RADIATION 47 



renders attainable far more powerful foci of invisible rays 

 than could possibly be obtained by the method of Sir Wil- 

 liam 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 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, lumi- 

 nous and non-luminous, we can intercept the former by the 

 iodine, and do what we please with the latter. Experi- 

 ments of this character, not only with the iodine solution, 

 but also with black glass and layers of lamp-black, 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 improvements have been intro- 

 duced, 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 ex- 

 posure of the face of the thermo-electric pile to the focus 

 of invisible rays, dashed the needles of a coarse galvanom- 

 eter violently aside. It is now found that on substituting 



