d some rays being more refracted than
others, we procure an elongated image of the luminous beam, exhibiting
three distinct colors, red, yellow and blue, which are to be regarded
as primitives--and from their interblending, seven, as recorded by
Newton, and shown in the accompanying wood cut. These rays being
absorbed, or reflected differently by various bodies, give to nature
the charm of color. Thus to the eve is given the pleasure we derive in
looking upon the green fields and forests, the enumerable varieties of
flowers, the glowing ruby, jasper, topaz, amethist, and emerald, the
brilliant diamond, and all the rich and varied hues of nature, both
animate and inanimate.

[Illustration: Fig. 3 (hipho_3.gif)]

Now, if we allow this prismatic spectrum (b. Fig. 3.) to fall upon any
surface (as at c.) prepared with a sensitive photographic compound, we
shall find that the chemical effect produced bears no relation to the
intensity of the light of any particular colored ray, but that, on the
contrary, it is dispersed over the largest portion of the spectrum,
being most energetic in the least luminous rays, and ever active over
an extensive space, where no traces of light can be detected. Fig. 4,
will give the student a better idea of this principle. It is a copy of
the kind of impression which the spectrum, spoken of, would make on a
piece of paper covered with a very sensitive photographic preparation.
The white space a. corresponds with the most luminous, or yellow ray,
(5, Fig. 3) over limits of which all chemical change is prevented. A
similar action is also produced by the lower end of the red ray c; but
in the upper portion, however we find a decided change (as at d). The
most active chemical change, you will perceive, is produced by the rays
above the yellow a; viz. 4, 3, 2 and 1 (as at b) the green (4) being
the least active, and the blue (3) and violet (1) rays the most so, the
action still continuing far beyond the point b which is the end of the
luminous image.

[Illustration: Fig. 4 (hipho_4.gif)]

Suppose we wish to copy by the Daguerreotype, or Calotype process, any
objects highly colored--blue, red and yellow, for instance
predominating--the last of course reflects the most light, the blue the
least; but the rays from the blue surface will make the most intense
impression, whilst the red radiations are working very slowly, and the
yellow remains entirely inactive. This accounts for the difficulty