DOUBLE HOLOPHOTE APPARATUS FOR LIGHTHOUSES, ETC. 637 



other cones of rays which issue from the new flames producing hollow cylindrical 

 beams, surrounding those shown in the figure. 



If we wish to have a beam of great intensity, and of equal brightness through- 

 out, the holophote principle is inapplicable ; but in dispensing with the secondary 

 lenses and the lateral reflectors, we obtain a less complex and more manageable 

 apparatus, to which the name of Cratophote may be given, from the great inten- 

 sity of the light which it is capable of producing. 



This instrument is shown in Fig. 2, where M, N, 0, P, Q are five flames, the 

 light of which we wish to concentrate into one beam RR, MN, a concave mirror, 

 and AA, BB, CC, DD, EE, &c, five, or any other number, lenses. The lenses 

 BB, CC, DD, EE have the same focal length, but the focal length of AA is only one- 

 half that of the rest. The light of the first flame M, or the cone of rays MAA, 

 is refracted into the parallel beam RR. The light of the second flame N, or the 

 cone of rays NBB, is refracted by the lens BB into its conjugate focus M, and, 

 passing through it, falls upon the first lens AB, and is thrown by it into the 

 beam RR. 



In like manner all the other flames or cones of rays, OCC, PDD, and QEE. 

 are converged by the lenses which intercept them to their conjugate foci, and 

 finally enter the beam RR, into which the light of the five cones of rays is con- 

 centrated. 



By placing a concave mirror RS in the instrument, so as to reflect the cone 

 of rays QRS to Q, they will pass through all the other lenses into the beam RR. 

 In like manner the other cones of rays PEE, ODD, NCC, MBB will be converged 

 to the conjugate foci of the lenses which intercept them, and fall upon the mirror 

 RS, which will send them back by the same process into the beam RR. 



The intensity of the beam RR may be increased by diminishing the diameter 

 and the focal length of the lens AB, which, when placed nearer N, will refract 

 the cone of rays MAA into a beam of the same diameter as the lens. 



The intense beam of light produced by this apparatus is obviously very dif- 

 ferent in its character from the compound beam obtained from the single or the 

 double holophote. In the single holophote the beam consists of the central beam 

 RR, Fig. 1, and of the hollow cylindrical beam p'"p'", which, being less intense, 

 will not be seen at such a distance as RR. In the double holophote the beam 

 consists of the central beam RR, and of three cylindrical beams, each of which has 

 a different intensity, and will, therefore, cease to be seen at different distances. 

 It might, therefore, be desirable that each part of a lighthouse beam should have 

 the same intensity, and, consequently, the same penetrating power as in the 

 catoptric system, where the beam is produced by several parabolic reflectors of 

 the same size, placed close to each other. In the dioptric system we can only 

 approximate to this, and that very imperfectly, by combining several small holo- 

 photes, and thus producing a wide beam, in which we have several central beams, 



