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FOR THE SCIENCE OF CALORIFIC RADIATIONS. 531 
colour of painting, and cannot have, for our purpose, the same 
propriety of expression as the word chroa, colour of light, which 
__ is precisely the phenomenon to which allusion is made in the 
_ comparison in which we are engaged. But I will reply still more 
_ directly to the question :—First, I shall observe that it is possible 
to define coloration otherwise than by the immediate testimony 
of sight; in fact the coloured rays are not distinguished from 
one another by the mere difference of the sensations produced 
_ on our eyes, but by differences of strength between the modifi- 
s ' cations of which these rays are susceptible under the action of 
bodies: it is thus that the red rays are less refracted than the 
green, that they are sent back or transmitted by red bodies in 
" greater quantity than the green rays, or vice versd relatively to 
green bodies. Moreover, in certain cases these differences con- 
stitute the only distinctive characters of luminous rays. We 
_know, for example, that certain individuals are insensible to red 
light, or to speak more exactly, we know that red light seems 
to them a colour perfectly similar to that of green light; the red 
_and green rays can then only be distinguished in the eyes of 
these individuals by the differences of diffusion, of absorption, 
:% and of transmission, as just alluded to. Thus, when illumi- 
_ nating a room successively by a light transmitted first by a red 
glass and then by a green glass, and presenting in both cases 
a green cloth anda red cloth to the person who confounds 
the two colours together, he will immediately comprehend that 
the two kinds of light introduced in the ambient space, although 
_ perfectly alike to his eyes when they receive the ordinary light 
_ of day, are scarcely identical, since the red cloth, which is very 
bright when the room is lit by the red light, becomes sombre 
when the ambient space is illuminated by the green light; and 
that, on the contrary, the green cloth, which took a dark tint in 
the first experiment, appears very bright in the second. Analo- 
zous proofs might be had by means of two coloured media, the 
one green and the other red, which would furnish a strong and 
Q feeble transmission, first of one kind, then of the opposite 
= 
ipht. But calorific radiations give pr ecisely differential cha- 
racters of the same kind; the expression colour of heat, then, 
has in it nothing which should offend, and it even satisfies ever 4 
principle of the most rigorous logic. 
_ The order adopted for the study of light, and the classification 
