. Lighthouse construction and illumination; . ^ Mr. A. Stevenson also extended the helical principle to the astragals ofthe lantern in a design for Start Point, in Orkney, in 1846, but it Avas notadopted. Mr. Douglass, independently, made a similar proposal in 1864, andwas the first to publish it and introduce it into lighthouses. Mr. Douglassslantern will be described subsequently, and is shown on Plate XII, 76 LIGHTHOUSE ILLUMINATION. 23. Mr. A. GordonsCatadioptric Reflector (Civ, Eng. Jour.,1847).—The next improvement was d for increasing the power of the para-bolic reflector dioptrically (
. Lighthouse construction and illumination; . ^ Mr. A. Stevenson also extended the helical principle to the astragals ofthe lantern in a design for Start Point, in Orkney, in 1846, but it Avas notadopted. Mr. Douglass, independently, made a similar proposal in 1864, andwas the first to publish it and introduce it into lighthouses. Mr. Douglassslantern will be described subsequently, and is shown on Plate XII, 76 LIGHTHOUSE ILLUMINATION. 23. Mr. A. GordonsCatadioptric Reflector (Civ, Eng. Jour.,1847).—The next improvement was d for increasing the power of the para-bolic reflector dioptrically (Figs. 47, ^c.^;;^. IMPROVEMENTS ON FRESNELS REVOLVING LIGHT. 77 48). Mr. Gordon placed in front of a paraboloid, a h c, fourof the outer rings of Fresnels annular lens, cl e g h, for thepurpose of intercepting some of the rays which escape theaction of the reflector, wliile the beam from the paraboloiditself passed through the central void, e g, between the lenti-cular rings. Though so far a step in the right direction, thedefects are very obvious,— 1st, The escape of the cone of rayse f g through the central void. 2d, The extremely short focaldistance of the reflector, which was found on trial to pro-duce a cone having a divergence of no less than 60°; and3d, The loss by metallic reflection from the paraboloid. 24. Lconor Fresnels improved Revolving Light.—M. pulDlished an improvement on his brothers light,in his Phares etFanaux des Cotesde France, Paris,1842, by enlarg-ing and altering theposition of the tra-pezoidal inclinedlenses (Fig. 49), and thus increas- ?! Fig. 49. ;#- ing their
Size: 1212px × 2062px
Photo credit: © Reading Room 2020 / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1800, bookdecade1880, bookpublisherlondo, bookyear1881
