42 FRAGMENTS OF SCIENCE. 



luminous and non-luminous, we can intercept the for- 

 mer by the iodine, and do what we please with the lat- 

 ter. Experiments of this character, not only with the 

 iodine solution, but also with black glass and layers of 

 lamp-black, were publicly performed at the Eoyal Insti- 

 tution in the early part of 18G2, 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 in- 

 tercepted by the glass. The obvious remedy here is to 

 employ rock-salt lenses instead of glass ones, or to aban- 

 don 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 prin- 

 ciple, 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. 



