1826.] Scientific Notices^^Miscellaneous. 235 



blue, green, orange, yellow, and was greatest of all in the red, R, 

 at the other end of the spectrum. But what was most remark- 

 able, the heat was found to extend beyond the limits of the 

 spectrum, and reached its maximum, not within the red rays, 

 but at a point s in the dark space, about half an inch beyond 

 their outer boundary, from which it diminished in both direc- 

 tions. This conclusion has been long received in Britain as 

 indisputable, and now finds a place in all our elementary work 

 on Natural Philosophy. Prof^ Leslie was led, however, to 

 question its accuracy, by an experiment he made twenty year 

 ago, and which was substantially the same with that made the 

 other day, except that, instead ofa common double-convex lens, 

 he had now the advantage of employing one of Fresnel's lenses 

 (surrounded by concentric prismatic rings) belonging to the 

 Northern Lights, with which he was accommodated by Mr. 

 Stevenson. The result was stated brie% in a note to the article 

 Climate, published in the Supplement to the Encyclopaedia 

 Britannica eight years ago, but not in a way to attract the 

 attention which the subject merited. The experiment is 

 remarkably simple, yet extremely well calculated to bring Her- 

 schel's doctrine to the test. 



Let L in figure 2 represent a double-convex lens of 20 inches 

 diameter. If the middle of this lens (marked by dotted lines) is 

 covered with a sheet of opaque paper, the uncovered rim i i, 

 two inches broad, will form a circular prism, which, if it were 

 extended in a straight hne, would be five feet long. If the lens 

 thus covered is exposed to the sun, the rays or pencils of rays 

 a z, which pass through the rim, will be refracted exactly as they 

 are in the spectrum, but they necessarily converge, and thus the 

 heat and fight of a prism five feet long can be accumulated in a 

 small point. Let a piece of paper be held at W a little before 

 the focus (or behind, for it answers equally either way), so as to 

 receive the circular ring of light x z, the red rays will be seen at 

 the outside of the ring at ?• (fig. 3, where the dotted lines repre- 

 sent a small segment of the luminous ring, and X Y the paper) 

 and the violet at the inside, v. The orange and indigo rays may 

 also be faintly discerned ; but as light in a state of great inten- 

 sity always becomes white, of whatever rays it may be composed, 

 so the other colours which should occupy the intermediate 

 space (between v and r), blue, green, and yellow, are lost in one 

 intense and dazzling white. We have here, in short, the colours 

 of the solar spectrum, but in a state of great concentration. Let 

 us now substitute for the piece of paper a stick of black sealing- 

 wax, with the surface roughened; and let this be placed across 

 the luminous ring, in the position W, fig. 2, or X Y, fig. 3. In 

 the course of a minute, the surface of the wax begins to shine 

 about y (fig. 3), and then melts. The fusion extends towards v 

 and towards /•, but always stops at the extreme edge of the red 



