. Lighthouse construction and illumination; . Fig. 66. Fig. 67. parallehses one half of the light; the other half is inter-cepted by a portion of a paraboloid, h e f i, and a portion ofdioptric spherical mirror e f, whose action is limited to raysso near the horizontal axis that none fall on the burner andare lost. All that is wanted to make this instrumentfaultless is to substitute glass for the remaining portion ofmetallic reflector (c h f i) which is used, and this is accom-plished in the design next to be described. 9. Back Frisms.—The maximum possible deviation oflight by means of the Fre
. Lighthouse construction and illumination; . Fig. 66. Fig. 67. parallehses one half of the light; the other half is inter-cepted by a portion of a paraboloid, h e f i, and a portion ofdioptric spherical mirror e f, whose action is limited to raysso near the horizontal axis that none fall on the burner andare lost. All that is wanted to make this instrumentfaultless is to substitute glass for the remaining portion ofmetallic reflector (c h f i) which is used, and this is accom-plished in the design next to be described. 9. Back Frisms.—The maximum possible deviation oflight by means of the Fresnel reflecting prisms of crownglass is limited to about 90°, beyond which the criticalangle would be overpassed, and the rays transmittedthrough the prisms, instead of being reflected by 1867 Mr. Brebner and myself designed what wetermed back prisms, by means of which, rays may bemade to deviate from their original direction for about130°, so that by their use the lighthouse engineer becomesvirtually independent of the critical
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Keywords: ., bookcentury1800, bookdecade1880, bookpublisherlondo, bookyear1881
