ther waves, sown in our
atmosphere, the light scattered by those particles would be exactly
such as we observe in our azure skies. And, indeed, when this light is
analyzed, all the colours of the spectrum are found in the proportions
indicated by our conclusion.

By its successive collisions with the particles the white light is
more and more robbed of its shorter waves; it therefore loses more and
more of its due proportion of blue. The result may be anticipated. The
transmitted light, where moderate distances are involved, will appear
yellowish. But as the sun sinks towards the horizon the atmospheric
distance increases, and consequently the number of the scattering
particles. They weaken in succession the violet, the indigo, the blue,
and even disturb the proportions of green. The transmitted light under
such circumstances must pass from yellow through orange to red. This
also is exactly what we find in nature. Thus, while the reflected
light gives us, at noon, the deep azure of the Alpine skies, the
transmitted light gives us, at sunset, the warm crimson of the Alpine
snows.

But can small particles be really proved to act in the manner
indicated? No doubt of it. Each one of you can submit the question to
an experimental test. Water will not dissolve resin, but spirit will;
and when spirit which holds resin in solution is dropped into water,
the resin immediately separates in solid particles, which render the
water milky. The coarseness of this precipitate depends on the
quantity of the dissolved resin. Professor Bruecke has given us the
proportions which produce particles particularly suited to our present
purpose. One gramme of clean mastic is dissolved in eighty-seven
grammes of absolute alcohol, and the transparent solution is allowed
to drop into a beaker containing clear water briskly stirred. An
exceedingly fine precipitate is thus formed, which declares its
presence by its action upon light. Placing a dark surface behind the
beaker, and permitting the light to fall into it from the top or
front, the medium is seen to be of a very fair sky-blue. A trace of
soap in water gives it a tint of blue. London milk makes an
approximation to the same colour, through the operation of the same
cause: and Helmholtz has irreverently disclosed the fact that a blue
eye is simply a turbid medium.

Sec. 12. _Artificial Sky_.

But we have it in our power to imitate far more closely the natural
conditions of this problem