a layer of dissolved iodine,
sufficiently opaque to cut off the light of the midday sun, was almost
absolutely transparent to the invisible calorific rays. [Footnote:
Professor Dewar has recently succeeded in producing a medium highly
opaque to light, and highly transparent to obscure heat, by fusing
together sulphur and iodine.]

By prismatic analysis Sir William Herschel separate the luminous from
the non-luminous rays of the sun, and he also sought to render the
obscure rays visible by concentration. 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 invisible 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 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 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 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 thermoelectric pile to the focus of invisible rays,