. Lighthouse construction and illumination; . s was not only the first appli-cation of total reflection to lighthouses, but was the first optical- combination, which, for the purpose required, was both geo-metrically and physically perfect (excepting of course theinevitable loss due to the divergence of the exfocal rays ofthe flame), leaving in fact no room for improvement; and,accordingly, this beautifvil instrument continues till now inuniversal use. In Figs. 37, 38, E E represent the cylindric refractor,/ the focus, and p p the totally reflecting prisms placed attop and bottom. 17. Fresncls
. Lighthouse construction and illumination; . s was not only the first appli-cation of total reflection to lighthouses, but was the first optical- combination, which, for the purpose required, was both geo-metrically and physically perfect (excepting of course theinevitable loss due to the divergence of the exfocal rays ofthe flame), leaving in fact no room for improvement; and,accordingly, this beautifvil instrument continues till now inuniversal use. In Figs. 37, 38, E E represent the cylindric refractor,/ the focus, and p p the totally reflecting prisms placed attop and bottom. 17. Fresncls Revolving Light, 1822.—In Figs. 39 and 40,which showFresnels form of revolving light, the central burnerB is surrounded by annular lenses L, and a compoundarrangement of inclined trapezoidal lenses 1 and plane silveredmirrors ]\I. The inclined lenses fit closely to each other,forminga pjrramidal dome, and the light, intercepted by them, is sent fkesnels dioptric combinations. 69 upwards in beams of a trapezoidal section, until, falling on. B--- the plane mirrors M, they are bent so far do^vnwards as toemerge with their axes r r parallel to the axes R E of therectangular lenses below. If, then, all these optical agentsare made, by the wheelwork N, to move together round thecentral lamp B, the sailor will receive the full flash wheneverthe axes of the emerging beams are pointed towards him,and he will be in darkness when they are turned away fromhim. Though all the rays are, or as Fresnel pointed out,might be intercepted and sent in the required directions byplacing similar inclined lenses and plane mirrors belowthe central lenses in order to intercept the light which 70 LIGHTHOUSE ILLUMINATION. escapes striking on the burner of the lamp, yet the design,unlike that of his fixed light, is very far from being perfect;not only because metallic reflection is employed, but becausetwo agents are needed for all but the central portion of therays, thus causing a large loss, to which F
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Keywords: ., bookcentury1800, bookdecade1880, bookpublisherlondo, bookyear1881
