288 Dr. Herschei/s Experiments on the Refrangibility 



be in the illumination, while No. 3 remained a standard. The 

 result proved as follows. 



No. i. 



No. 2. 



No. 3 



48 



48 



47i 



48i - 



48 



47i 



48| - 

 49 



- 48i - 



- 48^ - 



47i 

 47i 



Here the thermometer No. 1 rose 1 degree, in 15 minutes; 

 and No. 2 rose ■§- degree, in the same time. 



From these last experiments, I was now sufficiently persuaded, 

 that no rays which might fall beyond the violet, could have 

 any perceptible power, either of illuminating or of heating ; and 

 that both these powers continued together throughout the pris- 

 matic spectrum, and ended where the faintest violet vanishes. 



A very material point remained still to be determined, which 

 was, the situation of the maximum of the heating power. 



As I knew already that it did not lie on the violet side of the 

 red, I began at the full red colour, and exposed my thermometers, 

 arranged on a line, so as to have the ball of No. 1 in the midst 

 of its rays, while the other two remained at the side, unaffected 

 by them. 



No. 1. 





No. 2. 



No. 3 



48i - 



- 



48i - 



' 48 



55i 



- 



48i- - 



48 



55i 



- 



48^ - 



48 



Here the thermometer No. 1 rose 7 degrees, in 10 minutes, by 

 an exposure to the full red coloured rays. 



I drew back the stand, till the centre of the ball of No. 1 was 



