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which, whether they reach the retina or not, fail to excite
vision, we give the name of invisible or obscure rays. All
non-luminous bodies emit such rays. There is no body in
Nature absolutely cold, and every body not absolutely cold
emits rays of heat. But to render radiant heat fit to affect
the optic nerve a certain temperature is necessary. A cool
poker thrust into a fire remains dark for a time, but when
its temperature has become equal to that of the surrounding
coals it glows like them. In like manner, if a current of
electricity of gradually increasing strength be sent through
a wire of the refractory metal platinum, the wire first be-'
comes sensibly warm to the touch; for a time its heat aug-
ments, still, however, remaining obscure; at length we can
no longer touch the metal with impunity; and at a certain
definite temperature it emits a feeble red light. As the
current augments in power the light augments in brilliancy,
until finally the wire appears of a dazzling white. The
light which it now emits is similar to that of the sun.

By means of a prism Sir Isaac Newton unravelled the
texture of solar light, and by the same simple instrument
We can investigate the luminous changes of our platinum
wire. In passing through the prism all its rays (and they
are infinite in variety) are bent or refracted from their
straight course; and as diiferent rays are differently re-
fracted by the prism, we are by it enabled to separate one
class of rays from another. By such prismatic analysis Dr.
Draper has shown that, when the platinum wire first begins
to glow, the light emitted is a pure red. As the glow
augments the red becomes more brilliant, but at the same
time orange rays are added to the emission. Augmenting
the temperature still further, yellow rays appear beside the
orange, after the yellow green rays are emitted, and after
the green come, in succession, blue, indigo and violet rays.
To display all these colors at the same time the platinum
wire must be white-hot .° the impression of whiteness being

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