. Lighthouse construction and illumination; . Fig. 140. FOG-SIGNALS. 249 fractors. The rays escaping below the lenses JJ of the lowerapparatus are returned again to the focus/ by pieces of sphe-rical mirror m, while at the top of the lower lenses or re-fractors there are placed Back Prisms P (Chap. III.) These. Fig. 141. prisms will, in this case, have conjugate foci situate in theupper and lower flames (/and/), so that the light escapingabove the lenses or refractors of the lower flame f and belowthose of the upper flame / will be intercepted and rays escaping above the lenses or
. Lighthouse construction and illumination; . Fig. 140. FOG-SIGNALS. 249 fractors. The rays escaping below the lenses JJ of the lowerapparatus are returned again to the focus/ by pieces of sphe-rical mirror m, while at the top of the lower lenses or re-fractors there are placed Back Prisms P (Chap. III.) These. Fig. 141. prisms will, in this case, have conjugate foci situate in theupper and lower flames (/and/), so that the light escapingabove the lenses or refractors of the lower flame f and belowthose of the upper flame / will be intercepted and rays escaping above the lenses or refractors of the upperflame /will be intercepted by similar prisms p ii, the conjugatefoci of wliich will be situate respectively in the upper flame,and in a point/ equidistant above the prisms P. The raysconverging towards this upper focus f will be intercepted bya parabolic conoid C c, of novel form, presenting its convex, andnot as hitherto its concave, surface to the rays. This com- 250 LIGHTHOUSE ILLUMINATION. pound arrangement for the upper rays might be avoided bythe ordinary cupola of reflecting prisms; but what has beendescribed, though less perfect, is more compact. The opticalproperty of this instrument is to render parallel the convergingrays of light which fall upon it. In this way the se
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Keywords: ., bookcentury1800, bookdecade1880, bookpublisherlondo, bookyear1881
