384 DISTRIBUTION OF HEAT IN THE SPECTRUM. [MEMOIR XXVIII. 



greatly exceeds that in the visible spectrum ; and still 

 more recently Dr. Tyndall, examining the spectrum of 

 the electric light through rock - salt, showed that the 

 curve indicating the distribution " in the region of the 

 dark rays beneath the red shoots suddenly upward in a 

 steep and massive peak, a kind of Matterhorn of heat, 

 which quite dwarfs by its magnitude the portion of the 

 diagram representing the visible radiation." These in- 

 vestigations were made under unexceptionable circum- 

 stances ; the beam of electric light had practically under- 

 gone no atmospheric absorption, and the optical refract- 

 ing train was of rock-salt. 



Sir J. Herschel had shown in 1840 that when the 

 sun's rays are dispersed by a flint-glass prism, the dis- 

 tribution of the heat towards the less refrangible region 

 is not continuous, but there are three maximum points. 

 These points, as shown by Dr. Tyndall, do not exist in 

 the spectrum of electric light, the decline of which is 

 continuous; they are therefore to be attributed to the 

 absorptive disturbance which the sun's rays have under- 

 gone. Quite recently (1871), M. Lamansky has succeed- 

 ed in identifying these interruptions by the aid of the 

 thermo- multiplier. In his Memoir he states that, with 

 the exception of Foucault and Fizeau, in their well- 

 known experiments on the interference of heat, no one 

 has made reference to these lines, and that all experi- 

 menters describe the heat -curve as a continuous one 

 (Philosophical Magazine, April, 1872). 



I may therefore be excused for remarking at this point 

 that the three lines in question were not only observed 

 by me nearly thirty years ago, but that an engraving 

 of them was published in that journal, in a memoir an- 

 nouncing the discovery of fixed lines in the invisible 

 portions of the spectrum (May, 1843). It will be seen, 

 from an inspection of that engraving, that these lines 



