X. LIGHTHOUSES, ETC. 539 



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 w T ho 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 stria; ; but M. Soleil 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, cemented 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 m diameter. This proposal was adopted, and M. Soleil undertook 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, cemented 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 

 tried in public on 13th April 1821. It was placed on the top of the Obser- 

 vatory buildings, together with two large reflectors, one by Leuoir, the other 

 by Bordier-Marcet. The Lighthouse Committee, of which Mr. Becquey, 

 director-general of the " Pouts et Chaussees," was chairman, went to the 

 summit of Montmartre to judge of the effect. The result confirmed the 

 inventor's previsions, and every one allowed the superiority of the lens over 

 the reflectors. Meanwhile, Fresnel had already thought of improving these 

 first essays. He had invented a machine for constructing circular rings, and 

 M. Soleil was instructed to make eight large lenses constructed on annular 

 principles. He soon finished some of them, and in September 1821 the 

 Lighthouse Committee determined to try their effect at long distances. 

 Fresnel had fitted up on the top of the " Arc de 1'Etoile " a revolving appa- 

 ratus xipon which were fixed two of these annular lenses, four polygonal 

 lenses previously constructed, and four semi-polygonal lenses. At the focus, 

 a four-wicked lamp was burning. The committee then went to Chateuay, a 

 village situated N.N.E. of Paris, 24^- kilometres distance from the " Arc de 

 1'Etoile." The experiment took place during the night of 7/8 September 

 1821, and the results were adjudged as very satisfactory. The eight annular 

 lenses that had just been constructed form part of the first flashing-light 

 apparatus of the 1st class that Fresnel himself fixed on the watch-tower of 

 Cordouan, and which has lighted the entrance to the Gironde for more than 

 30 years. (The lens exhibited under No. 3 comes from this apparatus.) 



If an idea is to be formed of the progress made in the science of lenticular 

 lighthouses from its origin to later times, the three lenses before mentioned 

 should be compared with the lenses of modern construction exhibited under 

 Nos. 11, 12, and 13. 



