RADIATION. 43 



The pure bisulphide of carbon, which is the solvenl 

 of the iodine, is perfectly transparent to the luminous, 

 and almost perfectly transparent to the dark, rays of the 

 electric lamp. Supposing the total radiation of the 

 lamp to pass through the transparent bisulphide, while 

 through the solution of iodine only the dark rays are 

 transmitted. If we determine, by means of a thermo- 

 electric pile, the total radiation, and deduct from it 

 the purely obscure, we obtain the value of the purely 

 luminous emission. Experiments, performed in this 

 way, prove that if all the visible rays of the electric 

 light were converged to a focus of dazzling brilliancy, 

 its heat would only be one-eighth of that produced at 

 the unseen focus of the invisible rays. 



Exposing his thermometers to the successive colours 

 of the solar spectrum, Sir William Herschel determined 

 the heating power of each, and also that of the region 

 beyond the extreme red. Then drawing a straight line 

 to represent the length of the spectrum, he erected, at 

 various points, perpendiculars to represent the calorific 

 intensity existing at those points. Uniting the ends 

 of all his perpendiculars, he obtained a curve which 

 showed at a glance the manner in which the heat was 

 distributed in the solar spectrum. Professor Miiller of 

 Freiburg, with improved instruments, afterwards made 

 similar experiments, and constructed a more accurate 

 diagram of the same kind. We have now to examine 

 the distribution of heat in the spectrum of the electric 

 light ; and for this purpose we shall employ a par- 

 ticular form of the thermo-electric pile, devised by 

 Melloni. Its face is a rectangle, which by means of 

 movable side-pieces can be rendered as narrow as de- 

 sired. We can, for example, have the face of the pile 

 the tenth, the hundredth, or even the thousandth of an 

 inch in breadth. By means of an endless screw, this 



