SINGLE VISION. 



the apparent directions of the spot, and conse- 

 quently upon those of external objects, which, 

 as far as 1 know, have not been mentioned by 

 any other person. 



1. The spot is always seen single, whether 

 the surface, upon which it is projected, be 

 touching the face, or at the greatest distance 

 from us j and the reason is plain. For the 

 parts of the retinas, by whose affections from 

 the luminous body it is occasioned, are those 

 likewise which receive the pictures of objects, 

 placed at the intersection of the optic axes; 

 and as such objects always appear single, so 

 must also the spot. The fact indeed is so open 

 to observation, and its cause so easily shown, 

 that I should scarcely have thought of men- 

 tioning it, had not Dr. Darwin* lately told us, 

 that the spot is seen double, as often as the 

 eyes are directed to an object more or less 

 distant than the luminous body which gave rise 

 to it. With respect to our different assertions 

 upon this point, I shall only say, that I have 

 made the experiment, I believe, upward of an 

 hundred times, uniformly with the same result; 

 and that, if the spot ever appears double, this 

 must be from some cause very wide of a change 



* Philosoph. Transact, for 1786, p. 318. Dr. Darwin 

 indeed, says, p. 341, that Buffon had observed the same 

 fact ; but it is evident he has mistaken that author's meaning, 



