RADIATION. 21 



the optic nerve a certain temperature is necessary. A cool 

 poker thrust into a fire remains dark for a time, but when 

 its temperature has become equal to that of the surround- 

 ing coals, it glows like them. In like manner, if a cur- 

 rent of electricity, of gradually increasing strength, be sent 

 through a wire of the refractory metal platinum, the wire 

 first becomes sensibly warm to the touch; for a time its 

 heat augments, still however remaining obscure; at length 

 we can no longer touch the metal with impunity; and at a 

 certain definite temperature it emits a feeble red light. As 

 the current augments in power the light augments in 

 brilliancy, until finally the wire appears of a dazzling 

 white. The light which it now emits is similar to that of 

 the sun. 



By means of a prism Sir Isaac Newton unraveled the 

 texture of solar light, and by the same simple instrument 

 we can investigate the luminous changes of our platinum 

 wire. In passing through the prism all its rays (and they 

 are infinite in variety) are bent or refracted from their 

 straight course; and, as different rays are differently 

 refracted by the prism, we are by it enabled to separate 

 one class of rays from another. By such prismatic analy- 

 sis Dr. Draper has shown, that when the platinum wire 

 first 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 plati- 

 num wire atan 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 cur- 

 rent 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 ap- 

 pearance? It will be proved in the sequel that they main- 



