OF RADIANT HEAT THROUGH DIFFERENT BODIES. 35 



1 to infinity when we consider the effects produced by sources of a low 

 temperature. 



Hitherto we have made no account of the colours [of the diather- 

 manous bodies], or, rather, have considered them only in relation to the 

 diminution of transparency, or to the greater or less opacity which they 

 always cause in diaphanous substances*. 



We must now examine them more particularly, and determine their 

 influence on transmission. Such is the object of the fourth table. The 

 tints of those kinds of glass marked with an asterisk are the purest, and 

 approach nearest to those prismatic colours that bear the same names. 

 Of this I ha^'e satisfied myself by the following experiments. Having 

 by means of a heliostat introduced a horizontal sheaf of solar rays 

 into a dark chamber, I divided it into two by causing it to pass through 

 two apertures made in an opake screen. I contrived to make one of 

 the sheaves fall on a vertical prism, and the other on a coloured glass 

 which I wished to try. Thus the solar spectrum was seen cast on one side, 

 and a coloured spot in the line of the direct rays. To bring this spot into 

 contiguity with the corresponding colour of the spectrum, I placed behind 

 the glass a second vertical prism which turned about until the desired 

 effect was obtained. The two analogous tints are alwaj's easily com- 

 pared when they are near each other, and at the same time we are able 

 to judge whether the colour of the glass be more or less pure by the 

 new tints which are always developed in tlie passage of the coloured rays 

 of the glass through the prism. Of fourteen colours selected from several 

 species of glass, I have found but five making any near approach to the 

 prismatic colours and producing very feeble secondaiy tints. These tints 

 were absolutely imperceptible only in the case of red glass. 



There is another mode (and it has not been overlooked) of appretia- 



* I was lately told by an eminent philosopher, that to think of comparing the 

 intensities of different colours would be as absurd as it would be to institute a 

 comparison between heterogeneous elements. Waiving all inquiry as to the 

 correctness of such an assertion, I beg leave to remark that in certain cases 

 it is unanimously agreed that a tint is more or less clear than another tint of a 

 different kind, without giving rise to any metaphysical ideas opposed to the ge- 

 neral opinion. Let us take, for instance, the solar spectrum. Has it not been 

 always held that the maximum of brightness is to be found in the yellow, and 

 that on each side of it luminous intensity decreases ? The principle put forward 

 by me seems equally plain. When I assert that colours always introduce some 

 opacity into diaphanous bodies, no one is at a loss for my meaning. Put some 

 pure water between two parallel plates of colourless glass : let an observer be 

 placed at one side, and at the other a piece of writing, which is to be moved just 

 80 far from its first position as to become illegible. Now, for the water substi- 

 tute wine or oil or any other diaphanous liquid 7nore or less coloured ; the di- 

 stance at which the writing may be read will become less in proportion to the 

 greater depth of the colour independently of its kind. Thus when the writing 

 will be legible at the same distance through a yellow and a red liquid, these two 

 media will, in respect to us, be equally transparent. 



D 2 



