RADIATION. 293 



To display all these colors at tlie same time the platinum wire must be ivJiife- 

 liot ; the impression of whiteness being in fact produced by the simultaneous 

 action of all these colors on the optic nerve. 



In tlie experiment just described we began with a jilatinuiii wire at an ordi- 

 nary temperature, and gradually raised it to a white heat. At the begiiuiing, 

 and before the electric current had acted at all upon the wire, it emitted invisi- 

 ble rays. For some time after the action of the current had connnenced, 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 jiroved in the 

 se(juel that they maintain themselves in the radiation ; that a ray once emitted 

 continues to be emitted when the temperature is increased, and hence the emis- 

 sion from our platinum wu'e, even when it has attaiui'd its maximum brilliancy, 

 consists of a mixture of visible and invisible rays. If, instead of the platinum 

 "Wire, the eailh itself were raised to incandescence, the obscare radiation which 

 it now emits would continue to i)e emitted. To reach incandescence the ])lauet 

 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 jjlanets of our system are included in the present radia- 

 tion of the sun. 



The great pioneer in this domain of science was Sir William Ilerschel. 

 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 scdar spectrum. 

 Exposing thermometers to the successive colors he determined their heating 

 power, and found it to augment from the violet or most refracted end to the red 

 or least refracted end of the spectrum. But he did not stop here. Pushing his 

 thermometers into the dark space beyond the red, ho found that, though the 

 light had disappeared, the radiant heat falling on the instruments was more 

 intense than that at any visible part of the spectrum. In fact. Sir William 

 Ilerschel showed, and his results have been verified by various philosophers 

 since his time, that besides its luminous rays, the sun pours forth a multitude 

 of other rays more powerfully calorific than the luminous ones, but entirely 

 unsuited to the jjurposes of vision. 



At the less rel'rangiltle end of the solar spectrum, then, the range of the sun's 

 radiation is not limited by that of the eye. The same statement aj)plies to the 

 more refrangilile end. Kitter discovered the extension »jf the spectrum into the 

 invisible region beyond the violet; and, in recent times, this ultra-violet emis- 

 sion has had jieculiar interest conferred u])on it by the admirable researches of 

 Professor Stokes. The comi)lete s})ectrum of the sun consists, therefore, of 

 three distinct parts: 1st, of ultra-red rays of high heating pcjwer, but unsuited 

 to the purposes of vision ; 2d, of luminous rays which display the following 

 succession of colors : red, orange, yellow, green, blue, indigo, violet ; 3d, of 

 ultra-violet rays which, like the ultra-red ones, are incompetent to excite vision, 

 but, unlike them, possess a very feeble heating power. In consequence, how- 

 ever, of their chemical energy, these ultra-violet rays are of the utmost import- 

 ance to the organic world. 



II. — ORIGIN AND CHARACTER OF RADIATION. THE ETHER. 



W^hen we see a platinum wire raised gradually to a white heat and emitting 

 in succession all the colors of the s})ectrum, we are siiiqdy conscious of a series 

 of changes in the conditi(m of our eyes. We do not see the actions in which 

 these successive colors originate, but the mind irresistibly infers that the appear- 

 ance of the colors corresponds to certain contemporaneous changes in the wire. 



