. Lighthouse construction and illumination; . erthelessrequire condensing agents, and will therefore most appro-priately be described here. 1. Figs. 94 and 95 show straight refracting or reflectingprisms, which revolve and intercept certain of the rays froma central fixed light apparatus, so as to produce perfectdarkness over the sectors they subtend, while they spread therays which they intercept, uniformly over, and thus strengthenthe intermediate sectors, which are illuminated directly bythe central apparatus. The peculiar property of this arrange-ment is that the power is increased in prop
. Lighthouse construction and illumination; . erthelessrequire condensing agents, and will therefore most appro-priately be described here. 1. Figs. 94 and 95 show straight refracting or reflectingprisms, which revolve and intercept certain of the rays froma central fixed light apparatus, so as to produce perfectdarkness over the sectors they subtend, while they spread therays which they intercept, uniformly over, and thus strengthenthe intermediate sectors, which are illuminated directly bythe central apparatus. The peculiar property of this arrange-ment is that the power is increased in proportion to theduration of the intervening periods of darkness. Thus,neglecting the loss by absorption, etc., the power is doubledwhen the periods of light and darkness are equal, trebledwhen the dark periods are hvice as long as the light, andso on in proportion, while in every case the rays arespread uniformly over each illuminated sector. The con- CONDENSING INTERMITTENT LIGHTS. 121 densing intermittent apparatus which was first proposed in. f4 \\ A z: ;-T7 Xll. Fig. 94. Plan Fig. 95. Elevation 1872,^ was not employed till 1878 at Barrahead, in theHebrides, The Cabot Island Hght, in Newfoundland, is alsoon the same principle, and it is reported that it affordsgreat satisfaction. The apparatus for these lights was verycorrectly made by Messrs. Barbier and Fenestre, who ex-hibited a duplicate at the late Paris International Exhibition,for which they received a medal; and, though not myselfan exhibitor, I was also awarded a gold medal. , Lemonnier, et Cie., published a description ofapparatus on the same principle in 1879, and they haveintroduced it at several places. 2. Condensing Intermittent Lights of unequal periods.—Plate XIII. shows an example of the apparatus where the con-tinuity of the light is broken up into long and short periods ^ Royal Scot. Society of Arts, vol. viii. 122 LIGHTHOUSE ILLUMIXATIOX. of darkness alternately; and Plate XIV. shows an
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