

and the union of the Optic Nerves. 145 



brain in the above instance ; nor is it easy to ascertain how 

 mental emotions sometimes suddenly cause those symptoms to 

 cease. 



The function of vision does not unfrequently suffer from all 

 these causes ; but surely we have not legitimate reason to con- 

 clude, that when hemiopsia, or half sight has been produced, 

 the cause must be lesion of just half the optic nerve, or of the 

 whole nerve on one side, between its origin and the union on 

 the sella turcica. On the contrary, lesion, or disease of one 

 nerve at the point just mentioned has not been attended with 

 hemiopsia. 



Many have attempted to show how single vision with two 

 eyes is obtained, and also to explain the phenomena of hemiop- 

 sia, or half sight. It would be endless to advert to the opi- 

 nions of all who have written on these subjects; among the 

 most eminent we find Berkeley. The theory which his reason- 

 ing seems intended to support, ascribes single vision to the cor- 

 rections which the impressions made on the retinas by visible 

 objects habitually receive from the sense of touch ; so that the 

 mind was supposed to acquire by degrees the habit of know- 

 ing that objects were single, though the impression on the or- 

 gans of sight were double. 



The assistance and correction which the sense of sight re- 

 ceives from the touch, is supposed to be shown by the case of 

 the boy born blind, and restored to sight by Cheselden's ope- 

 ration. The same case is adduced as a proof of the gradual 

 manner in which the knowledge of the effects of light and shade, 

 as indicative of the figures of objects, was slowly acquired, and 

 by remembering the errors of sight which the touch corrected. 

 This boy saw objects single at the first moment his sight was 

 restored ; consequently, (if the lens had never transmitted light 

 prior to the operation,) this case is a proof that the faculty of 

 single vision is not acquired by habit, but is an original function. 



Smith and Reid maintained, that single vision arises from the 

 two pictures of objects falling on corresponding points of the re- 

 tinas. The former of those authors believed the faculty of single 

 vision to be acquired by habit, the latter considered it an origi- 

 nal power possessed independent of habit. Cheselden's case, 

 just noticed, tends to decide this point. Smith says, " When the 



VOL. IX. NO. I. JULY 1828. K 



