Experiments on the Refrangibility of the invisible Rays of the Sun. By William Herschel, LL.D. F.R.S. Read April 24, 1800. (Phil. Trans. 1800, p. 284.]
In a paper read to the Society at a former meeting, the Doctor announced some observations which seemed to indicate that there are two sorts of rays proceeding from the sun; the one the calorific rays, which are luminous and refrangible into a variegated spectrum; and the other the invisible rays, which produce no illumination, but create a sensible degree of heat, and appear to have a greater range of refrangibility than the colorific rays. To the latter he assigns the name of radiant heat. Having lately had some favourable opportunities to prosecute this investigation, he here delivers an account of the series of experiments he made on the subject, which seem to him to confirm the above conjecture. The mode of conducting these experiments was simply this:—
On a horizontal tablet covered with white paper, and divided into squares, for the conveniency of measurement, a part of the extreme colour of a prismatic spectrum was suffered to fall, the remainder of the coloured rays passing by the edge of the tablet, so as not to interfere with the experiment.
Three thermometers were placed on the tablet, at different distances from the termination of coloured rays. The general results of the ten experiments here described were as follows:—
From the four first it appears, that there actually are rays coming from the sun which are less refrangible than any of those which affect the sight; that they are invested with a high power of heating bodies, but with none of illuminating objects, which probably is the reason why they have hitherto escaped unnoticed.
The fifth and sixth experiments showed that the power of heating is extended, though in a feeble degree, to the utmost limits of the most refrangible or visible purple rays, but not beyond them ; and that it is gradually increased as the coloured rays grow less refrangible. And from the four last experiments we gather that the maximum of the heating power resides among the invisible rays without the prismatic spectrum, and is probably about half an inch beyond the last visible one, or from the confines of the red ray. These like-wise show that the sun’s invisible rays, in their less refrangible state, still exert a heating power, considerably beyond this maximum, fully equal to that of the red-coloured light; and that consequently, if we may infer the quantity of the efficient from the effect produced, these invisible rays of the sun probably far exceed the visible ones in number.
The inferences deduced from these results are, that the range of refrangibility of radiant heat, or colorific rays, when dispersed by a prism, begins at the purple-coloured light, where they are most refracted, and have the least efficacy; and that their refrangibility lessens and their power increases as they approach the confines of the red-coloured light, but that these confines are not the limits of
