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 have satisfied myself by the following experiments. Having 
by means of a heliostat’ ititroduced 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 sereen. I couerived 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 colotr 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 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 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 
prismatie colours and producing very feeble seeondary 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, J] 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 
gure 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 substi- 
tute wine or oil or any other diaphanous liquid morejor.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. 
Dye, 
