LENSES FOR LIGHT-HOUSES. 



269 



at hazard. In occupying himself with the same problem, 

 Fresnel, with his habitual penetration, perceived at the 

 first glance where the difficulty lay. He saw that the 

 lenticular light-houses could only become superior to 

 those with reflectors, by increasing considerably the in- 

 tensity of the flame which supplied the illumination ; 

 or by giving to the lenses enormous dimensions which 

 seemed to surpass all that any ordinary work could ac- 

 complish. He observed also that the lenses must have 

 a very short focal length ; that, in making them according 

 to the usual forms, they had too great a thickness, too 

 small a transparency ; and that their weights were consid- 

 erable, and pressed too much on the machinery for making 

 them rotate, so as speedily to bring on its destruction. 



To avoid this excessive thickness of the ordinary lenses, 

 their enormous weight, and want of transparency, which 

 were its consequences, they were replaced by others of a 

 peculiar form, which Buffon had imagined for another 

 purpose, and which he called lenses by steps. (L&ntilles 

 a echelons.)* It is possible at the present day to con- 



* The nature of these lenses a echelons will be understood at once 

 from the annexed sketch, where this construction is represented in 

 front view and in section. The effect of one continuous lens is made 



up by a combination of separate pieces, instead of one large lens as 

 indicated by the dotted outline. See Brewster's Optics. Cab. Cyclop. 

 p. 322. 



