Chap. XI. A N T I E N T METAPHYSICS. 151 



rate upon that organ of fenfe, and the progrefs of the communication 

 of the motion from the objedt to the mind, are better known with re- 

 fpe£t to this fenfe, than with refped: to any other ; for here we have 

 difcovered that the rays of Hght, which are refleded from the object, 

 are received in the pupil of the eye, there refracted and converged, fo 

 as to form a picture of the objedt in the retina or bottom of the eye, 

 and which, by the optic nerve being conveyed to the brain, the feat, as- 

 is fuppofed, of all fenfation, is perceived by the mind. 



From this account of vifion, it is evident, that the mind is not con- 

 verfant with the vifible objedt itfelf, but only with the image, or ,^*,. 

 A<.», as the Epicureans called it, thrown off from the objed. And this 

 is made ftill more evident by the common experiment of a room 

 darkened, and a lens put in a hole in the window-fhutter, which re- 

 ceives the rays from the objeifls without, and, refracting them in the 

 fame manner as the pupil of the eye does, makes a picture of them 

 upon the oppofite wall. Here it is evident, that the mind- only per- 

 ceives images of objeds : Nor is the cafe much altered when we come 

 out of a dark room and walk abroad ; for we carry with us a kind of 

 camera obfcura, in which we perceive only the images of the objeds as 

 they are painted upon the retina of the eye. Still, therefore, we know 

 nothing of them except thofe idols of them, which, according to Epi- 

 curus*s notion, fly off from their furface *, and, fo far from penetra- 

 ting into the ii fide, or efience of them, we cannot even fee the 

 furface of them near, but it muft be at the diftance of what is called ^ 

 dijlin^i vifion* 



Alchough 



• In this matter of vifion^ the Epicureans appear to me to have come nearer the 

 truth than the other philofophers of antiquity, and even the mathematicians ; for, ac- 

 cording to Euclid, vifion is produced by rays that come from the eye, not by rays 

 that come to the eye from the objed. See Euclid's Optics, and what I have further 

 faid upon this fubjed in the Origin and Progrefs of Language, voj. i. page 26. edit. 2- 



