. Lighthouse construction and illumination; . Fig. 87. for Lamlash, near the island of Arran, in the Firth of Clyde,constructed in 1876, in which the twin prisms (6, 3) werefirst applied. Their action will be easily understood withoutfurther description, by the numbers shown on the diagram. 8. Fixed Condensing Lights for more thaj^ One SectorOF Unequal Eange. 1. Isle Oronsay Apparahis. (Lighted in 1857.) —Isle Oronsay is situate in the narrow Sound of Skye, andthroughout nearly the whole of the illuminated arc of 167°,it does not requu-e to be seen at a greater distance than threeor four miles
. Lighthouse construction and illumination; . Fig. 87. for Lamlash, near the island of Arran, in the Firth of Clyde,constructed in 1876, in which the twin prisms (6, 3) werefirst applied. Their action will be easily understood withoutfurther description, by the numbers shown on the diagram. 8. Fixed Condensing Lights for more thaj^ One SectorOF Unequal Eange. 1. Isle Oronsay Apparahis. (Lighted in 1857.) —Isle Oronsay is situate in the narrow Sound of Skye, andthroughout nearly the whole of the illuminated arc of 167°,it does not requu-e to be seen at a greater distance than threeor four miles, while in one direction (down the Sound to-wards A and B, Fig. 88), it can be seen for about fifteen FIXED CONDENSING LIGHTS. 113 miles, and in another up the Sound (towards C and D), itcan be seen for about seven Fig. 88. The apparatus shown in horizontal section (Fig. 89) wasdesigned in order to reinforce the ordinary fixed light apparatusover the two sectors of greater range. This is effected by dis-tributing in those directions the spare light of 193°, whichwould otherwise have been lost on the landward side, or, ifreturned through the focus in the ordinary manner by meansof a spherical mirror, would have equally strengthened thatportion of the light which is already sufficiently formula for condensing the light unequally for differentarcs and distances has already been given (4). But themagnitude of the apparatus on which the visual angledepends, forms an element of some importance, especiallyin narrow seas, such as the Sound of Sleat. It is obvious,however, that the influence of this element must be circum-scribed within certain limits. The Oronsay light was there-fore, after due consideration, allocated nearly in the arith-metical ratios of the distances, and some such allo
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