44,2 Dr. Herschel's Experiments on the solar, and 



thermometer No. 4, and of 3 J in No. 1 ; while No. 2 remained 

 stationary. 



I tried now all the three angles of a prism of whitish glass : 

 they were of 63, 62, and 55 degrees; and I found invisible 

 rays of heat to accompany all the visible spectra given by these 

 angles. 



I tried a prism of crown glass, having an angle of 30 degrees ; 

 and found invisible heat rays as before. 



I tried a prism of flint glass, with so small an angle as 19 

 degrees, and again found invisible heat rays. 



I made a hollow prism, by cementing together three slips of 

 glass of an equal length, but unequal breadth, so as to give me 

 different refracting angles : they were of 51 , 6V 30', and 66° 

 30'. Then, filling it with water, and receiving the spectrum, 

 when exposed to the sun, as usual, on the table, I placed the 

 thermometer No. 1 at ,45 inch behind the visible red colour, 

 and No. 5 in the situation of the standard. The refracting angle 

 of the prism Was 6V 30' ; and, in five minutes, the thermometer 

 received if degrees of heat from the invisible rays. On trying 

 the other angles, I likewise found invisible heat rays, in their 

 usual situation beyond the red colour. 



Now, setting aside a minute inquiry into the degrees of heat 

 occasioned by these invisible rays, I shall here only consider 

 them as an additional part, annexed to the different quantities 

 of heat which are found to go along with the visible spectrum ; 

 in the same manner as if, in the spectrum of light, another colour 

 had been added beyond the red. Then, as from the foregoing 

 experiments it appears, that a change of the refracting medium, 

 and of the angle by which the refractions were made, occasioned 

 no alteration in the relative situation of the additional part AG, 



