clouds, the solar light is not reflected by the sky in the
proportions which produce white. The sky is blue, which indicates an
excess of the shorter waves. In accounting for the colour of the sky,
the first question suggested by analogy would undoubtedly be, Is not
the air blue? The blueness of the air has, in fact, been given as a
solution of the blueness of the sky. But how, if the air be blue, can
the light of sunrise and sunset, which travels through vast distances
of air, be yellow, orange, or even red? The passage of white solar
light through a blue medium could by no possibility redden the light.

The hypothesis of a blue air is therefore untenable. In fact the
agent, whatever it is, which sends us the light of the sky, exercises
in so doing a dichroitic action. The light reflected is blue, the
light transmitted is orange or red. A marked distinction is thus
exhibited between the matter of the sky, and that of an ordinary
cloud, which exercises no such dichroitic action.

By the scientific use of the imagination we may hope to penetrate this
mystery. The cloud takes no note of size on the part of the waves of
aether, but reflects them all alike. It exercises no selective
action. Now the cause of this may be that the cloud particles are so
large, in comparison with the waves of aether, as to reflect them all
indifferently. A broad cliff reflects an Atlantic roller as easily as
a ripple produced by a seabird's wing; and in the presence of large
reflecting surfaces, the existing differences of magnitude among the
waves of aether may disappear. But supposing the reflecting
particles, instead of being very large, to be very small in comparison
with the size of the waves. In this case, instead of the whole wave
being fronted and thrown back, a small portion only is shivered off.
The great mass of the wave passes over such a particle without
reflection. Scatter, then, a handful of such minute foreign particles
in our atmosphere, and set imagination to watch their action upon the
solar waves. Waves of all sizes impinge upon the particles, and you
see at every collision a portion of the impinging wave struck off; all
the waves of the spectrum, from the extreme red to the extreme violet,
being thus acted upon.

Remembering that the red waves stand to the blue much in the relation
of billows to ripples, we have to consider whether those extremely
small particles are competent to scatter all the waves in t