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STRUCTURE AND LIGHT OF THE SKY. 263

we should have the solarlight reflected; in two others un-
reflected. In fact, out of such a solitary beam, traversing
the unilluminated air, we should be able to extract every
effect shown by our incipient cloud. In the production of
such clouds we virtually carry bits of the sky into our
laboratories, and obtain with them all the effects obtainable
1n the open firmament of heaven.

The real sky is, as I have said, less perfect than our
artificial one may be made. For, mingled with the infini-
tesimal particles which constitute the true matter of the
sky, there are others too coarse to scatter perfectly po-
larized light at right angles to the solar beams. Hence,
when the brilliancy of the sky is diminished to the utter-
most, there is still a residue of light; the extinction is
partial, and not total, as in the case of our incipient cloud.
Let us consider this matter. The perfect polarization can
only be produced by excessively minute particles; imagine
them growing gradually larger as they actually do in our
experiments. The extinction by the Nicol is perfect as
long as the polarization is complete. But what would you
expect? Manifestly, that after a time the polarization
would cease to be perfect. But here again the relation of
the size of the particles to the size of the waves must come
into play. In relation to the blue waves the particles are
larger than in relation to the red; the blue waves, there-
fore, will be the first liberated from a condition dependent
on the smallness of the particles. They will first escape
from the trammels of polarization; and on their liberation
they exhibit an azure far purer and more brilliant than that
produced by the first precipitation of the particles. Could
we overarch ourselves with a sky of this color for a single
day, it would make us discontented with our present lack-
lustre firmament ever afterward. It will be observed that
in all these cases reason and experiment go hand in hand,
the one predicting, the other verifying; every such verifi-

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