le Spectrum.
There are, however, rays of light beyond both ends of the spectrum which
do not affect the optic nerves of the eye, and therefore are invisible
to sight. The rays in the spectrum which lie beyond the red are termed
ultra-red rays, while those beyond the violet are called ultra-violet
rays. It can be proved the former are rich in heating power, while the
latter possess great chemical power. By means of an instrument known as
the thermo-electric pile, or thermopile, the various heating power of
the whole spectrum, visible and invisible, can be determined.

Let us look for a moment at these invisible or dark rays. Strictly
speaking, all light is invisible, as we cannot see light itself, we can
only see it by reflection. We have seen that light is due to a wave
motion in the Aether, but we cannot see that wave motion, neither can we
see the Aether itself, so that it is not strictly correct to call a ray
visible or invisible. We have, however, accepted the terms in relation
to the rays of the spectrum, to distinguish between the invisible or
obscure rays of the spectrum and the visible rays. It was Sir W.
Herschel who first discovered the existence of these invisible waves. He
passed a thermometer through the various colours of the solar spectrum,
and then noted the temperature of each colour. He did not, however, stop
at the limit of the visible spectrum, but experimented with his
thermometer beyond its limits, and then found that beyond the red rays
there were other rays, the ultra-red rays, which possessed greater
heating power than any other rays of the spectrum. Thus his experiments
proved, that side by side with the luminous or light waves, there were
other rays, which, though they possessed greater heating power, yet were
not able to excite the optic nerve, and so produce the sensation of
sight.

From these facts we learn that the solar spectrum may be divided into
three parts--

1. The red or ultra-red end of the spectrum which possesses the greatest
heating power.

2. The central part, yellow and green, which is the greatest in luminous
power or light waves.

3. The violet or ultra-violet end, which possesses great chemical or
actinic power as it is sometimes termed.

We have already seen (Art. 69) that the same aetherial waves which give
rise to heat, also give rise to light, and that the only physical
difference between heat and light is, that the waves which cause the
phenomena of heat are of