unt for the production of these colours from a ray of
light, Newton supposed that such a ray is actually made up of seven
distinct colours, which being mixed in proper proportions neutralize or
destroy each other. In order to account for the decomposition of the ray
of white light by the prism, and for the lengthened form of the
_spectrum_, as it is called, he supposed that each of the seven coloured
rays was capable of being bent by the prism in a different manner from
the rest. Thus, in the figure, the red appears to be less bent out of
the direction of the original ray than the orange--the orange less than
the yellow, and so on until we arrive at the violet, which is bent most
of all.

It is scarcely necessary to remark, that these views were found to be
correct, except as regards the number of colours in the solar spectrum;
for it is now ascertained, with tolerable certainty, that there are only
three primitive or pure colours in nature, and these are _red_, _yellow_,
and _blue_; and it is supposed that by mingling two or more of these
colours in various proportions, all the colours in nature are produced.

Now, to apply this explanation to the production of the rainbow, which is
usually seen under the following circumstances:--The observer is placed
with his back to the sun, and at some distance before him rain is
falling,--the air between the sun and the rain being tolerably clear. He
then often sees two circular arcs or bows immediately in front of him.
The colours of the inner bow are the more striking and vivid of the two.
Each exhibits the same series of colours as in the spectrum formed by the
prism; namely, _red_, _orange_, _yellow_, _green_, _blue_, _indigo_, and
_violet_; but the arrangement of these colours is different in the two
bows, for while in the inner bow the lower edge is violet and the upper
red, in the outer bow the lower edge is red and the upper violet. The
production of both bows is due to the refraction and reflexion of light,
the drops of rain forming, in fact, the prism which decomposes the white
light of the sun. The colours in the rainbow have the same proportional
breadth as the spaces in the prismatic spectrum. "The bow is,
therefore," as Sir D. Brewster remarks, "only an infinite number of
prismatic spectra, arranged in the circumference of a circle; and it
would be easy, by a circular arrangement of prisms, or by covering up all
the central part of a large lens, to produce a s