THE ARCHIPELAGO OP BREHAT. 135 



these lenses than in the case of the metallic reflectors. 

 This mode of lighting was not therefore attended by 

 any practical results, while even the name of the 

 person with whom the arrangement originated is 

 now forgotten. 



With the view of overcoming the difficulty to 

 which we have referred, Fresnel conceived the idea of 

 decomposing his lenses into several elements. The 

 central glass was an ordinary lens of small dia- 

 meter and consequently of inconsiderable thickness. 

 He formed the others of prisms, which were ar- 

 ranged round the centre in concentric circles, whose 

 curvatures were so calculated that their focus co- 

 incided with that of the lens itself. It would have 

 been impossible to cut and polish such large 

 glass circles; Fresnel therefore constructed them of 

 separate pieces which he cemented together with isin- 

 glass. Thus was realised one of the conceptions of 

 our illustrious Buffon *, whose genius seems to have 

 comprehended all departments of science. He, too, 

 had conceived the idea of constructing poly zonal lenses, 

 but supposing it necessary that the different parts of 

 the series should all be formed of one piece, he had 

 regarded this plan as impracticable. The merit of the 

 invention, belongs therefore, exclusively to Fresnelf , 

 who was ignorant of Buffon's suggested plan till he 

 had himself realised his own theoretical conceptions. 



* [A notice of the life and works of Buffon is given in the 

 Appendix, Note XL] 



f [M. de Quatrefages is probably not aware that Sir David 

 Brewster described a method of constructing large lenses of several 

 pieces in 1812, ten years before the publication of Fresnel's cele- 

 brated memoir.] 



K 4 



