Experiments on the refrangibility of the invisible rays of the sun

Abstract

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.