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 diathermanous 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[1].
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 have 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 always easily compared 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 the 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 secondary 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 general 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 so far from its first position as to become illegible. Now, for the water substitute wine or oil or any other diaphanous liquid more or less coloured; the distance 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.
