MEMOIR XXVIIL] DISTRIBUTION OF HEAT IN THE SPECTRUM. 



little more than half the space they do in the diffraction ; 

 while the green, blue, indigo, and violet, from the fixed 

 line E to H, occupy nearly double the space in the pris- 

 matic that they do in the diffraction spectrum. The gen- 

 eral result is, that in the prismatic the less refrangible 

 regions are much compressed, and the more refrangible 

 much dilated. And it is plain that the same will hold 

 good in a still greater degree for any invisible rays that 

 are below the red and above the violet respectively. 



Now, if a thermometer of any kind were carried in 

 succession from the greatly dilated, more refrangible re- 

 gions to the greatly condensed, less refrangible, could the 

 measures obtained be accepted as expressing the true 

 distribution ? The thermometric surface being invariable, 

 would it not receive in the less refrangible spaces more 

 than its proper amount of heat, and in the more refrangi- 

 ble less than its proper amount ? 



If we should admit that the distribution of heat in a 

 correctly formed spectrum is uniform, it is plain that 

 measures made by the use of a prism would not substan- 

 tiate that admission. The concentration to which I have 

 alluded as taking place in the less refrangible region 

 would give an exaggerated, an increased heat for that 

 region ; and, on the contrary, the dilatation of the more 

 refrangible would give an exaggerated diminution of 

 heat for that space. But if it were possible to make sat- 

 isfactory heat measures on the diffraction spectrum, in 

 which the colored spaces and fixed lines are arranged ac- 

 cording to their wave-lengths, the admission would be 

 substantiated. 



In view of these facts, I did attempt, many years ago, 

 to make heat measures on the diffraction spectrum. But 

 so small is the heat that, as may be seen in the Philo- 

 sophical Magazine (March, 1857), the results were un- 

 satisfactory. More recently I have tried another meth- 



