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- 
