56 M. MELLONI ON THE IMMEDIATE TRANSMISSION 
This apparatus being placed horizontally on a table, the centre C was 
brought within a little distance of the bottom of a vertical prism(N) of 
rock salt, so that when the ruler CD was properly placed the refracted 
parcel of hot rays fell on all the points of the linear pile. 
By establishing the electric communications with the galvanometer 
and moving the ruler over the graduated are, the point at which the 
deviation of the magnetic index attained its greatest value was easily 
determined. The radiating source was then changed while everything 
else was allowed to remain in the same state. We had now a calorific 
action more or less intense than the preceding; but in order to obtain 
the maximum of effect it was necessary to slide the ruler in one direction 
or the other. Thus, for instance, when I commenced the experiment 
with the incandescent platina, that is, when I had found the correspond- 
ing position of the pile that gives the greatest galvanometric deviation, it 
was necessary to move the ruler about two lines towards B, on the side 
to which the most refrangible rays are directed, if I substituted the 
Locatelli lamp for the platina. But if I substituted for the platina a 
plate of copper heated to 390° I was obliged to slide the ruler three lines 
towards A, in the direction of the less refrangible rays. The action of 
the boiling water in this experiment was too feeble to be compared with 
that of any of the three other sources. 
The refraction and constant transmission of the calorific rays through 
the rock salt being placed beyond the possibility of doubt, we imme- 
diately see the use that may be made of this substance in investigating 
the nature of radiant heat. If, for instance, it is proposed to propagate 
to great distances the action of a heated body of small dimensions, we 
are now certain that we have only to place the body at the focus of a 
lens of rock salt, which will refract the calorific rays and make them form 
a real pharos of heat by issuing in a direction parallel to the axis. Is 
it desired that extremely feeble rays emanating from any source should 
be rendered perceptible? Let them be received on a lens of this sub- 
stance having a thermoscopic body placed in its focus. In this manner 
we may, with the aid of an ordinary differential thermometer with small 
balls, obtain very decided indications of the heat issuing from a vessel 
filled with tepid water and placed at a great distance. In short, rock 
salt formed into lenses and prisms acts upon calorific rays in a manner 
perfectly analogous to that in which optical instruments act upon lumi- 
nous rays. It constitutes then the érwe glass of radiant heat, and there- 
fore the only glass that should be employed in appreciating the effects 
of its intensity. All other transparent bodies are but partial and in- 
complete transmitters of heat, totally intercepting calorific rays of a 
certain kind. It is easy to conceive, from these considerations, with 
what serious disadvantages those persons have had to contend who have 
undertaken to investigate the composition of solar heat with common 
