. Lighthouse construction and illumination; . tportion 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, a
. Lighthouse construction and illumination; . tportion 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 allocationappears warranted from nautical considerations connectedwith the locality. Moreover, there are difficulties in con-struction, and also in connection with the amount of available I 114 LIGHTHOUSE ILLUMINATIOX. space in the lantern which had to be taken into arc down the Sound could not have been made smallenough without cutting through the central bulls eyeof the lens, and this would not have been Fig. 89. In Fig. 8 9, A is a sector of 16 7° of a fixed light sector subtends the entire arc A C, in Fig. 88, whichis all that has to be illuminated, so that the rest of thelight on the landward side, amounting to 193°, is sparelight, of which 129° are parallelised by the portion of theholophotal apparatus B, and after falling on a series of twelveequal and similar straight prisms «, are again refracted, but inthe horizontal plane only; and lastly, after passing througha focal plane (independent for each prism), they emergein a series of 12 equal wedges, each having a horizontal FIXED CONDENSING LIGHTS. 115 divergence of about 10°, and in all of wliicli the rays arerespectively parallel to those of the directly diverging sectora, from the main apparatus, which is therefore strength-ened. Each of these supplemental prisms spreads its lightover the whole arc A B (Fig. 88), and the portion a ofthe main apparatus does the s
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Keywords: ., bookcentury1800, bookdecade1880, bookpublisherlondo, bookyear1881
