398 Objeffs Jeen Jingle ly two Eyes. [Book IX. 



fenfe j thefe rays give aljfo the idea of different colours, 

 according to the properties of the bodies which tranf- 

 iriit or reflect them, upon the principles formerly de- 

 monft rated. 



It is very difficult to explain how it happens that 

 two diftinct images are painted upon both eyes, and 

 yet that we only perceive a fingle object. This dif- 

 ficulty has been attempted to be foived by having 

 recourfe to the power of habit ; but I confefs I can- 

 not help being of opinion with Dr. Reid, that the 

 correfpondence of the centers of the two eyes, on 

 which fingle vifion depends, does not arife from cuftom, 

 but from fome natural ccnftitution of the optic nerves. 

 The cafe of the young man born blind, who was 

 couched by Mr. Chefelden, and who faw fingly with 

 both eyes, immediately upon receiving his fight, is 

 ve*y properly adduced by that refpectable author in 

 favour of this fuppofition. He alfo found, that three 

 young gentlemen, whom he endeavoured to cure of 

 fquinting, faw objects fingly, as fqon as they were 

 brought to direct the centers of both eyes to the fame 

 object, though they had never been ufed to do fo from 

 their infancy ; he remarks too, that there are cafes in 

 which the fulled conviction of an object being Tingle 

 will never make the object appear fo, even by the 

 longeft practice, as in the cafr of looking through a 

 multiplying glafs *. 



In thofe who fquint, the diflance between the two 

 pupils is confiderably lefs than in other perfons, for 

 when the pupil of the undiftorted eye is feated in the 

 middle of the orbit, as in looking diredly forwards, 

 the pupil of the other eye is drawn clofe to the nofe, 

 fo that the two axes are never pointed at the fame ob- 



* Reid's Inquiry into Human Mind, p. 267. 



