58 Dr BREWSTER on the Construction of Polyzonal Lenses, 



is refracted by the lenses AB and LL, and thus strengthens 

 every part of the main column of light LRLR. 



The cone of rays FAC, and FED fall upon the lenses AC, 

 and BD, and are refracted into parallel beams, which are thrown 

 into horizontal directions a R b R, jfR e R, by the plane mirrors 

 a 6, ef. In like manner, the cones FCG, FDE are thrown into 

 the parallel beams cR </R, ^Rg-R. The cone of rays FGM be- 

 ing reflected back to F by the mirror GM, will pass through the 

 lens BD, and strengthen the beam/R e R, as if it had radiated 

 from F, and in the same way, the cone FNE, reflected by NE, will 

 add to the intensity of the beam a R b R. All the other mirrors 

 and lenses not seen in the section, will, in like manner, refract 

 and reflect the light which falls upon them into horizontal beams, 

 so that the main column LRLR will be surrounded on all sides 

 with a concentric cylinder of light. The beam might be still 

 farther widened by another zone of lenses, and another set of 

 mirrors, which would throw the cones FGM, FEN into a hori- 

 zontal line, but it is decidedly preferable to throw that light into 

 the beams a R b R, and/R e R. 



By the construction now described, we have obviously the 

 power of throwing into one horizontal beam all the sphere of light 

 which radiates from a luminous source, with the exception of 

 what falls between the lenses, which cannot amount to two-tenths 

 of the sphere. In parabolic reflectors only six-tenths of the 

 sphere of light falls upon the reflecting surface, so that the com- 

 bination of lenses and mirrors, has, in this respect, a remarkable 

 superiority, arising from the luminous focus being actually en- 

 veloped by the refracting and reflecting surfaces. 



The allowance of two-tenths of the whole sphere of light for 

 what is lost between the lenses, is sufficiently large ; but it may 

 be reduced even to one-tenth, if, instead of making the lenses 

 circular, we form them into a real zone, each lens, placed on the 

 surface of the sphere, being comprehended between two paral- 

 lels of longitude and two parallels of latitude. In this way the 



