30 FRAGMENTS OF SCIENCE. 



are added to the emission. Augmenting the tempera- 

 ture 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 colours at the same time the platin- 

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

 ness being in fact produced by the simultaneous action 

 of all these colours on the optic nerve. 



In the experiment just described we began with a 

 platinum wire at an ordinary temperature, and gradu- 

 ally 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 them- 

 selves in the radiation; that a ray once emitted contin- 

 ues 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-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 kind of obscure rays emitted 

 by all the planets of our system are included in the 

 present radiation of the sun. 



