$32 M. MELLONI ON THE POLARIZATION OF HEAT. 
nary heat; the rest pursues its way without losing its radiating state, is 
afterwards dispersed, and falls upon a second lens with a shorter focus, 
placed beyond the first at a distance precisely equal to its own principal 
focal distance. The rays received by this second lens in a state of di- 
vergence are parallel to each other when they leave it; and form a pencil 
of condensed heat, which enters the thermoscopic case, and finally 
reaches the pile, which stands at a suitable distance from the aperture. 
The section of the pencil being a little less than that of the pile, all its 
parts concur in the production of the thermoscopic effect, and thus we 
lose the calorific effect of none of the rays that issue from the polar- 
izing system. 
It is very important to observe, that the common centre of the two 
superposed plates of tourmaline is not placed exactly in the common 
focus of the two conjugate lenses, but a little nearer to the second, in 
order that the portion of heat absorbed by those plates, and radiated on 
the second lens, may be necessarily refracted in diverging rays whose 
action becomes weaker, and is completely destroyed at a short distance, 
without influencing the thermoscopic body, which is thus affected only 
by the heat arising from direct transmission. That this condition is 
actually fulfilled may be ascertained by blackening the tourmalines, or 
by substituting for them any other plates well covered with lampblack ; 
for the index of the galvanometer reassumes its natural position in equi- 
librio, and retains it whether the communication with the calorific 
source be established or intercepted. 
By this simple contrivance we cause very minute plates of tourmaline 
to transmit a bundle of rays almost as broad as the surface of the first 
lens, and then bring all the emergent rays, and these alone, pure, and 
unmixed with even the smallest particle of the caloric derived from the 
heating of the plates, to produce their effect on the thermoscope. 
Combining a lens of two inches and a half in diameter and three 
inches in the focus, with a lens of fourteen lines, I obtain a quantity of 
rays emerging from the tourmalines, such as, in several instances, pro- 
duces a deviation of the needles, amounting to between 60° and 80°, at 
the distance of a metre from the small flame of a Locatelli lamp fur- 
nished with a reflector. This energetic action, though necessary for 
the experiments which I had in view, is then too great; but there 
is an easy way of reducing it as much as we wish: we have only to 
render the rays more or less divergent by a proper approximation of the 
lenses. 
The plates of tourmaline are adjusted to the central part of two pa- 
rallel covers of cork filling the interior of a round box, which is suffi- 
ciently shallow, has a small circular aperture at the centre, and is sup- 
ported at the proper height by a metallic screen with a similar aperture. 
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