Johnson's natural philosophy and key to philosophical charts : illustrated with 500 cuts; being reduced photographic copies of all the diagrams contained in the author's philosophical series of indestructible school charts ; for the use of schools and families (1878)
towers erectedalong the coast, upon the tops of which are large lanterns, lighted,at night, as guides to mariners. In early times light-houses were illuminated by fires, made of wood,coal, or other substances; subsequently by means of oil-lamps, placedin foci of concave reflectors. But the metal… … This consists of substituting, for the reflectors, plano-convex lenses,in the principal foci of Avhich are placed powerful lamps, with severalconcentric Avicks. The difficulty of making large plano-convex lenses,together with their great absorption of light, led to the adoption of asystem of…
Physics · Astronomy
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