If we suppose the action of sulphuric acid analogous to that of water, but not so energetic, we shall see the reason why, with the prism of acid, the maximum takes place in the orange. In short, the very glass of which the common prisms are made must operate in a similar manner, and cause in each ray a loss inversely proportioned to its refrangibility. Therefore, if we employed in the construction of the common prism a substance less active than common glass, the losses sustained by the less refrangible rays would be diminished in a greater ratio; so that they would gain on the more refrangible rays, and the maximum would pass in a direction opposite to the preceding, that is, fr»m the violet to the red.
This is exactly the result obtained by Herschel, Englefield, and Seebeck by operating on prisms of flint glass; for the maximum was transferred to the obscure space quite close to the last red stripe of the spectrum.
Let us compare these effects with the numbers which represent the calorific transmissions. We shall find that the maximum of heat, in passing from the yellow, where it is found when we use a prism of water, departs from it always in the same direction in proportion as the substances of the prisms substituted for the water are more diathermanous. It passes a little out of the spectrum when, instead of crown, we employ flint glass. Admitting then the correctness of such a theory, the line of greatest heat must pass quite beyond the colours into a space far distant from the red limit if we employ rock salt, a substance possessing a far greater diathermancy as compared with flint glass than flint glass does as compared with crown. I tried the experiment; it was completely successful. I found that the maximum of temperature in the spectrum derived from the prism of salt was thrown into the dark space as far at least from the last band as the blue is (in an opposite direction) from the red. At the moment I cannot assign more exact measures; for in the first place I operated with very small prisms, and when I subsequently obtained larger pieces the season did not allow me to reconsider and study the result more nicely. But the effect has been so marked in the experiment which I made, and so invariable in several successive repetitions, that I look upon it as decisive, and have not the least doubt as to the removal of the maximum of temperature to the last band of the red rays in the spectrum produced with rock salt[1].
The distribution of the degrees of temperature in the solar spectrum
- ↑ I have since obtained the same results with five prisms of rock salt whose angles of refraction vary between 30° and 70°. These prisms have been made out of several pieces taken from the mines of Cordona, Wieliecza, and Vicq: they have been cut in different directions relatively to the axis of crystallization. I shall give the numerical data in a work in which it is intended to treat specially of the analysis of the caloric solar rays.
