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SCIENTIFIC USE OF THE IMAGINATION. 143

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. You can
cause it to separate in thick clots or in exceedingly fine
particles. Professor Brilcke 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 trans-
parent solution is allowed to drop into a beaker containing
clear water kept 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 distinctly blue. It is not
perhaps so perfect a blue as I have seen on exceptional
days, this year, among the Alps, but it is a very fair sky-
blue. A trace of soap in water gives a tint of blue. Lon-
don, and I fear Liverpool milk, makes an approximation to
the same color through the Operation of the same cause;
and Helmholtz has irreverently disclosed the fact that the
deepest blue eye is simply a turbid medium.

The action of turbid media upon light was illustrated
by Goethe, who, though unacquainted with the undula-
tory theory, was led by his experiments to regard the
firmament as an illuminated turbid medium with the dark-
ness of space behind it. He describes glasses showing a
bright yellow by transmitted, and a beautiful blue by re-
flected light. Professor Stokes, who was probably the first
to discern the real nature of the action of small particles
on the waves of ether, describes a glass of a similar kind.1

1 This glass, by reflected light, had a. color “ strongly resembling that
of a decoction of a horse-chestnut bark.” Curiously enough, Goethe
refers to this very decoction: “Man nehme einen Streifen frischer Rinde

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