36 >f- MELLOXr ON THE FREE TRANSMISSION' 



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 compa- 

 rison 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 im- 

 perceptible. 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 trans- 

 mit a quantity of red rays less than that which passes through the red 

 glass. 



It is knovin that in the solar spectrum produced by a prism of com- 

 mon glass, the greatest heat is found in the red, and that the interme- 

 diate temperatures continually decrease until we come to the violet. Does 

 this calorific distribution in the coloured rays, separated by the refract- 

 ing power of the prism, exist also when they are separated by the ab- 

 sorptive power of the colouring matter? 



In order to ascertain this we have only to compare, at the diflerent 

 temperatures of the spectrum, the numbers which represent the calo- 

 rific transmissions of our five coloured glasses ; they are as follows : 



violet red ^^^^llow blue green 



S3, 47^^^^ 34, 33, 26. 



The order of the ^rours 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 



