548 THE NERVOUS SYSTEM. 



When the eyes, on the other hand, are directed to the more distant 

 object, the nearer one is no longer at the point of fixation. For the 

 right eye, its image will appear to the left of the line of sight, and 4 

 for the left eye to the right of this line. It therefore becomes double \ 

 and indistinct. 



Thus, in ordinary binocular vision every object but one appears 

 double and indistinct. This circumstance is so little noticed that it 

 never causes confusion of sight, and even requires a special experi- 

 ment to demonstrate its existence. The reason for its passing unob- 

 served is twofold. . First, the attention is naturally concentrated upon 

 the object at the point of fixation. When this point is shifted, each 

 new object upon which it falls appears single ; and thus the idea of a 

 double image, even if indistinctly suggested at any time, is at once 

 dispelled by the movement of the eyes in that direction. Secondly, 

 an object placed toward either side will form a double image, since its 

 apparent position is different for the two eyes. But the obliquity of 

 its rays, and consequently the indistinctness of its image, will be greater 



FIG. 145. FIG. 146. 



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



for the right eye than for the left, or vice versa ; and the notice of the' 

 observer, if drawn to it at all, is occupied with the more distinct of 

 the two images, to the exclusion of the other. The fact becomes 

 palpable only in such an experiment as the above, where the bodies 

 examined are in the same linear range, so that the double images pro- 

 duced are equal in intensity, and sufficiently contrasted with surround- 

 ing objects to attract attention. 



Double vision may be produced at any time by pressure at the outer 

 angle of one eye, so as to alter its position in the orbit, the other eye 

 remaining fixed. But in this case the whole field of vision is displaced, 

 and all objects are doubled indiscriminately. This form of double vis- 

 ion is produced, in vertigo or intoxication, by irregular action of the 

 muscles of the eyeball. 



Appreciation of Solidity and Projection. When both eyes are 



