168 Dr Brewster on the Construction of Polyzonal Lenses 
adopted in the great burning lens of Parker, 32 inches in dia- 
meter ; but in the polyzonal burning lenses we may avail our- 
selves of the second lens with peculiar effect, by giving it the 
form of a meniscus (See Fig. 6.), which, though bounded by 
spherical surfaces, has the singular property of converging homo- 
geneous rays to a mathematical point, provided the distance of 
the focus of the great burning glass from the centre of the first 
surface of the small lens is to the radius of the first surface as 
the Sine of the angle of incidence is to the Sine of the angle of 
refraction. The aberration of the principal lens being almost 
removed by its polyzonal construction, and the second glass 
entirely free of that source of error, we may expect from such 
an apparatus effects of the most powerful kind 
The munificence of Sovereigns and of public Societies, has 
been so frequently displayed during the last century, in the con- 
struction of instruments, beyond the reach of individual zeal, 
that we cannot omit the present opportunity of suggesting the 
formation of a National Burning Apparatus, to the construction 
of which all the scientific institutions in the kingdom might con- 
tribute. Each society, for example, might undertake the for- 
mation of a single zone of the national lens, which zone would 
of itself be an useful burning glass, and might be afterwards 
completed when the society’s funds enable it to do so. These 
numerous zones might then be combined for the purpose of oc- 
casional experiments, under the direction of our most distin- 
guished philosophers, and we cannot doubt but that discoveries 
would thus be made which would form an epoch in the history 
of the arts and sciences. The fusion and combination of refrac- 
tory materials could not fail to produce the most interesting com- 
pounds, and create processes in the useful arts of which we can 
at present form no conception. The reduction to fluidity of simple 
and compound minerals, and the phenomena exhibited by their 
union, and by their slow cooling, would throw the clearest light 
on many of the most perplexing questions in geology, while 
every branch of general physics would derive new resources 
from the energy of such an irresistible agent. 
The expence of a polyzonal apparatus five ox six feet in dia- 
meter, even in the more compound form of the Burning Sphere , 
• f } — j ) ; — j — ! — ' 
m The meniscus may form oiie side of a glass sphere for holding the product 
combustion, as shewn in Fig. 6. 
