on the terrestrial Rays that occasion Heat. 317 



ljth Experiment. Refraction of the invisible Rays of solar Heat. 



I covered one half of Mr. Dollond's burning lens with paste- 

 board, and threw the prismatic spectrum upon that cover;* 

 then, keeping the last visible red colour one-tenth of an inch 

 from the margin of the pasteboard, I let the invisible rays be- 

 yond the spectrum fall on the lens. In the focus of the red 

 rays, or a very little beyond it, I had placed the ball of the ther- 

 mometer No. 1 ; and, as near to it as convenient, the small one 

 No. 2. Now, that the invisible solar rays which occasion heat 

 were accurately refracted to a focus^ may be seen by the follow- 

 ing account of the thermometers. 



No. 1. No. 2. 



In the Focui- Near the Focus. 



?' 57 57 



1 102 57 



Here, in one minute, these rays gave 45 degrees of heat to 

 the thermometer No. 1, which received them in the focus, while 

 the other, No. 2, suffered no change. 



It is remarkable, that notwithstanding I kept the red colour 

 of the spectrum ~ of an inch upon the pasteboard, a little of 

 that colour might still be seen on the ball of the thermometer. 

 This occasioned a surmise, that possibly the invisible rays of the 

 sun might become visible, if they were properly condensed ; I 

 therefore put this to the trial, as follows. 



iSth Experiment. Trial to render the invisible Rays of the Sun 

 visible by Condensation. 

 Leaving the arrangement of my apparatus as in the last 

 experiment, I withdrew the lens, till the last visible red colour 



* See Plate XIV. 



