. Lighthouse construction and illumination; . 2d, The extremely short focaldistance of the reflector, which was found on trial to pro-duce a cone having a divergence of no less than 60°; and3d, The loss by metallic reflection from the paraboloid. 24. Lconor Fresnels improved Revolving Light.—M. pulDlished an improvement on his brothers light,in his Phares etFanaux des Cotesde France, Paris,1842, by enlarg-ing and altering theposition of the tra-pezoidal inclinedlenses (Fig. 49), and thus increas- ?! Fig. 49. ;#- ing their focal distance. A very considerable improvementwas thus effect
. Lighthouse construction and illumination; . 2d, The extremely short focaldistance of the reflector, which was found on trial to pro-duce a cone having a divergence of no less than 60°; and3d, The loss by metallic reflection from the paraboloid. 24. Lconor Fresnels improved Revolving Light.—M. pulDlished an improvement on his brothers light,in his Phares etFanaux des Cotesde France, Paris,1842, by enlarg-ing and altering theposition of the tra-pezoidal inclinedlenses (Fig. 49), and thus increas- ?! Fig. 49. ;#- ing their focal distance. A very considerable improvementwas thus effected, but he still retained metallic reflectionand double agents for the upper rays. 25. Holoplwtal System—These improvements come nextin order of date, but for the sake of clearness and unity weshall first describe the remaining French improvements onFresnels revolving light. 26. M. Lepautesform of Revolving Light.—M. Lepaute,the collaborateur of A. Fresnel, gave a design in 1851, inwhich, in order to avoid the use of double agents, he. 78 LIGHTHOUSE ILLUMINATION. increased the height of the lens, and reduced proportionallythe angle subtended by the iixed light prisms above andbelow. In this way he extended the powerful part of thelight probably farther than was consistent with favourableangles of incidence of rays falling near the top and bottomof such elongated lenses. The apparatus could thereforeparallelise the rays in the vertical plane only by its upperand lower prisms. Of course, if he had been acquaintedwith the holophotal prisms subsequently to be described, hecould have parallelised the light in every plane from top tobottom of the apparatus. In M. Lepautes letter to the Board, of 28th July 1851, he states that hisdesign received the approbation of the Commissioners ofLights in France; and he adds that The French Adminis-tration is aljout to order from the undersigned an apparatusof the first order of this description of flashes for everyminute, t
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Keywords: ., bookcentury1800, bookdecade1880, bookpublisherlondo, bookyear1881
