bly exist among the light-giving waves. And
if we take the whole range of solar radiation into account--its
non-visual as well as its visual waves--I think it probable that the
force, or energy, of the largest wave is more than a million times
that of the smallest.

Turned into their equivalents of sensation, the different light-waves
produce different colours. Red, for example, is produced by the
largest waves, violet by the smallest, while green is produced by a
wave of intermediate length and amplitude. On entering from air into
a more highly refracting substance, such as glass or water, or the
sulphide of carbon, all the waves are retarded, but the smallest ones
most. This furnishes a means of separating the different classes of
waves from each other; in other words, of analysing the light.

Sent through a refracting prism, the waves of the sun are turned aside
in different degrees from their direct course, the red least, the
violet most. They are virtually pulled asunder, and they paint upon a
white screen placed to receive them 'the solar spectrum.' Strictly
speaking, the spectrum embraces an infinity of colours; but the limits
of language, and of our powers of distinction, cause it to be divided
into seven segments: red, orange, yellow, green, blue, indigo, violet.
These are the seven primary or prismatic colours.

Separately, or mixed in various proportions, the solar waves yield all
the colours observed in nature and employed in art. Collectively,
they give us the impression of whiteness. Pure unsifted solar light
is white; and, if all the wave-constituents of such light be reduced
in the same proportion, the light, though diminished in intensity,
will still be white. The whiteness of snow with the sun shining upon
it, is barely tolerable to the eye. The same snow under an overcast
firmament is still white. Such a firmament enfeebles the light by
reflecting it upwards; and when we stand above a cloud-field--on an
Alpine summit, for instance, or on the top of Snowdon--and see, in the
proper direction, the sun shining on the clouds below us, they appear
dazzlingly white. Ordinary clouds, in fact, divide the solar light
impinging on them into two parts--a reflected part and a transmitted
part, in each of which the proportions of wave-motion which produce
the impression of whiteness are sensibly preserved.

It will be understood that the condition of whiteness would fail if
all the waves were diminis