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SCIENTIFIC USE OF THE IMAGINATION. 139

soluble antecedent, and that you will manfully and woman-
fully prolong your investigations of the ether and its waves
into regions which have been hitherto crossed by the
pioneers of science alone.

Not only are the waves of ether reflected by clouds,
by solids, and by liquids, but when they pass from light air
to dense, or from dense air to light, a portion of the wave-
motion is always reflected. Now our atmosphere changes
continually in density from top to bottom. It will help
our conceptiOns if weregard it as made up of a series of thin
concentric layers, or shells of air, each shell being of the
same density throughout, and a small and sudden change
of density occurring in passing from shell to shell. Light
would be reflected at the limiting surfaces of all these shells,
and their action would be practically the same as that of the
real atmosphere. And now I would ask your imagination
to picture this act of reflection. What must become of the
reflected light ? The atmospheric layers turn their convex
surfaces toward the sun ; they are so many convex mirrors
of feeble power, and you will immediately perceive that the
light regularly reflected from these surfaces cannot reach
the earth at all, but is dispersed in space.

But though the sun’s light is not reflected in this fashion
from the aerial layers to the earth, there is indubitable evi-
dence to show that the light of our firmament is reflected
light. Proofs of the most cogent description could be here
adduced; but°we need only consider that we receive light
at the same time from all parts of the hemiSphere of heav-
en. The light of the firmament comes to us across the di-
rection of the solar rays, and even against the direction of
the solar rays ; and this lateral and Opposing rush of wave-
motion can only be due to the rebound of the waves from
the air itself, or from something suspended in the air. It is
also evident that, unlike the action of clouds, the solar light
is not reflected by the sky in the proportions which produce

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