80 FRAGMENTS OF SCIENCE. 



more brilliant, but at the same time orange rays are 

 added to the emission. Augmenting 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 colours 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 colours on the optic nerve. 



In the experiment just described we began with a 

 platinum 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 kind of obscure rays emitted by all the 



