VIII. LIGHTHOUSES, &C. 451 



6. Lens with catadioptric rings, constructed in . 1825. 



7. Model of apparatus with catadioptric rings. Fresnel, 1825. 



8. Burner with 4 wicks, constructed after experiments 



made by Arago and Fresnel in . . . 1820. 



9. Burner with 2 wicks, with external covering, by 



Henry Lepaute ..... 1845. 



10. Burner with 4 wicks, storied, for mineral oils. 



11. Large annular lens of Barbier and Tenestre . 1876. 



12. Lenticular pann el for flashing lights of Henry Lepaute 1876. 



1 3. Apparatus for revolving electric lights, of Sanker, 



Lemonnier & Co. . . . . 1876. 



Lighthouse Service of France, Paris. 



2202a. Echeloned Lenses. 



Lighthouse Service of France, Paris. 



No. 1. First essay of echeloned lens, polygonal form. Invented by 

 A. Fresnel, and constructed under his direction in 1819. 



No. 2. First echeloned lens, polygonal; form, for flashing lights of the first 

 class. Invented by A. Fresnel, and constructed in 1820. 



No. 3. First echeloned lens, annular form, for flashing lights of the first 

 class. Invented by A. Fresnel, and proceeding from the lenticular apparatus 

 fixed on the tower of Cordouan in 1821. 



When Fresnel conceived the idea of substituting in lighthouses large glass 

 lenses for metallic reflectors, he thought of composing these lenses of several 

 pieces, and of calculating the curves of these different pieces so as to rectify 

 their spherical divergence. He demonstrated his plan before the Lighthouse 

 Committee in August 1819, three months only after his appointment on the 

 Committee, and on the 19th of October following he was granted the sum of 

 500 fr. for constructing a trial lens. He consulted the optician Soleil, who 

 seconded him with much good will, but who could only put at his disposal 

 the limited appliances then in use. Glass was at this time worked still by 

 hand, and shaped only into plane or spherical forms. Fresnel admitted that 

 the lens should be flat on one side ; that the different gradients, instead of 

 forming circular rings, should be defined by polygons and divided into a certain 

 number of pieces, each of which should receive on its echeloned side a sphe- 

 rical surface properly calculated. Another difficulty arose from the glass 

 factories being unable to supply in sufficient size pieces of crown glass free 

 from bubbles and striae ; but Fresnel discovered the way of re-smelting glass 

 without altering its transparency. 



He first constructed a trial lens of 35 centimetres diameter (the one exhi- 

 bited under No. 1). It was given by Soleil to the Academy of Sciences, and 

 deposited at the " Conservatoire des Arts et Metiers." It is composed of 21 

 pieces, glued together, and fixed upon a pane serving as a support. 



Emboldened by this first success, Fresnel proposed to the Lighthouse Com- 

 mittee, at their sitting of 31st December 1820, to order the construction of a 

 lenticular revolving light apparatus for the Cordouan lighthouse. The prin- 

 cipal part of this apparatus was to include eight square lenses of 76 centi- 

 metres, forming together an octagonal prism inscribed within a cylinder of 

 2 ra diameter. This proposal was adopted, and Mr. Soleil vindertook the 

 construction of these eight polygonal echeloned lenses. (One of them is 

 exhibited under No. 2.) It is to be seen that it was composed of 100 pieces 

 of glass, glued together, and that the flat pane, which in the trial lens serves 

 as a support, has been done away with. One of these new lenses was first 



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