04* On the Catoptrical and Dioptrical 



common to emeralds of other figures, the prohibition to en- 

 grave eonea e emeralds in particular, would have been ridi- 

 culous and without any reason. It is therefore established 

 by the authority of Pliny, that the antients knew that the 

 concave figure of transparent bodies gave them the property 

 of rendering the sight more clear and distinct; and that 

 they knew how to give them this figure, and how to palish 

 them. 



82. " Further: the antients were acquainted with the pro- 

 perty by which concave mirrors enlarge bodies by reflection. 

 Thev made such mirrors from pure curiosity, and in order 

 to have the pleasure of seeing objects magnified by them. 

 They even made very large ones for this purpose. To be 

 convinced of this fact, we have only to read attentively the 

 1 6th chapter of the 1st book of Seneca's Natural Questions. 

 (t is also evident from the 5th chapter of the same book, 

 that the property of concave mirrors in inverting the images 

 of objects was known to the antients. They knew likewise, 

 that a spectator may stand before a concave mirror, in such 

 a position, as to sec the image in the air between the mir- 

 ror and the eye. For Artemidorus of Parinum, cited by 

 Seneca, in the same book, chapter 4th, says : "Si speculum 

 concctrum feceris, quod sit sectce pilce pars, si extra medium,^ 

 ronstiteris, qukinique juxta te steterint, universi a te vide- 

 luntur prop lores tibi quam speculo : that is, " If you make 

 a concave speculum, which is the portion of a sphere, and 

 if vou stand beyond the middle of it (or further from the 

 mirror than the centre) you will see those who stand by you 

 nearer to vou than the speculum. 



83. u The art of making large mirrors was not unknown 

 to the antients. For Quintitian, in the last chapter of his 2d 

 book De Oratore, and Plutarch, in the life of Demosthenes, 

 tell us, that this famous Athenian orator, when young, used 

 to declaim before a large mirror, in order the better to re- 

 gulate his gestures. Seneca also, towards the end of the 

 2 7 th chapter of his above-cited book, says, that mirrors 

 were made as large as the human body. 



61." The antient Peruvians, in the time of the Incas, had 

 the art of making mirrors plane, convex, and concave, of two 

 kinds of stone, capable of receiving a fine polish ; and also, 

 according to some, of a composition of several metals now 

 unknown. They were all plane on one side, and concave 

 or convex on the other. Don Antonio de Utloa, who saw a 

 great number of them in South America, says, " They were 

 ai well finished as if those Peruvians had had all the neces- 

 sary machinery, and great knowledge in optics.'' He saw 



2 one 



