wo THE PHILOSOrHY 



the fenfe by which the perceiving nerves of any kind affc' 

 afFe<5led, is not an image or idea of the object. The idea of 

 rednefs has nothing in common with the lead refrangible 

 portions of Hght feparated from the other fix coloured rays 

 of which white light is compofed. The pain of burning 

 reprefents not to the mind any thing of that fwift and fubtle 

 matter by which the nervous threads are broken or deflroy- 

 ed. There is nothing in the idea of a fharp found, from a 

 cord of a certain length, which can inform the mind that this 

 eord vibrates 2000 times in a fecond. * 



Another queftion with regard to vifion has been much 

 agitated by philofophers. Becaufe a feparate image of every 

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

 that we naturally fee all objedls double *, that we learn to cor- 

 real this error of vifion by the fenfe of touching ; and that, 

 if the fenfe of feeing were not conftantly re^ified by that of 

 touching, we fhould be perpetually deceived as to the pofi- 

 tion, number, and fituation of obje£ls. The Count de Bufibn 

 mentions the real fa61:, though he afcribes it to a wrong caufe. 



< When two images/ fays he, ' fall on correfponding parts of 



< the retina, or thofe parts which are always afiefted at the 

 « fame time, obje<Sts appear fingle, becaufe we are accujlomed 

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



* of objects fall upon parts of the retinae which are not ufual- 

 ' ly afFe6ted at the fame time, they then appear double, be- 



< caafe we have not acquired the hah'it of rectifying this un- 



< ufual fenfution. Mr. Chefielden, in his anatomy, relates 

 ' the cafe of a man who had been afte£led with a firabifmus;, or 



* fc^uinting, in confequence of a blow on the head. This man 



* faw every obje(St double for a long time : But he gradually 

 « learned to correct this error of vifion, with regard to ob- 

 ^ je«5ls which were fan:iiiiar to him \ and, at laft, he faw eve- 



♦ For a more ample difcuflion of this point, fee lialler, Phyfiol, torn. 2, ;— 

 and Dr. Reid's Inquirj'. 



