withstanding its marvellous
sensibility to certain impressions of this class, it is singularly
obtuse to other impressions.

Nor does the optic nerve embrace the entire range even of radiation.
Some rays, when they reach it, are incompetent to evoke its power,
while others never reach it at all, being absorbed by the humours of
the eye. To all rays 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
becomes sensibly warm to the touch; for a time its heat augments,
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 different rays are differently
refracted 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
sensibly 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
colours at the same time the platinum wire must be _white-hot_: the
impression of whiteness being in fact produced by the simultaneous
action of all these c