ting the influence of colour in diaphanous media. It consists in causing corresponding rays of the spectrum to pass through the glasses. The passage is attended only with a very inconsiderable loss when the tints are very pure. Now by fixing one side of my five plates of glass at proper distances on the margin of a sheet of pasteboard exposed to the coloured sheaf of the prism, I found that each prismatic ray traversed glass of the same colour without suffering any loss of intensity. At least, the alteration produced by these glasses on the corresponding solar rays was nearly the same in all cases. This fact is inferred from a comparison of the prismatic rays which fall on the wall directly and those which reach it after having passed through the coloured pieces of glass. The shadows brought by the latter rays are so very light as to be almost imperceptible. In every other case they are very strongly marked. If for instance we substitute the violet for the red, the spot on the wall is almost dark; if the violet be not perfectly pure, it will not at least transmit a quantity of red rays less than that which passes through the red glass.
It is known that in the solar spectrum produced by a prism of common glass, the greatest heat is found in the red, and that the intermediate temperatures continually decrease until we come to the violet. Does this calorific distribution in the coloured rays, separated by the refracting power of the prism, exist also when they are separated by the absorptive power of the colouring matter?
In order to ascertain this we have only to compare, at the different temperatures of the spectrum, the numbers which represent the calorific transmissions of our five coloured glasses; they are as follows:
| violet | red | yellow | blue | green |
| 53, | 47, | 34, | 33, | 26. |
The order of the colours considered relatively to their degrees of heat and the numerical relations of those degrees are so altered that the violet light, which in the spectrum possesses a temperature twenty-five or thirty times lower than that of the red light, appears here of a higher temperature. Such a difference is not to be explained by supposing that, in the transmission of the violet glass, there passes a great quantity of red rays; for it should, on this hypothesis, be found to transmit them in a greater proportion than they are transmitted by the red glass; which, according to the preceding experiments, is impossible.
These facts seem to be opposed to the opinion of those philosophers who hold that in luminous heat the same rays simultaneously excite the two sensations of light and heat, but would be easily comprehended if we supposed caloric and light to be two distinct agents. In the latter case we should say that in the prism the refractive force acts unequally on the different caloric rays, as it does, in a greater or less degree, on
