A solar beam of light is a bundle of rays; a ray being the smallest
portion of light which can emanate from a luminous body.
Each of these rays possesses distinctive characters, both as regards
their chemical functions and colors.
Sir Isaac Newton proved that the white light emitted [p085] by
the sun is not so simple as it appears, but is composed of vivid
colors, as shown by his beautiful experiment, the Analysis of Light,
which is exemplified by the use of a glass prism. (See fig. 1.)
[Illustration: FIG. 1.]
The ray of light A, E, being admitted into a darkened room through a
hole A, in the shutter, would fall upon the wall at E. As soon as
the prism, B, C, is placed in the path of the sunbeam so as to allow
it to fall on one of its angles B, the ray will be refracted, or
bent out of its course so as to pass through the prism (as in the
line D) and not in the same line, A, E, that it would have done had
the prism not been interposed.
Another effect also takes place; the ray of white light is
decomposed into its component colors, and if you stand at a short
distance from the prism, you will see that these colors are spread
out in a triangular form, the base of which is on the wall and the
apex at the angle C of the prism. Remove the prism and it is seen
that the splendid display of colors upon the wall has disappeared,
and a round spot of white light, E, is seen below the place occupied
by the spectrum.
The colored image on the wall is called the prismatic or solar
spectrum, which, according to Sir Isaac Newton, is composed
of seven different colors. The color at the lowest portion of
the image is red and the one at the other end is violet, the
intermediate parts being occupied by five other colors, and
the whole arranged [p086] according to the table below, the
proportion of each color having been measured by Fraunhofer with
the greatest care, with the results placed opposite to each,
corresponding with the 360 degrees of a circle, the red ray being
the least and the violet the most refracted of this chromatic
image.