a 

 great change would be occasioned in the perspective view of the object, supposing 

 it to possess an uneven surface; whilst it scarcely affects the image formed by a 

 picture. In the same manner, a person who only possesses one eye obtains, 

 by a slight motion of his head, the same idea of the form of body which another 

 would acquire by the simultaneous use of his two eyes. 



889. The appreciation of the distance of objects may be easily shown to be 

 principally derived from the association, in the Mind, of visual and tactile sen- 

 sations; assisted, in regard to near objects, by the sensations derived from the 

 muscles of the eyeballs. How much our right estimation of the relative dis- 

 tances of objects not too far removed from the eye depends upon the joint use 



