298 RADIATION. 



brought, by a converging lens, the ultra-red rays to a focus, but by this conden- 

 sation ho obtained no light. The solution of iodine offers a means of filtering 

 the solar beam, or, failing it, the beam of the electric lamp, which renders attain- 

 able more powerful foci of invisible rays than could possibly be obtained in the 

 above experiment by Sir William Herschel ; for to form his spectrum he was 

 obliged to operate upon solar light which had passed through a narrow slit or 

 through a small aperture, the amount of the obscure heat admitted being limited 

 by this circumstance. But with our opaque solution we may employ the entire 

 surface of the largest lens, and, having thus converged the rays, luminous and 

 non-luminous, we can intercept the fonner by the iodine, and do what we please 

 with the latter. Experiments of this character, not only with the iodine solution, 

 but also with black glass and layers of lampblack, were publicly performed at 

 the Royal Institution in the early part of 1862, and the effects at the foci of 

 invisible rays then obtained were such as had never been witnessed previously. 

 In the experiments here referred to, glass lenses were employed to concentrate 

 the rays. But glass, though highly transparent to the luminous, is in a high 

 degree opaque to the invisible heat-rays of the electric lamp, and hence a large 

 portion of those rays was intercepted by the glass. The obvious remedy here is 

 to employ rock-salt lenses instead of glass ones, or to abandon the use of lenses 

 wholly and to concentrate the rays by a metallic mirror. Both of these improve- 

 ments have been introduced, and, as anticipated, the invisible foci have been 

 thereby rendered more intense, ^'he 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 thenuo-electric pile a combustible 

 body, the invisible rays are competent to set that body on fire. 



VI. — ^VISIBLE AND INVISIBLE EATS OF THE ELECTEIC LIGHT. 



We have next to examine what proportion the non-luminous rays of the elec- 

 tric 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 bisul- 

 phide of carbon, which is the solvent of the iodine, is perfectly transparent to 

 the luminous, and almost periectly transparent to the dark rays of the electric 

 lamp. Through the transparent bisulphide the total radiation of the lamp may 

 be considered to pass, while through the solution of iodine only the dark rays 

 are transmitted. Determining, then, by means of a thermo-electric pile, the 

 total radiation, and deducting from it the purely obscure, we obtain the amount 

 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 daz- 

 zling brilliancy, its heat would only be one-ninth 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 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 per- 

 pendiculars, 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 dis- 

 tribution of heat in the spectnun of the electric light ; and for this purpose we 

 shall employ a particular form of the thermo-electric pile, devised b^ Melloni. 

 Its face is a rectangle, which, by means of movable side pieces, can be rendered 

 as narrow as desired. 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 



