ght 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 reflection from the sky and that
from an ordinary cloud, which exercises no such dichroitic action.

The cloud, in fact, takes no note of size on the part of the waves of
ether, but reflects them all alike. Now the cause of this may be that
the cloud-particles are so large in comparison with the size of the
waves of ether as to scatter them all indifferently. A broad cliff
reflects an Atlantic roller as easily as it reflects a ripple produced
by a sea-bird's wing; and, in the presence of large reflecting
surfaces, the existing differences of magnitude among the waves of
ether may also disappear. But supposing the reflecting particles,
instead of being very large, to be very small, in comparison with the
size of the waves. Then, instead of the whole wave being fronted and
in great part thrown back, a small portion only is shivered off by the
obstacle. Suppose, then, such minute foreign particles to be diffused
in our atmosphere. Waves of all sizes impinge upon them, and at every
collision a portion of the impinging wave is struck off. All the waves
of the spectrum, from the extreme red to the extreme violet, are thus
acted upon; but in what proportions will they be scattered? Largeness
is a thing of relation; and the smaller the wave, the greater is the
relative size of any particle on which the wave impinges, and the
greater also the relative reflection.

A small pebble, placed in the way of the ring-ripples produced by
heavy rain-drops on a tranquil pond, will throw back a large fraction
of each ripple incident upon it, while the fractional part of a larger
wave thrown back by the same pebble might be infinitesimal. Now to
preserve the solar light white, its constituent proportions must not
be altered; but in the scattering of the light by these very small
particles we see that the proportions _are_ altered. The smaller waves
are in excess, and, as a consequence, in the scattered light blue will
be the predominant colour. The other colours of the spectrum must, to
some extent, be associated with the blue: they are not absent, but
deficient. We ought, in fact, to have them all, but in diminishing
proportions, from the violet to the red.

We have thus reasoned our way to the conclusion, that were particles,
small in comparison to the size of the e