540 SEC.' 12. APPLIED MECHANICS. 



Fixed Light Apparatus. 



No. 4. First fixed light apparatus of 0-50 m diameter, invented by A. Fresnel, 

 and constructed in 1824. 



Fresnel, after his appointment to the Lighthouse Committee of 1819, first 

 gave his attention to flashing lights ; meanwhile, he had thought about ob- 

 taining fixed lights, and in the first design of the lenticular lighthouse that 

 he submitted to the committee on 31st October 1820, he indicated, as 

 a solution, the .use of cylindrical lenses ; but the Lighthouse Committee 

 had thrown aside the fixed light system as possessing less power than re- 

 volving lights, and as being liable to be mistaken for the incidental lights of 

 the coast. The committee altered this decision later, and Fresnel then in- 

 vented the system of fixed light apparatus (0.50 m diameter), exhibited under 

 No. 3. In this apparatus the lenticular drum, which should be cylindrical, 

 so as to give an uniform subdivision of light, shows a polygonal form of 

 16 fascets, because no lathe was then known for making cylindrical pieces. 

 The upper part is made up of two lenticular zones in the shape of a 16-panel 

 cupola, every element of which is coupled with a plane mirror. The lenses 

 unite in parallel fasces the rays emitted by the light, and the mirrors reflect 

 them in the direction of the horizon. 



A similar system, but having one lenticular zone only, is fitted at the lower 

 part. The lamp has two wicks, and stands upon a plate raised or lowered by 

 a jack between three leaders. With the polygonal form that had to be 

 adopted, there were 16 directly receiving more light than the intermediate 

 parts, but Fresnel, while constructing the instrument, discovered the means of 

 greatly lessening this inequality, by alternating the shining directions of the 

 lenticular cylinders with those of the two other parts. This first trial of a 

 fixed light apparatus was demonstrated by Fresnel before the Academy of 

 Sciences of Paris, at their sittings of 3rd 'May 1 824. It was then inaugurated 

 on the 1st February 1825 in the port of Dunkirk. 



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. G. 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. Researches 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 Bordier-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 



