RADIATION. 31 



wholly, and to concentrate the rays by a metallic mirror. 

 Both of these improvements have been introduced, and, as 

 anticipated, the invisible foci have been thereby rendered 

 more intense. The mode of operating remains however 

 the same, in principle, as that made known in 1862. It 

 was then found that an instant's exposure of the face of 

 the thermo-electric pile to the focus of invisible rays 

 dashed the needles of a coarse galvanometer violently aside. 

 It is now found that on substituting .for the face of the 

 thermo-electric pile a combustible body, the invisible rays 

 are competent to set that body on fire. 



6. Visible and Invisible Rays of the Electric Light. 



We have .next to examine what proportion the non- 

 luminous rays of the electric light bear to the luminous 

 ones. This the opaque solution of iodine enables us to do 

 with an extremely close approximation to the truth. 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 deter- 

 mine, by means of a thermo-electric pile, the total radia- 

 tion, and deduct from it the purely obscure, we obtain the 

 value of the purely luminous emission. Experiments per- 

 formed in this way prove that if all the visible rays of the 

 electric light were converged to a focus of dazzling bril- 

 liancy, its heat would only be one-eighth of that produced 

 at the unseen focus of the invisible rays. 



Exposing his thermometers to the successive colors of the 

 solar spectrum, Sir William Herschel determined the heat- 

 ing 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 perpendicu- 

 lars, he obtained a curve which showed at a glance the 

 manner in which the heat was distributed in the solar spec- 

 trum. Professor Miiller of Freiburg, with improved in- 

 struments, afterward made similar experiments, and con- 

 structed a more accurate diagram of the same kind. We 

 have now to examine the distribution of heat in the spec- 



