SEEING. 13 



lures are bright or obscure, in proportion to the distances of 

 the objects represented. When objects are very remote, 

 their pictures on the retina are so faint, that they are entirely 

 obliterated by the vigorous and lively impressions of nearer 

 objects, with which we are every way surrounded. On the 

 other hand, when near objects emit a feeble light only, com- 

 pared with that which proceeds from a remote object, as, for 

 example, when we view luminous bodies in the night, then 

 very distant objects make distinct pictures on the retina, and 

 become perfectly visible. Hence a man ? by placing himself 

 in a dark situation, and looking through a long tube, without 

 the intervention of a glass, may make a kind of telescope, 

 which will have a considerable effect, even during the day. 

 For the same reason, a man at the bottom of a deep pit can 

 see the stars at noon. 



Another question with regard to vision has been much 

 agitated by philosophers. Because a separate image of every 

 object is painted on the retina of each eye, it was concluded, 

 that we naturally see all objects double ; that we learn to 

 correct this error of vision by the sense of touching ; and that 

 if the sense of seeing were not constantly rectified by that of 

 touching, we should be perpetually deceived as to the posi- 

 tion, number, and situation of objects. The Count de Buffon 

 mentions the real fact, though he ascribes it to a wrong cause. 

 ' When two images,' says he, ' fall on corresponding parts 

 of the retinae, or those parts which are always affected at the 

 same time, objects appear single, because we are accustomed 

 to judge of them in this manner. But when the images of 

 objects fall upon parts of the retinae which are not usually 

 affected at the same time, they then appear double, because 

 we have not acquired the habit of rectifying this unusual sen- 

 sation. Mr. Cheselden, in his anatomy, relates the case of a 

 man who had been affected with a strabismus, or squinting, 

 in consequence of a blow on the head. This man saw every 

 object double for a long time ; but he gradually learned to 

 correct this error of vision, with regard to objects which were 

 familiar to him ; and, at last, he saw every object single as 

 formerly, though the squinting was never removed. This is 

 a proof still more direct, that we really see all objects double, 

 and that it is by habit alone we learn to conceive them to be 

 single.' * 



In this, and other passages, the Count de Buffon has pointed 

 out the genuine cause (or ultimate fact) why we see ob- 



* Buffon, Vol. III. p. 7. Trans. S. 



