304 Dr. Herschei/s Experiments on the solar, and 



Stb Experiment. Reflection, and Condensation, of the invisible 



solar Rays. 



I made an apparatus for placing the small steel mirror at any 

 required angle;* and, having exposed it to the prismatic spec- 

 trum, so as to receive it perpendicularly, I caused the colours to fall 

 on one half of the mirror, which, being covered by a semicircular 

 piece of pasteboard, would stop all visible rays, so that none of 

 them could reach the polished surface. On the pasteboard were 

 drawn several lines, parallel to the diameter, and at the distance 

 of one- tenth of an inch from each other ; that, by withdrawing 

 the apparatus, I might have it at option to remove the last 

 visible red to any required distance from the reflecting surface. 

 In the focus of the mirror was placed the thermometer No. 2. 

 I covered now also the other half of the mirror, till the thermo- 

 meter had assumed the temperature of its situation. Then, 

 withdrawing the apparatus out of the visible spectrum, till the 

 last tinge of red was one-tenth of an inch removed from the 

 edge of the pasteboard, and the whole of the coloured image 

 thus thrown on the semicircular cover, I opened the other half 

 of the mirror, for the admission of invisible rays. The result 



was as follows. 



No. 2. 



In the Focus of invisible Heat. 



O' 61 



1 80 



Here, in one minute, the thermometer rose 19 degrees. I 

 covered the mirror. 



2' 72 



3 67 



4 64 



* See Plate XII. Fig. 2. 



