164 Dr Brewster on the Construction of Polyzonal Lenses 
F, and produce a degree of heat fully one-half of what was pro 
duced by the direct refracted rays of the sun through the lens 
A. A similar effect will be produced by the mirror RS and 
lens D, the mirror TU and lens C, the mirror V W and lens E, 
and by all the other mirrors and lenses’ which are not seen in the 
section. The effect may be still farther increased by the addi- 
tion of a large lens at X, X. As the angle which the surface of 
each mirror forms with the axis of its corresponding lens is a 
constant quantity, the mirrors may be all fixed to the general 
frame of the sphere, and therefore the only adjustment which 
the instrument will require, is to keep the axis of the lens A 
parallel to the direction of the solar rays. 
cfi In order to estimate the advantages of this construction, 
let us compare its effects with those of a solid lens, which ex- 
poses the same area of glass to the incident rays. 
66 1. In the burning sphere, almost the only diminution of 
light is that which arises from reflection by the plane mirrors, 
and which may be estimated pretty accurately at one-half of the 
incident light ; but this loss can be amply compensated by add- 
ing a few more lenses. 
“ % 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. 
<c 3* In the burning sphere, the lenses may be obtained of 
much purer glass than can be got for a solid lens ; and there- 
fore, c&teris paribus , they will transmit more light. 
66 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. 
66 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 dif- 
ferent individuals may be combined for any occasional experi- 
ment , in which a great intensity of heat is requisite” 
