. Lighthouse construction and illumination; . Fig. 27. C^ A^ and B C, C^ B^ are placed back to back, so as to point inopposite directions, and a third, M N, is placed within thesecond, but pointing in the same dnection as the this arrangement possesses some advantages, espe-cially in the smaller divergence, due to the greater focaldistance of the mirror M N, as compared with that of theordinary paraboloid, there is still a large amount of Lght (thecone A F A) which wholly escapes interception. 11. Bordier Marccfs Fanal SicUrcd.—In order strictly to equalise a fixed lightover the w
. Lighthouse construction and illumination; . Fig. 27. C^ A^ and B C, C^ B^ are placed back to back, so as to point inopposite directions, and a third, M N, is placed within thesecond, but pointing in the same dnection as the this arrangement possesses some advantages, espe-cially in the smaller divergence, due to the greater focaldistance of the mirror M N, as compared with that of theordinary paraboloid, there is still a large amount of Lght (thecone A F A) which wholly escapes interception. 11. Bordier Marccfs Fanal SicUrcd.—In order strictly to equalise a fixed lightover the whole horizon,Marcet also jDroposedhis very ingeniousfanal sidAral (Fig. 2 8),which is generated bythe revolution of aparabolic jDrofile p pround its parameter asa vertical axis, instead. Fig. 28. 62 LIGHTHOUSE ILLUMINATION. of roimd a horizontal axis as in former reflectors. Thevertices of the parabolas are cut off, so as to permit ofa common focus for the flame. The rays will therefore bereflected by this instrument parallel to the horizontal axis,but in the vertical plane only, while the natural divergenceof the light in azimuth will not be interfered with. Bythis excellent contrivance the light was, for the first time,spread equally round the horizon in one continuous a large portion of the rays in the vertical plane are stillallowed to escape past the lips of the reflector, and this loss takes place all round the circle, eventhough the radiant w^ere reduced toa mathematical point. 12. Mr. IF. F. Barloids Reflector(Fig. 29).—In the London Trans-actions of 1837, Mr. Barlow, ,in order to intercept more lightthan by the common parabola, ^Dro-posed to place in front of thatinstrument and opposite to theflame /, a small spherical mirrorX y z, m as to ca
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Keywords: ., bookcentury1800, bookdecade1880, bookpublisherlondo, bookyear1881
