OF NATURAL HISTORY. i8 



J 



Another queftion with regard to vifion has been much agitated by 

 philofophers. Becaufe a feparate image of every objedl is painted 

 on the retina of each eye, it was concluded, that we natur;illy fee all 

 objeds double ; that we l^arn to correct this error of vifion by the 

 fenfe of touching ; and that, if the fenfe of feeing were not con- 

 ftanily redified by that of touching, we fhould be perpetually de- 

 ceived as to the pofition, number, and fituation of objedts. The 

 Count de BufFon mentions^ the real fad, though he afcribes it to a 

 wrong caufe. ' When two' images,' fays he, ' fall on cor re/ponding 

 ' parts of the retinae, or thofe parts which are always afFeded at the 



* fame time, objeds appear fmgle, becaufe we are accujlomed to judge 



* of them in this manner. jBut, when the images of objeds fall up- 

 ' on parts of the retinae which are not ufually afFeded at the fame 

 ' time, they then appear double, becaufe we have not acquired the 



* habit of redifying this unufual fenfation. Mr Cheflelden, in his 

 ' anatomy, relates the cafe of a man who had been affeded with a 



* ftrabifmus, or fquinting, in confequence of a blow on the head. 



* This man faw every objed double for a long time : But he gra- 



* dually learned to corred this error of vifion, with regard to objeds 



* which were familiar to him ; and, at laft, he faw every objed 



* fingle as formerly, though the fquinting was never removed. This 



* is a proof ftill more dired, that we really fee all objeds double, 



* and that it is by hah'it alone we learn to conceive them to be 

 ' fingle *.' 



In this, and other paflages, the Count de Buffon has pointed out 

 the genuine caufe (or ultimate fad) why we fee objeds fingle with 

 two eyes. He tells us, that, though a diftind image is painted on 

 each retina, whenever thefe images are painted on correfponding 

 points of the retinae, an objed is perceived to be fingle. It is equal- 

 ly 



* Buffon, vol. 3. pag. 7.. Tranflah 



