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



We are now, in the same manner, to find a figure to express 

 the heating power of the refracted prismatic rays, or what may 

 be called the spectrum of heat. In order to determine the 

 length of our base, I examined the extent of the invisible rays, 

 and found, that at a distance of two inches beyond visible red, 

 my thermometer, in a few minutes, acquired 1^ degree of heat. 

 The extent of the coloured spectrum at that time, or the line 

 which answers to GQ in my figure, measured 2,997 i ncnes - If 

 two inches had been the whole of the extent of the invisible 

 part, it might be stated to be in proportion to the visible one 

 as 2 to 3 ; but we are to make some allowance for a small space 

 required beyond the last ordinate, that the curve of the heating 

 power drawn through it may reach the base ; and indeed, at 2^ 

 inches beyond visible red, I could still find-§- degree of heat. It 

 appears therefore sufficiently safe, to admit the base of the spec- 

 trum of heat AQ, to be to that of the spectrum of light GQ, as 

 5* to 3 ; or, conforming to the Newtonian figure before men- 

 tioned, the base of which is 3,3 inches, as 57-^- to 33. Now, if 

 we assume for the maximum of heat, an ordinate of an equal 

 length with that which was fixed upon for the maximum of 

 light, it will give us a method of comparing the two spectra 

 together. Accordingly, I have drawn the several ordinates 

 B, C, D, E, F, G, H, I, K, L, M, N, O, P, of such lengths as, 

 from experiments made on purpose, it appeared they should be, 

 in order to express the heat indicated by the thermometer, when 

 placed on the base, at the several stations pointed out by the 

 letters. 



A mere inspection of the two figures, which have been drawn 

 as lying upon one another, will enable us now to see how very 

 differently the prism disperses the heat-making rays, and those 



