30 FRAGMENTS OF SCIENCE. 
to examine the substance in its nsnal solid condition. It 
however dissolves freely in bisulphide of carbon, There is 
no chemical union between the liquid and the iodine; it is 
simply a case of solution, in which the uncombined atoms 
of the element can act upon the radiant heat. When per- 
mitted to do so, it' was found that a layer of dissolved 
iodine, sufficiently opaque to cut off the light of the mid- 
day sun, was almost absolutely transparent to the invisible 
calorific rays.* 
By prismatic analysis Sir William llerschel separated 
the luminous from the non-luminous rays of the sun, and 
he also sought to render the obscure rays visible by con- 
centration. Intercepting the luminous portion of his 
spectrum he brought^ by a converging lens, the ultra-red 
rays to a focus, but by this condensation he 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 attainable far more powerful foci of invisi- 
ble rays than could possibly be obtained by the method of 
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 being limited by this circum- 
stance. But with our opaque solution we may employ the 
entire surface of the largest lens, and having thus con- 
verged the rays, luminous and non-luminous, we can 
intercept the former 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 
* Professor Dewar has recently succeeded in producing a medium 
highly opaque to light, aud highly transparent to obscure heat, by 
fusing together sulphur and iodine! 
