750 PHILOSOPHICAL TRANSACTIONS. [ANNO 1800. 



the same experiments we collect, that the illuminations of yellow and green are 

 equal to each other, and not much inferior to the maximum ; this giveN us the 

 ordinates k and m. Then, by the rest of the same experiments, we Obtain also 

 the ordinates h, i, n, o, p, with sufficient accuracy for the purpose here intended. 

 All these being applied to the middle of the spaces which belong to their respective 

 colours, we have the figure gr»g, representing what may be called the spectrum 

 of illumination. 



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 2 inches beyond the visible red, my ther- 

 mometer in a few minutes acquired 1°^- of heat. The extent of the coloured 

 spectrum at that time, or the line which answers to Ga in the figure, measured 

 2.997 inches. If 2 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 

 24- inches beyond visible red, I could still find 4- degree of heat. It appears there- 

 fore sufficiently safe, to admit the base of the spectrum of heat aq, to be that of 

 the spectrum of light gg, as 5^ to 3 ; or conforming to the Newtonian figure before 

 mentioned, the base of which is 3.3 inches, as 574 to 33 « Now if we assume for 

 the maximum of heat, an ordinate of an equal length with that which was fixed on 

 for the maximum of light, it will give us a method of comparing the 2 spectra 

 together. Accordingly I have drawn the several ordinates b, c, d, e, p, 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 in- 

 spection of the 2 figures, which have been drawn as lying on each other, will 

 enable us now to see how very differently the prism disperses the heat-making rays, 

 and those which occasion illumination, over the areas asqa, and grog, of our 2 

 spectra ! These rays neither agree in their mean refrangibility, nor in the situa- 

 tion of their maxima. At r, where we have most light, there is but little heat; 

 and at s, where we have most heat, we find no light at all ! 



Exper. 21. The Sines of Refraction of the Heat-making Rays, are in a 

 Constant Ratio to the Sines of Incidence. I used a At j inch At , inch Standard# 



prism with a refracting angle of 6* 1°; and placing the Min. N°4. N° 1. N° 2. 

 thermometer N° 4 half an inch, and N° 1 one inch, 67 66 6 J 



beyond the last visible red colour, I kept N° 2 by 5 6*9 67 63J 



the side of the spectrum, as a standard for tempera- 8 9 * '* * 



ture. Here in 8 minutes, the thermometer at half an inch from visible colour, 

 rose 5-J- degrees; and, at 1 inch from the same, the other thermometer rose 3±; 

 while the temperature, as appears by N° 2, remained without change. 



