. Lighthouse construction and illumination; . -\^ a ~ — d cos ~+h- —r sin 45°6. Dioptric Holopliotal Revolving Ligld, 1850.—Applica-tion of total Itcficction to Revolving Lights.—Sections of thefront half of the dioptric holophote (Fig. 62) are equallyapplicable to a revolving light where there is a central burner,which is for that purpose enclosedby a glass cage made up of thosesections. This form of revolvinglight is shown in Fig. 64, and isgeometrically and physically per-fect, because it intercepts all therays by single agents, and theseare glass. The central lenses,L L in tlie original de
. Lighthouse construction and illumination; . -\^ a ~ — d cos ~+h- —r sin 45°6. Dioptric Holopliotal Revolving Ligld, 1850.—Applica-tion of total Itcficction to Revolving Lights.—Sections of thefront half of the dioptric holophote (Fig. 62) are equallyapplicable to a revolving light where there is a central burner,which is for that purpose enclosedby a glass cage made up of thosesections. This form of revolvinglight is shown in Fig. 64, and isgeometrically and physically per-fect, because it intercepts all therays by single agents, and theseare glass. The central lenses,L L in tlie original design of1850 (Fig. 64), instead of beingof a rectangular form are circular,so that the reflecting and refract-ing zones are concentric at theirjunctions, by which some light issaved. When the light is not required to show all. Ficr. 64. HOLOPHOTAL SYSTEM. 89 round the horizon, a portion of a dioptric spherical mirrormay be placed behind the flame on the laud side, and therays reflected by it used for strengthening the oppositeseaward arc. Mr. J. T. Chance, in referring to the relative advantagesof Fresnels revolving light and of the holophotal, says atp. 14 of his paper on ojotical apparatus: — In Fresnelsrevolving apparatus, as the focal distance of the accessorylenses is less than one half of the shortest focal distance inthe system of reflecting zones, the intensity of the lightissuing from the former would be scarcely more-^ than one-fourth of that transmitted by the latter; and in additionto this cause of inferiority is the loss arising at the mirrors;so that, on the whole, the modern plan (holophotal) must givelight ^ve or six times more intense than that of the former(Fresnels) arrangement. The holophotal revolving light is now in imiversal was first introduced in 1850 by Messrs. Stevenson o
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