38 Dr BREWSTER on the Construction of Polyzonal Lenses, 



which arises from the difficulty of keeping the segments in their 

 place, yet, when used for lighthouses or burning-instruments, the 

 very purpose to which they are applied, we are confined to dia- 

 meters of a moderate size. Under these circumstances, it may 

 be desirable to introduce into the parallel or convergent beam 

 a greater quantity of light than what passes through the lens. 

 This may be effected by a catadioptric combination of lenses 

 and mirrors, which I described in 1811, and which, when applied 

 to lighthouses, possesses the advantage of throwing into one pa- 

 rallel beam almost every ray of light which diverges from the 

 luminous source. 



For the purpose of applying these, or lenses of any form, 

 to produce powerful effects as burning instruments, I proposed 

 the subsequent combination, under the name of a Burning 

 Sphere. The following is the passage from the Encyclopaedia : 



" In order to construct a burning instrument which shall, in 

 a great measure, be unlimited in its power, we must combine the 

 principles both of reflection and refraction. We are not aware 

 that any instrument of this kind has ever been proposed ; and 

 we are the more surprised at this, as the proper combination of 

 lenses and mirrors must naturally suggest itself to any one who 

 considers the limits which are set to the construction of single 

 lenses, and the disadvantages, either of a theoretical or a practi- 

 cal nature, to which they are liable. 



" The lenses A, B, C, D, E, Plate III. Fig. 5., which may be of 

 any diameter and focal length, are so placed in the spherical sur- 

 face AMN, that their principal foci exactly coincide in the point 

 F. If any of the lenses have a different focal length from the 

 rest, the coincidence of its focus with that of the other may be 

 easily effected, by varying its distance from F. The whole sphe- 

 rical surface, whose section is AMN, except a small opening for 

 admitting the objects to be fused, may be covered with lenses, 



