ter case it was reddish, being seen mainly by transmitted light.
Such smoke was not in exactly the condition to give us the glow of the
Alps, but it was a step in this direction. Bruecke's fine precipitate
above referred to looks yellowish by transmitted light; but, by duly
strengthening the precipitate, you may render the white light of noon
as ruby-coloured as the sun, when seen through Liverpool smoke, or
upon Alpine horizons. I do not, however, point to the gross smoke
arising from coal as an illustration of the action of small particles,
because such smoke soon absorbs and destroys the waves of blue,
instead of sending them to the eyes of the observer.

These multifarious facts, and numberless others which cannot now be
referred to, are explained by reference to the single principle, that,
where the scattering particles are small in comparison to the
aethereal waves, we have in the reflected light a greater proportion
of the smaller waves, and in the transmitted light a greater
proportion of the larger waves, than existed in the original white
light. The consequence, as regards sensation, is that in the one ease
blue is predominant, and in the other orange or red. Our best
microscopes can readily reveal objects not more than 1/50000th of an
inch in diameter. This is less than the length of a wave of red
light. Indeed a first-rate microscope would enable us to discern
objects not exceeding in diameter the length of the smallest waves of
the visible spectrum. [Footnote: Dallinger and Drysdale have recently
measured cilia 1/200000th of an inch in diameter. 1878.] By the
microscope, therefore, we can test our particles. If they be as large
as the light-waves they will infallibly be seen; and if they be not so
seen, it is because they are smaller. Some months ago I placed in the
hands of our President a liquid containing Bruecke's precipitate. The
liquid was milky blue, and Mr. Huxley applied to it his highest
microscopic power. He satisfied me that had particles of even
1/100000th of an inch in diameter existed in the liquid, they could
not have escaped detection. But no particles were seen. Under the
microscope the turbid liquid was not to be distinguished from
distilled water. [Footnote: Like Dr. Burdon Sanderson's 'pyrogen,'
the particles of mastic passed, without sensible hindrance, through
filtering-paper. By such filtering no freedom from suspended
particles is secured. The application of a condens