•ict. v.] DISSERTATION SECOND. 175 



though they were still far from the truth. They conceiv- 

 ed vision to be performed in consequence of certain simu- 

 lacra, or images continually thrown off from the surfaces 

 of bodies, and entering the eye. This was the substitute 

 in their philosophy for rays of light, and had at least the 

 merit of representing that which is the medium of vision, 

 or which forms the communication between the eye and 

 external objects, as something proceeding from the latter. 

 The idea of simulacra, or spectra, flying off continually 

 from the surfaces of bodies, and entering the eye, was per- 

 haps as near an approach to the true theory of \ision, as 

 could be made before the structure of the eye was under- 

 stood. 



In the arts connected with opticks, the ancients had 

 made some progress. They were sufficiently acquainted 

 with the laws of reflection to construct mirrors both plane 

 and spherical. They made them also conical; and it ap- 

 pears from Plutarch, that the fire of Vesta, when extin- 

 guished, was not permitted to be rekindled but by the rays 

 of the sun, which were condensed by a conical speculum 

 of copper. The mirrors with which Archimedes set fire 

 to the Roman gallies have been subjects of much discus- 

 sion, and the fact was long disbelieved, on the ground of 

 being physically impossible. The experiments of Kircher 

 and Buffon showed that this impossibility was entirely ima- 

 ginary, and that the effect ascribed to the specula of the 

 Greek geometer might be produced without much difficul- 

 ty. There remains now no doubt of their reality. A pas- 

 sage from Aristophanes ' gives reason to believe that, in 

 his time, lenses of glass were used for burning, by collect- 

 ing the rays of the sun ; but in a matter that concerns the 

 history of science, the authority of a comick poet and a 



1 In Nubibus, Act. 2, sc. 1, v. 20. 



