. Lighthouse construction and illumination; . ce on which they impinge, and byrefraction they are made to j)ass through the glass, but inaltered directions. The size of the flame produces divergencewith refractors, and the amount of this divergence for any • J- p 4-1 1 • 1 / Radius of flame \ point or tJie lens — Sm (^dlst. of poiut from centre of flarnej • 13. Peesxels Optical Agents. 1. Annular Lens.—So far back as 1748, the celebratedBuffon suggested a new form of lens for turning order to save the loss of heat by absorption, which musttake place with the suns rays in passing th
. Lighthouse construction and illumination; . ce on which they impinge, and byrefraction they are made to j)ass through the glass, but inaltered directions. The size of the flame produces divergencewith refractors, and the amount of this divergence for any • J- p 4-1 1 • 1 / Radius of flame \ point or tJie lens — Sm (^dlst. of poiut from centre of flarnej • 13. Peesxels Optical Agents. 1. Annular Lens.—So far back as 1748, the celebratedBuffon suggested a new form of lens for turning order to save the loss of heat by absorption, which musttake place with the suns rays in passing through the thickglass of a large lens whose outer profile is continuouslyspherical, he proposed to grind out of a solid piece of glass,a lens in steps or concentric zones, so as to reduce to aminimum the thickness, as shown in Fig. 31. This ideawas carried into execution by the Abb^ Eochon in , the Academician, in his Eloge de Buffon, in1773 (Paris edition, 1804, p. 35), proposed the capital 64 LIGHTHOUSE Section Fis. 31. improvement of building up Buffons stepped lens in sejmrate ringSj so as to correct, to alarge extent, the sphericalaberration, which is the di-vergence from the parallelof rays emitted by a lenshaving a spherical David Brewster in 1811also described in the Edin-burgh Encyclopsedia (article Burning Instruments ) thesame mode of buildinglenses and of correcting the aberration. But all thesewriters designed their lenses for turning purjDoses only,and not with any view to operating on light, and stillless to lighthouse illumination. In 1822 Augustin Fres-nel, celebrated alike as a mathematician and physicist, a]Dpa-rently ignorant both of Buffons and Condorcets proposals,described (Memoire sur un nouveau systeme dJ^clairagedes Phares, 1822), and afterwards constructed a similarlens, but for lighthouse purjDOses, in which the centres ofcurvature of the different rings receded from the axis of theinstrument accordin
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