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180 FRAGMEN TS 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 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. 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

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