r 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. The phenomena certainly occur as if our atmosphere were
a medium rendered slightly turbid by the mechanical suspension of
exceedingly small foreign particles.

Here, as before, we encounter our sceptical 'as if.' It is one of the
parasites of science, ever at hand, and ready to plant itself and
sprout, if it can, on the weak points of our philosophy. But a strong
constitution defies the parasite, and in our case, as we question the
phenomena, probability grows like growing health, until in the end the
malady of doubt is completely extirpated. The first question that
naturally arises is this: 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 dissolve it; and when spirit holding 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 either in thick clots or in exceedingly fine
particles. 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, 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
may be seen on exceptional days among the Alps, but it is a very fair
sky-blue. A trace of soap in water gives a tint of blue. London, and
I fear Liverpool, milk makes an approximation to the same colour,
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