2 PROFESSOR TYNDALL ON CALORESCENCE. 



and the results of his observations are rendered graphically in fig. 2. Here the area 

 A B C D represents the invisible, while C D E represents the visible radiation. 



Fig. 1. 



Fig. 2. 



Spectrum of Sun (Herschel) reduced. Spectrum of Sun (Miiller). 



With regard to terrestrial sources of heat, it may be stated that all such sources 

 hitherto examined emit those obscure rays. Melloni found that 90 per cent, of the 

 emission from an oil-flame, 98 per cent, of the emission from incandescent platinum, and 

 99 per cent, of the emission from an alcohol-flame consists of obscure rays*. The visible 

 radiation from a hydrogen-flame is, according to my own experiments, too small to admit 

 of measurement. With regard to solid bodies, it may be stated generally that, when 

 they are raised from a state of obscurity to vivid incandescence, the invisible rays emitted 

 in the first instance continue to be emitted with augmented power when the body glows. 

 For example, with a current of feeble power the carbons of the electric lamp may be 

 warmed and caused to emit invisible rays. But the intensity of these same rays may 

 be augmented a thousandfold by raising the carbons to the temperature necessary for 

 the electric light, flere, in fact, the luminous and non-luminous emission augment 

 together, the maximum of brightness of the visible rays occurring simultaneously with 

 the maximum calorific power of the invisible onesf . 



At frequent intervals during the past ten or twelve years I have had occasion to expe- 

 riment on the invisible rays of the electric light, and have finally made them the sub- 

 ject of special investigation. The present paper contains a brief account of the inquiry. 

 I endeavour, in the first place, to compare the luminous with the non-luminous radia- 

 tion of the electric light, and to determine their relative energy ; I point out a method 

 of detaching the luminous from the non-luminous rays ; and afterwards describe various 

 experiments illustrative of the calorific power of the invisible rays, and of the transmu- 

 tations of which they are capable. 



§2. 



The instrument employed by Professor Muller in the investigation above alluded to, 

 was a form of the thermo-electric pile devised by Melloni for the examination of this 

 and kindred questions. Through the kindness of my friend Mr. Gassiot, a very beautiful 

 instrument of this kind, constructed by Ruhmkorff, has remained in my possession for 

 several years, and been frequently employed in my researches. It consists of a double 

 metallic screen, with a rectangular aperture in the centre — a single row of thermo-electric 

 * La Thermochrose, p. 304. f On this point see the Eede Lecture for 1865, p. 33 (Longmans). 



