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SCIENTIFIC USE OF THE IMAGINATION. 137

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 ana-
lyzing 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 vir-
tually pulled asunder, and they paint upon a white screen
placed to receive them “the solar spectrum.” Strictly
speaking, the Spectrum embraces an infinity of colors, 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 colors.

Separately, or mixed in various proportions, the solar
waves yield all the colors observed in nature and employed
in art. Collectively, they give us the impression of white-
ness. Pure unsifted solar light is white; and if all the
wave-constituents of such light be reduced in the same pro-
portion the light, though diminished in intensity, will still
be" white. The whiteness of Alpine 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 reflection, and when we lift
ourselves above a cloud-field—to an Alpine summit, for in-
stance, or to the top of Snowdon—and see, in the proper
direction, the sun shining on the clouds, they appear daz-
zlingly 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 conditions of whiteness
would fail if all the waves were diminished equally, or by
the same absolute quantity. They must be reduced pro-
portionately, instead of equally. If by the act of reflection

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