OF NATURAL HISTORY. 191 



* ry obje6l iingle as formerly, though the fqnintlng was never 



* removed. This is a proof ftill more direct, that we really 

 < fee all objects double, and that it is by habit alone we learn 



* to conceive them to be fingle*.' 



In this, and other pafTages, the Count de Buffon has point- 

 ed out the genuine caufe (or ultimate fa£l) why we fee ob- 

 jefls fingle with two eyes. He tells us, that, though a dif- 

 tincfl Image is painted on each retinae, whenever thefe images 

 are painted on correfponding points of the retinae, an object 

 is perceived to be fingle. It is equally true, that, when one 

 eye is diftorted by the finger, or any other caufe, in fuch a 

 manner that the images are painted on points of the ratinae 

 which do not correfpond, the object is perceived to be double. 

 Objedls which are much nearer, or much more remote, than 

 that to which both eyes are dlre£led, appear double. If a 

 candle is placed at the diftance often feet, and a rnan holds his 

 finger at arm's-length between his eyes and the candle, when 

 he looks at the candle, he fees his finger double, and w hen 

 he looks at his finger he fees the candle double. < In this 



< phaenomenon,' Dr. Reid properly remarks, < it is evident 



* to thofe who underftand optics, that the pictures of objects 



< which are feen double, do not fall upon points of the reti- 

 « nae which are fimilarly fituated, but that the pictures of 

 « objects feen fingle do fall upon points fimilarly fituated. 



* Whence we infer, that as the points of the two retinae, 



* which are fimilarly fituated with regard to the centres, do 



< correfpond, fo thofe which are difiimilarly fituated do not 



* correfpond. It is to be obferved, that although, in fuch 



< cafes as are mentioned in the lafi: phaenomenon, we have 

 « been accuftomed from infancy to fee objects double which 



< we know to be fingle ; yet cufiom, and experience of the 



* unity of the object, never take away this appearance of du- 

 i plicityf.* 



* Buffon, Tcl. 3, pa^, 7, Tranilat. f Dr. Reid's Inquiry, &c, page 187. 



