270 PRESNEL. 



struct the largest lenses of this kind, although we do not 

 yet know how to fabricate thick masses of glass free from 

 defects. It suffices to compose them of a number of dis- 

 tinct small pieces ; a plan proposed by Condorcet. 



I can here affirm that, at the moment when the idea of 

 these lenses " by steps " occurred to the mind of Fresnel, 

 he had not the least knowledge of the previous projects 

 of Buffon and Condorcet. But assertions of this kind 

 are interesting only to the author in regard to his own 

 claims, they have no value for the public. In its eyes 

 there is not, I will say more, there ought not to be 

 more than one inventor, he who first makes public the 

 discovery. After so large a concession, it may at least 

 be allowed me to remark that in 1820 there did not 

 exist a single lens of this construction in the physical 

 cabinets, and that besides, up to that time, lenses had 

 only been regarded as the means of producing great 

 effects of heat ; that it was Fresnel who created methods 

 to construct them with exactness and economy ; that it 

 was he, and he alone, who even imagined the application 

 of them to light-houses. This application, however (as I 

 have just pointed out), could never have led to any use- 

 ful result if it had not been combined with suitable modi- 

 fications of the lamp ; if the illuminating power of flame 

 had not been greatly augmented. This important part of 

 the system required special studies, numerous and deli- 

 cate experiments. Fresnel and one of his friends (Arago) 

 devoted themselves to the inquiry with ardour ; and their 

 common labour led to the construction of a lamp with 

 many concentric wicks, whose brilliancy was twenty-five 

 times that of the best lamps, with only a double current. 



In the glass lenses imagined by Fresnel, each lens 

 sent successively to all parts of the horizon a light equiv- 



