RADIATION 85 



begins to glow the light emitted is sensibly red. As the 

 glow augments the red becomes more ^brilliant, but at the 

 same time orange rays are added to the emission. Aug- 

 menting the temperature still further, yellow rays appear 

 beside the orange; after the yellow, green rays are emitted; 

 and after the green come, in succession, blue, indigo, and 

 violet rays. To display all these colors at the same time 

 the platinum wire must be white-hot: the impression of 

 whiteness being in fact produced by the simultaneous 

 action of all these colors on the optic nerve. 



In the experiment just described we began with a pla- 

 tinum wire at an ordinary temperature, and gradually 

 raised it to a white heat. At the beginning, and even 

 before the electric current had acted at all upon the wire, 

 it emitted invisible rays. For some time after the action 

 of the current had commenced, and even for a time after 

 the wire had become intolerable to the touch, its radiation 

 was still invisible. The question now arises, What be- 

 comes of these invisible rays when the visible ones make 

 their appearance? It will be proved in the sequel that 

 they maintain themselves in the radiation ; that a ray once 

 emitted continues to be emitted when the temperature is 

 increased, and hence the emission from our platinum wire, 

 even when it has attained its maximum brilliancy, consists 

 of a mixture of visible and invisible rays. If, instead of 

 the platinum wire, the earth itself were raised to incan- 

 descence, the obscure radiation which it now emits would 

 continue to be emitted. To reach incandescence the planet 

 would have to pass through all the stages of non-lumin :.s 

 radiation, and the final emission would embrace the rays 

 of all these stages. There can hardly be a doubt that, 

 from the sun itself, rays proceed similar in kind to those 



