THE SENSES. 549 



directed at a single object, its distance may be estimated with some 

 accuracy by the convergence of the visual axes required for its fixa- 

 tion. Another impression is also produced by binocular vision, when 

 an object, of appreciable volume and thickness, is viewed within a mod- 

 erate distance. Owing to the lateral separation of the two eyes, and 

 the convergence of their visual axes, they do not receive precisely the 

 same image. Both eyes will see the front of the object in nearly the 

 same manner ; but in addition the right eye will see a little of its right 

 side, and the left eye a little of its left side. This is illustrated in Figs. 

 145 and 146, representing an object as seen by the two eyes, at a dis- 

 tance 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 

 side to the right eye. As the central part of its mass is in the point 

 of fixation, at the junction of the visual axes, the object appears single. 

 But the images which it presents to the two eyes are not precisely 

 identical ; and the combination of these different images into one gives 

 the impression of solidity and projection. 



This effect is complete only when the object is within a moderately 

 short distance. For those which are remote, the convergence of the 

 visual axes, and the consequent difference in configuration of the im- 

 ages, become inappreciable, and the impression of solidity disappears. 

 At a distance of some miles even a large object, like a mountain, loses 

 its projection, and appears like a flattened mass against the horizon. 

 The pictorial representation of distant views is therefore often very 

 effective, the idea of remoteness in different parts of the landscape 

 being conveyed by appropriate intersections of outline and by varia- 

 tions in tone, color, and distinctness, like those due to the interposition 

 of the atmosphere. But a picture which aims to represent the solidity 

 of near objects can never deceive us in this respect, however elaborate 

 its details ; since its surface presents the same image to both eyes, and 

 it is consequently evident that the objects delineated have no real pro- 

 jection. But the appearance of solidity may be successfully imitated by 

 representing an object in two different positions. This is the principle 

 of the stereoscope. Two photographic pictures of the same object are 

 taken from different points of view, one of them representing it as it 

 would be seen by the right eye, and the other as it would be seen by 

 the left. With these pictures so placed in the stereoscope that each 

 eye has presented to it the appropriate view, the two images are com- 

 bined in the act of vision, producing the apparent effect of projection 

 and solidity. 



General Laws of Visual Perception. Beside the formation and com- 

 bination of optical images, there are certain phenomena connected with 

 visual perceptions in general which are of interest in the physiology 

 of sight. Some of these phenomena require special modes of investi- 

 gation, while others are made evident by comparatively simple means, 

 and are often important in their hygienic relations. 



Luminous impressions upon the eye continue for a short time after 



