eter into the dark space beyond it.
Here he found the temperature actually higher than in any part of the
visible spectrum. By this important observation, he proved that the
sun emitted heat-rays which are entirely unfit for the purposes of
vision. The subject was subsequently taken up by Seebeck, Melloni,
Mueller, and others, and within the last few years it has been found
capable of unexpected expansions and applications. I have devised a
method whereby the solar or electric beam can be so _filtered_ as to
detach from it, and preserve intact, this invisible ultra-red
emission, while the visible and ultra-violet emissions are wholly
intercepted. We are thus enabled to operate at will upon the purely
ultra-red waves.

In the heating of solid bodies to incandescence, this non-visual
emission is the necessary basis of the visual. A platinum wire is
stretched in front of the table, and through it an electric current
flows. It is warmed by the current, and may be felt to be warm by the
hand. It emits waves of heat, but no light. Augmenting the strength of
the current, the wire becomes hotter; it finally glows with a sober
red light. At this point Dr. Draper many years ago began an
interesting investigation. He employed a voltaic current to heat his
platinum, and he studied, by means of a prism, the successive
introduction of the colours of the spectrum. His first colour, as
here, was red; then came orange, then yellow, then green, and lastly
all the shades of blue. As the temperature of the platinum was
gradually augmented, the atoms were caused to vibrate more rapidly;
shorter waves were thus introduced, until finally waves were obtained
corresponding to the entire spectrum. As each successive colour was
introduced, the colours preceding it became more vivid. Now the
vividness or intensity of light, like that of sound, depends not upon
the length of the wave, but on the amplitude of the vibration. Hence,
as the less refrangible colours grew more intense when the more
refrangible ones were introduced, we are forced to conclude that side
by side with the introduction of the shorter waves we had an
augmentation of the amplitude of the longer ones.

These remarks apply not only to the visible emission examined by Dr.
Draper, but to the invisible emission which precedes the appearance of
any light. In the emission from the white-hot platinum wire now before
you, the lightless waves exist with which we started, only their
int