. Lighthouse construction and illumination; . Fiar. 21. 58 LIGHTHOUSE ILLUMINATION. Fig. 21, that the parabolic mirror a is at best but avery imperfect instrument, for even though the radiant were amathematical point, only about -^-l-th of the light would beintercepted by a mirror of the usual form, and the cone ofrays shown in Fig. 21, escaping past the lips of the mirror,would be therefore lost. It is also to be noted that photometric observations showthat after reflection the rays are not distributed uniformlyover the emerging cone, the density of which rapidly decreasestowards the edges. T
. Lighthouse construction and illumination; . Fiar. 21. 58 LIGHTHOUSE ILLUMINATION. Fig. 21, that the parabolic mirror a is at best but avery imperfect instrument, for even though the radiant were amathematical point, only about -^-l-th of the light would beintercepted by a mirror of the usual form, and the cone ofrays shown in Fig. 21, escaping past the lips of the mirror,would be therefore lost. It is also to be noted that photometric observations showthat after reflection the rays are not distributed uniformlyover the emerging cone, the density of which rapidly decreasestowards the edges. The mode of producing a fixed light on the catoptricsystem was by arranging a number of reflectors o (Fig. 22)aroimd a stationary frame or chandelier n. As abeady. mentioned, an ordinary paraboloid has about 14-| degrees ofdivergence, so that 25 reflectors were needed to light upcontinuously (though, as we have seen, not equally) thewhole horizon. If, again, the light was to be revolving, achandelier having 3 or 4 flat faces was employed (j), Figs. CATOrTEIC SYSTEM. 59 23 and 24), on each of wliich were fixed a certain number of
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