224 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK in. 



surfaces culled annular by geometers." Fresnel was anticipated 

 however in these improvements by Condorcet 1 in 1788, who not only 

 proposed to build the lens in separate pieces but to vary the curva- 

 tures of the different rings in order to correct the spherical aberration. 

 But Condorcet proposed his lens for burning purposes only, while 

 Fresnel was the first who applied the Buffon- Condorcet lens to 

 lighthouse illumination. To utilize in the most complete way 

 possible the rays of light emerging from the lamp placed at the 

 common focus of the set of lenses which compose a dioptric appa- 

 ratus, Fresnel had the idea of securing the upper rays which would 

 have been lost by trapezoidal lenses arranged all round the lamp with 

 such an inclination that the rays were reflected horizontally in the 



FIG. 156. Path of rays in Fresnel's catadioptric 

 lighthouse, with lenses and inclined mirrors. 



FIG. 157. Total reflection in the, prisms in 

 catadioptric lighthouses. 



direction rr, by mirrors MM above them. 'The light K from the 

 vertical lenses was thus greatly added to. Fig. 155 presents the plan 

 and section of a lenticular revolving apparatus such as Fresnel first 

 planned. But Fresnel did not stop with the improvement of the re- 

 volving apparatus. His design for fixed lights consisted of a cylindric 

 refracting hoop of glass completely surrounding the lamp, being the 

 solid which is generated by the revolution of a vertical section of his 

 annular lens round a vertical axis. By this instrument the light is 

 parallelized in the vertical plane only, so as to show a fixed light 



1 Histoire de 1'Acade'mie Royale des Sciences, Eloge de Buffon, Paris, 1791. 



