VIII. LIGHTHOUSES, &C. 453 



Catadioptric Rings* 



No. 5. First apparatus containing catadioptric rings, as well for fixed light 

 as for flashing lights, invented by A. Fresnel, for lighting the St. Martin 

 Canal, and constructed in 1825. 



No. 6. Annular lens, composed of dioptric and catadioptric elements, 

 similar to those of apparatus No. 5, and constructed at the same date. 



No. 7. Models in wood of an apparatus similar to No. 5, but on a larger 

 scale. Study of A. Fresnel in 1825. 



The last invention of A. Fresnel, that of the catadioptric rings, was pro- 

 moted by a request for information addressed to him by the Prefect of the 

 Seine in 1825. It was a question of applying to the lighting the quays of the 

 St. Martin's Canal more powerful lamps than those used commonly in the 

 city of Paris. This problem, to which Fresnel's attention was called, was the 

 same as that of the port-lights apparatus, of which he had postponed the study 

 because the sidereal reflectors of Borclier-Marcet were sufficient to supply the 

 wants of the service." 



The principal part of these small apparatus, that is, the lenticular cylinder, 

 offered no theoretical difficulty. It was to proceed out of an echeloned section 

 turning around the vertical axis ; the only thing was to construct it in circular 

 shape, because the polygonal shape would have been impossible for rings of 

 20 to 25 centimetres diameter. The question was not so easily solved as 

 regards the accessory parts intended to utilise the luminous rays passing out- 

 side the cylinder, because the reflectors used in the other classes must now be 

 reduced to too small dimensions. It was then that Fresnel thought of the 

 phenomenon known in optics under the name of " total reflection," and 

 imagined to substitute for the common reflectors glass rings, within which the 

 luminous rays should be reflected without appreciable loss. 



Fresnel's first conception for these circular rings was to direct the fasces 

 through which the luminous rays pass perpendicularly to these rays, so as 

 not to alter their direction ; the reflecting surface would then have preserved 

 the shape of the mirrors to be replaced, but hence resulted inconvenience, 

 and a too great weight of glass. Fresnel found out that an inclined direction 

 upon the rays could be given to these in and out going fasces, and thus these 

 inclinations be combined, as well as the shape of the reflecting surface, so as 

 to force the rays to emerge horizontally. The transverse section of the rings 

 then became triangular, instead of showing four sides, and the dimensions 

 lessened. 



The apparatus exhibited is that to which Fresnel first applied this invention. 

 Its diameter is reduced to 0'20 m ; the cylinder is generated by an echeloned 

 section composed of three elements and fills up a half circumference. The 

 rays passing above this cylinder are gathered by four total reflection rings, 

 and this is effected by turning 'around the vertical point of the focus the 

 section of the catadioptric triangles just spoken of. Thus is obtained a fixed 

 light apparatus, lighting up half the horizon. The lamps of the St. Martin's 

 Canal having to be erected at 70 metres distance, it became necessary to give 

 them a greater lateral than frontal intenseness. Fresnel succeeded in this 

 by placing on each side a half annular dioptric lens, generated by the rotation 

 of the section of the cylinder around an horizontal axis, parallel with the 

 longitudinal direction of the quay, but he had moreover the happy idea of 

 making the section of catadioptric triangles to revolve around this axis so as 

 to form an annular lens, collecting around the focus an angle of great 

 amplitude, and comprising at the same time dioptric and catadioptric rings. 



The manufacture of these different circular rings offered serious difficulty, 

 and Fresnel was obliged to set up a factory. 



A first apparatus was completed in 1826, and submitted to the Lighthouse 

 Committee towards the end of December. Four of these new lamps were 



