VOL. XC.] PHILOSOPHICAL TRANSACTIONS. 6*89 



shadow of their balls thrown on the intersection of these lines. Now, setting my 

 little stand on a table, I caused the prismatic spectrum to fail with its extreme co- 

 lour on the edge of the paper, so that none might advance beyond the first line. 

 In this arrangement, all the spectrum, except the vanishing last quarter of an inch, 

 which served as a direction, passed down by the edge of the stand, and could not 

 interfere with the experiments. I had also now used the precaution of darkening 

 the window in which the prism was placed, by fixing up a thick dark green curtain, 

 to keep out as much light as convenient. 



The thermometers being perfectly settled at the temperature of the room, I 

 placed the stand so that part of the red colour, refracted by the prism, fell on the 

 edge of the paper, before the thermometer N° 1, and about half way, or l| inch, 

 towards the 2d : it consequently did not come before that, or the 3d thermometer, 

 both which were to be my standards. During the experiment, I kept the last ter- 

 mination of visible red carefully to the first line, as a limit assigned to it, by gently 

 moving the stand when required; and found the ther- 

 mometers, which were all placed on the 2d line, af- 

 fected as annexed. Here the thermometer N° 1 rose 

 6*4- degrees, in 10 minutes, when its centre was placed 

 4. inch beyond visible light. 



In order to have a confirmation of this fact, I cooled the thermometer N° 1, 

 and placed N° 2 in the room of it: I also put N° 3 in the place of N°2, and N° 1 

 in that of N° 3 ; and having exposed them as before 

 arranged on the 2d line, I had the annexed result. 

 Here the thermometer N° 2 rose to 2£ degrees, in 1 2 

 minutes; and being much more sensible than N° J, it 

 came to the temperature of its situation in a short time; 

 but I left it exposed longer, on purpose to be perfectly assured of the result. Its 

 showing but 2$ degrees advance, when N° 1 showed 6± y has also been accounted 

 for before. 



It being now evident that there was a refraction of rays coining from the sun, 

 which, though not fit for vision, were yet highly invested with a power of occa- 

 sioning heat, I proceeded to examine its extent as 



follows. The thermometers were arranged on the 3d 46 *' 46 2 ' 45a ' 

 line, instead of the 2d; and the stand was, as before, 50 4,6% 46 



immersed up to the first, in the coloured margin of the *if J£* *5| 

 vanishing red rays. The result was thus. Here the 

 thermometer N° 1 rose 5£ degrees, in 1 3 minutes, at 1 inch behind the visible 

 light of the red rays. 



I now placed the thermometers on the 4th line, in- 

 stead of the 3d ; and, proceeding as before, I had the 

 annexed result. Therefore the thermometer N° 1 rose 

 3-f degrees, in 10 minutes, at H inch beyond the visible light of the red rays. 



vol. xviii, 4 T 



