334 Prof. Tyndall on Luminous and Obscure Radiation. 



21. To the mixed radiation which issues from solid bodies 

 at a very high temperature, the pure bisulphide of carbon is also 

 eminently transparent. Hence, as the bisulphide of carbon in- 

 terferes but slightly with the obscure rays issuing from a highly 

 luminous source, and as the dissolved iodine seems not at all to 

 interfere with them, we have in a combination of both substances 

 a means of almost entirely detaching the purely thermal rays 

 from the luminous ones. 



22. If vibrations of a long period, established when the radia- 

 ting body is at a low temperature, maintain themselves, as 

 indicated in paragraph 14, side by side with the new periods 

 which augmented temperature introduces, it would follow that a 

 body once pervious to the radiation from any source must always 

 remain pervious to it. We cannot so alter the character of the 

 radiation that a body once in any measure transparent to it 

 shall become quite opake to it. We may, by augmenting the 

 temperature, diminish the percentage of the total radiation trans- 

 mitted by the body; but inasmuch as the old vibrations have 

 their amplitudes enlarged by the very accession of temperature 

 which produces the new ones, the total quantity of heat of any 

 given refrangibility transmitted by the body must increase with 

 increase of temperature. 



23. This conclusion is thus experimentally illustrated. A cell 

 with parallel sides of polished rock-salt was filled with the solu- 

 tion of iodine, and placed in front of the camera within which 

 was the platinum spiral. Behind the rock-salt cell was placed 

 an ordinary thermo-electric pile, to receive such rays as had 

 passed through the solution. The rock-salt lens was in the 

 camera in front, but a small sheaf only of the parallel beam 

 emergent from the lamp was employed. Commencing at a very 

 low dark heat, the temperature was gradually augmented to full 

 incandescence with the following results : — 



Table II. 

 Appearance of spiral. Deflection. 



Dark 1 



Dark but hotter .... 3 



Dark but still hotter ... 5 



Dark but still hotter ... 10 



Feeble red 19 



Dull red 25 



Red 35 



Full red 45 



Blight red 53 



Very bright red .... 63 



Nearly white 69 



White ....... 75 



Intense white 80 



