far the purposes of Illumination in Lighthouses. 51 



In consequence of the weakness of the column of light, Red 

 is the only colour which has been used for distinguishing 

 lights ; but when the column of light is rendered strong by an 

 improved system of illumination, several other colours may be 

 used with great effect, and the power of varying the lights may 

 be thus widely extended. 



The only advantage which parabolic reflectors possess, as a 

 compensation for their numerous defects, is, that they receive a 

 very large part of the sphere of light which radiates from the 

 burner ; but this advantage is more nominal than real, for we 

 shall afterwards see that a smaller portion of the sphere well 

 reflected, or well refracted, into a parallel beam, will produce 

 a much more useful effect. 



* 



If any partiality for reflectors should still exist, they ought to 

 be made much larger, and should be built up of separate zones 

 and segments, like the polyzonal lenses *. The material should 

 be speculum metal, ground and polished upon pitch. The cen- 

 tral portion should be a spherical mirror of considerable radius, 

 and the other zones might be ground with annular surfaces, so ad- 

 justed as to afford a parallel beam of light. As such reflectors, 

 however, would still possess several of the inconveniences of the 

 present system, we shall content ourselves with merely allud- 

 ing to them, and shall proceed to the description of the New 

 Lenses. 



* That the reflectors for lighthouses are considered by competent judges to re- 

 quire improvement, appears from the following passages : " It is greatly to be de- 

 sired," says the Editor of the Bibliotheque Universelle for July 1826, " that the 

 perfection to which optical instruments have been brought, should be extended to 

 that branch of the science which has for its object the illumination of lighthouses." 



" From certain experiments now in progress," says Mr STEVENSON, " the 

 writer is in expectation that considerable improvements may be introduced in the 

 construction of reflectors ; and that additional modes of distinguishing the lighthouses 

 on the coast will be obtained." Account of the Bell Bock Lighthouse, p. 527. 



G 2 



