40 Dr BREWSTER on the Construction of Polyzonal Lenses, 



"2. In the solid lens, a great diminution of light arises from 

 the thickness of the central portions, and from the obliquity of 

 the parts at the circumference ; which, we conceive, will be fully 

 equal to the light lost by reflection in the burning sphere. 



" 3. In the burning sphere, the lenses may be obtained of 

 much purer glass than can be got for a solid lens ; and therefore, 

 c&teris paribus, they will transmit more light. 



" 4. Owing to the small size of each lens in the burning 

 sphere, the diminution of effect arising both from spherical aber- 

 ration, and from the aberration of colour, will be very much less 

 than in the solid lens. 



" 5. In the burning sphere, the effect is greatly increased, in 

 consequence of the shortness of the focal length of each lens, 

 and the greater concentration of the incident light. 



" 6. In the burning sphere, all kinds of lenses may be com- 

 bined. They may be made of any kind of glass, of any diame- 

 ter, and of any focal length ; and the lenses belonging to different 

 individuals may be combined for any occasional experiments in 

 which a great intensity of heat is requisite" 



To those who are acquainted with the laws of the distribution 

 of light which passes through lenses, or which falls upon reflec- 

 tors, it is scarcely necessary to say, that the very same appara- 

 tus which is best fitted for producing combustion from the solar 

 rays, is also best fitted for producing the column of illumina- 

 tion in lighthouses. The only difference between the two ope- 

 rations is, that, in the one case, the parallel rays of the sun pass 

 through the lens, and are refracted to its focus ; while, in the 

 other case, the rays pass from the focus, and are refracted by the 

 lens into a parallel beam. Hence, the Polyzonal Lens, and the 

 Burning Sphere above described, are peculiarly applicable for 

 the illuminating apparatus of lighthouses. This application of 

 these contrivances early presented itself to me ; and some time 

 between 1818 and 1820, I was in communication with Mr STE- 



