for Light-Houses and for Burning Glasses. 163 
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 
practical nature, to which they are liable. 
“ This instrument, which has been proposed by Dr Brewster, 
and which may properly be called a burning sphere, from the 
arrangement of the lenses, is represented in Plate IV. Fig. 5. 
which is merely a .section of the sphere, and represents only five 
of the lenses, and four of the mirrors. The lenses A, B, C, D, 
E, which may be of any diameter and focal length, are so placed 
in the spherical surface 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 spherical surface, whose section is AMN, ex- 
cept a small opening for admitting the objects to be fused, may 
be covered with lenses, having all their foci coincident at F ; 
though it will, perhaps, be more convenient to have the poste- 
rior part MN without lenses, and occupied by a mirror of near- 
ly the same radius FA as the sphere. The object of this mir- 
ror, is to throw back upon the object at F the light that passes 
by it without producing any effect. Each of the lenses, except the 
lens A, is furnished with a plane glass mirror, which may be either 
fixed to the general frame of the sphere, or placed upon a separate 
stand. When this combination is completed, the sphere is ex- 
posed to the sun, so that its rays may fall at right angles upon the 
lens A, which will, of course, concentrate them at F, and pro- 
duce a pretty intense heat. The plane mirror PQ, when pro- 
perly adjusted, will reflect the sun’s light perpendicularly upon 
the lens B, by which it will be refracted accurately to the focus 
l 2 
