118 SPECIAL SENSES. 



eye to be much superior to the other in acuteness of vision, 

 an object is fixed with the better eye, and its image is formed 

 upon the fovea. The image formed upon the retina of the 

 other eye is indistinct, and in many instances is habitually 

 disregarded ; so that, practically, the subject uses but one 

 eye, and presents the errors of appreciation which attend 

 monocular vision, such as a want of accurate estimation of 

 the solidity and distance of objects. 1 It is stated, as the rule, 

 that when strabismus of long standing is remedied, as far as 

 the axes of the eyes are concerned, by an operation, binocular 

 vision is not restored ; but the experiments necessary to the 

 accurate determination of this point are exceedingly delicate 

 and must be made with great care. 2 This is explained upon 

 the supposition that the functional power of the retina of the 

 affected eye has been gradually and irrecoverably lost from 

 disuse. In normal binocular vision, the images are formed 

 upon the fovea centralis of each eye ; that is, upon corre- 

 sponding points, which are, for each eye, the centres of dis- 

 tinct vision. 



It is hardly necessary to speculate with regard to the 

 reason why two images, one upon each retina, convey the im- 

 pression of a single object. We appreciate a sound with 

 both ears ; the impression of a single object is received by 

 the sensory nerves of two or more fingers ; the olfactory 

 nerves upon the two sides are simultaneously concerned in 

 olfaction ; and, in the same way, when we look at a single 

 object with both eyes, the brain appreciates a single image. 

 "We shall see, however, that the concurrence of both eyes is 

 necessary to the exact appreciation of distance and form ; 

 and, when the two images are formed upon corresponding 



1 Prof. H. D. Noyes has stated to me verbally, that, in some cases of stra- 

 bismus of long standing, there has seemed to exist binocular vision, which is to 

 be explained only upon the supposition that a new fovea centralis, as it were, 

 has been formed in one eye, by increasing the sensibility of the retina at a new 

 point through constant use. 



2 STELLWAG VON CAKION, Treatise on the Diseases of the Eye, New York, 

 1868, pp. 705, 713. 



