180 FRAGMENTS OF SCIENCE. 



tion, 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 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 J%ays 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. 

 Through the transparent bisulphide the total radiation of 

 the lamp may be considered to pass, while through the 



