therefore, the leaves appear like faintly blushing roses, and present
a very beautiful appearance. With the blue ammonia-sulphate of copper,
which transmits no red, this effect is not obtained.

As the year advances the crimson gradually hardens to a coppery red;
and in the dark green leaves of old ivy it is almost absent.
Permitting a beam of white light to fall upon fresh leaves in a dark
room, the sudden change from green to red, and from red back to green,
when the violet glass is alternately introduced and withdrawn, is very
surprising. Looked at through the same glass, the meadows in May
appear of a warm purple. With a solution of permanganate of potash,
which, while it quenches the centre of the spectrum, permits its ends
to pass more freely than the violet glass, excellent effects are also
obtained.[7]

This question of absorption, considered with reference to its
molecular mechanism, is one of the most subtle and difficult in
physics. We are not yet in a condition to grapple with it, but we
shall be by-and-by. Meanwhile we may profitably glance back on the web
of relations which these experiments reveal to us. We have, firstly,
in solar light an agent of exceeding complexity, composed of
innumerable constituents, refrangible in different degrees. We find,
secondly, the atoms and molecules of bodies gifted with the power of
sifting solar light in the most various ways, and producing by this
sifting the colours observed in nature and art. To do this they must
possess a molecular structure commensurate in complexity with that of
light itself. Thirdly, we have the human eye and brain, so organized
as to be able to take in and distinguish the multitude of impressions
thus generated. The light, therefore, at starting is complex; to sift
and select it as they do, natural bodies must be complex; while to