270 , FRESNEL. 
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- 
