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XLI. — Description of a Double Holophote Apparatus for Lighthouses, and 

 of a Method of Introducing the Electric or other Lights. By Sir David 

 Brewster, K.H., D.C.L., F.R S. (Plate XXXVIII.) 



(Read 29th April 1867.) 



In the year 1812 I described an apparatus, by which the light of the sun, or 

 of any luminous body concentrated in the focus of an improved lens, could be 

 returned by reflection from a spherical mirror into the same focus, thus in- 

 creasing the light and heat produced by the cone of refracted rays. The apparatus 

 contained also a double system of lenses and plain mirrors, by which additional 

 beams of light could be concentrated in the same focus. * 



In 1827 I described the very same apparatus as applied in the dioptric system 

 of lights, the whole of the light which issues from a lamp being thrown into one 

 wide and parallel beam, constituting what has been called a holophote, now in use 

 in every part of the world, -f- 



This apparatus consists of three different parts : — 



1st, Of a lens which refracts into a parallel beam of light a cone of rays, 

 which has for its base the surface of the lens, and for its apex the source of light. 



This use of a common lens was made in the lighthouse in the Isle of Portland 

 in 1789. 



2d, Of a spherical mirror placed behind the flame, which throws into the 

 parallel beam a similar cone of rays, whose base is the mirror, and whose apex is 

 the flame. 



3d, Of two systems of lenses and plane mirrors, by which the cones of rays, 

 which would otherwise be lost, are thrown into cylindrical beams, which widen 

 the principal parallel beam. 



If the single holophote has been found so useful in lighthouse illumination, a 

 double or even a triple holophote, in which all the lenses and mirrors may be re- 

 duced in size, must, in particular circumstances, have a peculiar value. 



A double holophote, in which the light of two flames is condensed into a wide 

 beam, is shown in Plate XXXVIII. Fig. 1, where F, F' are the two flames sur- 

 rounded by lenses either of one or more pieces, and by plane and spherical 

 reflectors, which may be made of speculum metal, of prisms, or of glass silvered 

 behind or before. The cone of rays FPS, issuing from the flame F, is converged 



* Edinburgh Encyclopaedia, Art. Burning Instruments. 

 f Edinburgh Transactions, 1827, vol. xi. pp. 55, 56. 



VOL. XXIV. PART III. 8 K 



