236 On the Catoptrical and Dioptrical 



metal, and concave, having a. focus by reflection; and it \i 

 the common opinion, that the antients knew nothing of the 

 foci by refraction of convex glasses/' 



60. " M. de Fontenelle and M. de la Hire prove satis- 

 factorily*, bv several passages horn Aristophanes and his 

 scholiast, ana from Pliny and Lactantius, that the antients 

 also knew the effects of convex glasses and globes of glass 

 in burning. In the History (of the Acad, of Sc.) which I 

 have ojuoted, we find all these passages ; to which I may add 

 another, from S. Clemens Alexandrhnus, Stromatum lib. vi. 

 cited by father Feijoo, Theat. Crit. torn. 9. p. 146 : " Viam 

 txcogiiat, (j7ia lux, qum a sole procedit, per vas vitreum, 

 aqua plenum, ignescat f. He devises a way by which the 

 light, proceeding from the sun through a glass vessel filled 

 with water, excites fire." Here also I may add another 

 passage from S. Isidore, of Seville X, in his Etymol. lib. xvi. 

 cap. 13, where, speaking of crystal, he says : " Hie oppo- 

 si,'us radiis solis, adeo rapit flam-mam, ut aridis fuvgis vel 



foliis ignem prcsbeai. When opposed to the rays of the 

 sun, it so urges flame as to set fire to dry agaric, or leaves. " 



61. u But Ml de Fontenelle, in the place above cited, 

 and other philosophers with him, suppose the antients to 

 have been ignorant of several things which they certainly 

 knew, as I shall prove in the following articles. 



62. (c In the first place, it is by no means certain, as 

 M. de Fontenelle alleges, that the mirrors of the antients 

 must have been concave. There is nothing to induce us to 

 believe that they did not know the manner of burning with 

 an assemblage of plane mirrors; and, if they had this know- 

 ledge, it is plain that their mirrors were not necessarily con- 

 cave. 



63. <c Although this invention of burning with plane 

 mirrors be regarded by some as very recent, yet it is, in 

 truth, very, antient. According to several modern authors, 

 the burning mirrors of Proclus and Archimedes were com- 

 posed of plane ones. If this opinion be correct, the anti- 



* In Hist, de VAcaJ. R. drs Sr. T708, p. 137, &c. Amst. edit. See 

 also Encycl. Brit, or Hutton's Diet. art. Burning Glass or Burning 

 M. nor . — Trans! a 'or. 



f The same passage is quoted by Rep;»ardt in his Orig. Ancien. de la 

 JPbyi. Nowv. torn. 1. p. 275, with this difference, that he begins it, " An 

 'Xi:og;!at, ha. Art devises a way," &'c. — Translator. 



\ Isidore of Seville, otherwise calied fsidorus Hispalensis, the latest of 

 those called antieiits in this memoir, flourished about 'he middle of the 

 seventh century. See Fallemont, El em. de I' Hist. t. iii. p. 350. A If at, 

 in the work now 'before me, p. 458. says, that this Isidore died A. D. 

 C 3 h . — Tran s I at or . 



quity 



