THE SENSE OF VISION. 801 



The eye, however, can aid itself in the perception of depth in ways which 

 the painter has not at his disposal. By the sense of effort associated with the 

 act of accommodation we are able to estimate roughly the relative distance of 

 objects before us. This aid to our judgment can, of course, be employed only 

 in the case of objects comparatively near the eye. Its effectiveness is greater 

 for objects not far from the near-point of vision, and diminishes rapidly as the 

 distance is increased, and disappears for distances more than two or three meters 

 from the eye. 



When the head is moved from side to side an apparent change in the rela- 

 tive position of objects at different distances is produced, and, as the extent of 

 this change is inversely proportional to the distance of the objects, it serves as 

 a measure of distance. This method of obtaining the " parallax " of objects 

 by a motion of the head is often noticeable in persons whose vision in one 

 eye is absent or defective. 



Binocular Vision. The same result which is secured by the comparison 

 of retinal images seen successively from slightly different points of view is 

 obtained by the comparison of the images formed simultaneously by any object 

 in the two eyes. In binocular vision we obtain a much more accurate idea of 

 the shape and distance of objects around us than is possible with monocular 

 vision, as may be proved by trying to touch objects in our neighborhood with 

 a crooked stick, first with both eyes open and then with one eye shut. When- 

 ever we look at a near solid object with two eyes, the right eye sees farther 

 round the object on the right side and the left eye farther round on the left. 

 The mental comparison of* these two slightly different images produces the 

 perception of solidity or depth, since experience has taught us that those objects 

 only which have depth or solidity can affect the eyes in this way. Conversely, 

 if two drawings or photographs differing from each other in the same way that 

 the two retinal images of a solid object differ from each other are presented, 

 one to the right and the other to the left eye, the two images will become 

 blended in the mind and the perception of solidity will result. Upon this fact 

 depends the effect of the instrument known as the stereoscope, the slides of 

 which are generally pairs of photographs of natural objects taken simultaneous- 



FIG. 260. To illustrate stereoscopic vision. 



ly with a double camera, of which the lenses are at a distance from each other 

 equal to or slightly exceeding that between the two axes of vision. The prin- 

 ciple of the stereoscope can be illustrated in a very simple manner by drawing 

 circles such as are represented in Figure 260 on thin paper, and fastening each 



51 



