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 lias 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 wliite-liot: 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 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 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- 
