RADIATION. 43 



The pure bisulphide of carbon, which is the solvent 

 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 col- 

 ours of the solar spectrum, Sir William Herschel deter- 

 mined 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, after- 

 wards 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 particular 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 



