XIV. Experiments on the refrangibility of the invisible rays of the sun

In that section of my former paper which treats of radiant heat, it was hinted, though from imperfect experiments, that the range of its refrangibility is probably more extensive than that of the prismatic colours; but, having lately had some favourable sunshine, and obtained a sufficient confirmation of the same, it will be proper to add the following experiments to those which have been given. I provided a small stand, with four short legs, and covered it with white paper. On this I drew five lines, parallel to one end of the stand, at half an inch distance from each other, but so that the first of the lines might only be ¼ of an inch from the edge. These lines I intersected at right angles with three others; the 2d and 3d whereof were, respectively, at 2½ and at 4 inches from the first.