VOL. XC.] PHILOSOPHICAL TRANSACTIONS. 751 



I now took a prism with an angle of 45°, and, placing the 

 thermometers as before, I had as follows: Here we also had, in 5 g 

 10 minutes, a rise of 7° in the thermometer N° 4, and of 3-f- in J> 1 

 N° 1 ; while N° 2 remained stationary. 



I tried now all the 3 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° ; and found invisible heat rays as before. I tried a prism of flint glass, with 

 so small an angle as J 9°, and again found invisible heat rays. 



I made a hollow prism, by cementing together 3 slips of glass of an equal 

 length, but unequal breadth, so as to give me different refracting angles : they 

 were of 51°, 62° 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 thermo- 

 meter N° 1 at .45 inch behind the visible red colour, and N° 5 in the situation 

 of the standard. The refracting angle of the prism was 62° 30' ; and in 5 minutes 

 the thermometer received 1-f- degrees of heat from the invisible rays. On trying 

 the other angles, I likewise found invisible heat rays, in their usual situation be- 

 yond 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 ad- 

 ditional 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 fore- 

 going 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, with respect to gq ; and as the part 

 Get is already known to follow the law of refraction we have mentioned, it is 

 equally evident that the additional heat of ag must follow the same law. We do 

 not enter into the dispersive power of different mediums with respect to heat, 

 since that would lead us farther than the present state of our investigation could 

 authorize us to go ; the following experiment however will show that, as with light 

 so with heat, such dispersive power must be admitted. 



Exper. 11. Correction of the Different Refrangibility of Heat, by contrary 

 Refraction in Different Mediums. — I took 3 prisms ; one of crown glass, having 

 an angle of 25° ; another of flint glass, with an angle of 24 ; and a third of crown 

 glass, with an angle of 10°. These being put together, as they are placed when 

 experiments of achromatic refractions are to be made, I found that they gave a 

 spectrum nearly without colour. The composition seemed to be rather a little 

 over adjusted; there being a very faint tinge of red on the most refracted side, 

 and of violet on the least refracted margin. I examined both extremes by 2 ther- 

 mometers ; keeping N° 3 as a standard, while N° 2 was applied for the discovery 

 of invisible rays ; but I found no heat on either side. After this, I placed N° 2 

 in the middle of the colourless illumination ; and in a little time it rose 2°, while 



