This apparatus being placed horizontally on a table, the centre was brought within a little distance of the bottom of a vertical prism() of rock salt, so that when the ruler 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 arc, 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 corresponding position of the pile that gives the greatest galvanometric deviation, it was necessary to move the ruler about two lines towards , 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 , 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 immediately 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 substance 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 luminous rays. It constitutes then the true glass of radiant heat, and therefore the only glass that should be employed in appreciating the effects of its intensity. All other transparent bodies are but partial and incomplete 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
