Page:Scientific Memoirs, Vol. 1 (1837).djvu/111

Rh of smoke result from the more or less thorough blending of the blond and the tawny; those of fire from Nos. 8, 9, and 10; the deep blue is produced by the Nos. 10, 11, 12, which are the deepest tints of the scale.

The first blond is properly that of light hair in childhood, and it is a fact worthy of remark, that as children grow older it becomes progressively deeper and deeper, in the order of the Nos. 2, 3, 4 in the scale. The perfect resemblance of the first tints on the scale to those which we observe about the moon when she is surrounded by clouds is equally remarkable: it seems in fact that this luminous appearance may be thus definitively explained. Tints of this kind do not arise from refraction and diffraction; they are produced only by means of thin plates: the luminous halo in question is therefore a phænomenon produced by thin plates.

This observation, combined with the fact that the tints exhibited by the clouds in every variety of aspect are almost all comprised in the first ring, leads to another consequence relative to the constitution of vesicular vapours. The measurements and experiments of Newton have shown what are the dimensions of the layers of air, of water, and of glass, which produce the colours of the several rings. The red of No. 10 is the last tint of the first ring: the indigo (No. 12) belongs to the second, and the thickness of the layer of water which produces it by reflexion is about the ten-millionth part of an English inch. As we know then, on the one hand, that the vesicular vapours are formed of water, and on the other that they do not reflect or transmit any tint beyond No. 12, we may conclude that their external film is in no case thicker than the ten-millionth part of an inch.

This result appears to me so decidedly certain as to be entitled to a place in science.

Second Ring. — From No. 11 to No. 28 (inclusive).

This interval commences at the deep violet No. 11, and extends to the lake-red No. 28. It comprises the most beautiful of all the gradations; namely,

Blue, Azure, Yellow, Orange, Red.

Newton places a green tint between the azure and the yellow. My scale exhibits no trace of green, and, with whatever attention I have examined Newton's second ring, I have never been able to perceive, in the place where the green should be found, anything but white tinged with azure and answering to the Nos. 15, 16, 17 of my scale. It is true that in the solar spectrum we meet with green in passing from the blue to the yellow: but the colours of the prism are simple, those of the thin plates are compound, and the order of their succession