RADIATION. 169 



in fact produced by the simultaneous action of all these 

 colors on the optic nerve. 



In the experiment just described we began with a plat- 

 inum 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 becomes 

 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 

 incandescence, 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- 

 luminous 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 which the dark earth pours nightly into space. In 

 fact, the various kinds of obscure rays emitted by all the 

 planets of our system are included in the present radiation 

 of the sun. 



The great pioneer in this domain of science was Sir 

 William Herschel. Causing a beam of solar light to pass 

 through a prism he resolved it into its colored constituents ; 

 he formed what is technically called the solar spectrum. 

 Exposing thermometers to the successive colors he deter- 

 mined their heating power, and found it to augment from 

 the violet or most refracted end, to the red or least refracted 

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