amount of white light is at C; the yellow contains
less of the chemical power than any other portion of the solar
spectrum. It has been found that the most intense heat is at the
extreme red, b.

Artificial lights differ in their color; the white light of burning
charcoal, which is the principal light from candles, oil and gas,
contains three rays--red, yellow, and blue. The dazzling light emitted
from lime intensely heated, known as the Drummond light, gives the
colors of the prism almost as bright as the solar spectrum.

If we expose a prepared Daguerreotype plate or sensitive paper to the
solar spectrum, it will be observed that the luminous power (the
yellow) occupies but a small space compared with the influence of heat
and chemical power. R. Hunt, in his Researches on Light, has presented
the following remarks upon the accompanying illustration:

[Illustration: Fig. 3 (amdg_3.gif)]

"If the linear measure, or the diameter of a circle which shall include
the luminous rays, is 25, that of the calorific spectrum will be 42.10,
and of the chemical spectrum 55.10. Such a series of circles may well
be used to represent a beam from the sun, which may be regarded as an
atom of Light, surrounded with an invisible atmosphere of Heat, and
another still more extended, which possesses the remarkable property of
producing chemical and molecular change.

A ray of light, in passing obliquely through any medium of uniform
density, does not change its course; but if it should pass into a
denser body, it would turn from a straight line, pursue a less oblique
direction, and in a line nearer to a perpendicular to the surface of
that body. Water exerts a stronger refracting power than air; and if a
ray of light fall upon a body of this fluid its course is changed, as
may be seen by reference to Fig. 4.

[Illustration: Fig. 4 (amdg_4.gif)]

It is observed that it proceeds in a less oblique direction (towards
the dotted line), and, on passing on through, leaves the liquid,
proceeding in a line parallel to that at which it entered. It should
be observed that at the surface of bodies the refractive power is
exerted, and that the light proceeds in a straight line until leaving
the body. The refraction is more or less, and in all cases in
proportion as the rays fall more or less obliquely on the refracting
surface. It is this law of optics which has given rise to the lenses
in our camera tubes, by which means we are enabled t