SENSE OF SIGHT. 



is illustrated in Figs. 206 and 20T, which represent a skull as seen by 

 the two eyes, when placed exactly in front of the observer at a distance 

 of eighteen inches or two feet ; rather more of the details on one side 

 being visible to the left eye, and rather more of those on the other 



Fig. 206. Fig. 207. 



AS SEEN BY THE LEFT EYE. AS SEEN BY THE RIGHT EYE. 



being visible to the right eye. As the central part of the mass is in the 

 point of fixation, at the junction of the two visual axes, the object 

 appears single. But the images which it presents to the two eyes are 

 not precisely identical in form ; and it is the combination of these dif- 

 ferent images into one which gives rise to the impression of solidity or 

 projection. 



But this effect is complete only when the object is situated within a 

 moderately short distance. For those which are comparatively remote, 

 the convergence of the visual axes, and consequently the difference in 

 the apparent configuration of the two images, become inappreciable, 

 and the optical impression of solidity disappears. At a distance of 

 some miles even a large object, like a mountain, loses its projection, 

 and presents the form of a flattened mass against the horizon. It is on 

 this account that pictorial representations of distant views are often 

 extremely effective ; the idea of successive remoteness in different parts 

 of the landscape being conveyed by appropriate intersection of the out- 

 lines and by variations in tone, color, and distinctness, like those due 

 to the interposition of the atmosphere. On the other hand, a picture 

 of near objects, which aims to represent their solidity, can never de- 

 ceive us in this respect, however elaborate may be the details of surface, 

 shadow, and color ; since the flat surface of the picture presents the 

 same image to both eyes, and it is consequently evident that the ob- 

 jects delineated have no real projection. But if two pictures of the 

 same object, taken in two different positions, be presented in such a 

 way that only one of them is seen by the right eye, and only the other 

 by the left, the same optical effect may be produced as by the object 

 itself, and the appearance of solidity and projection may be perfectly 



