re transparent, and the consequence is that, when white light
falls upon a mixture of yellow and blue powders, the green alone is
sent back to the eye. You have already seen that the fine blue
ammonia-sulphate of copper transmits a large portion of green, while
cutting off all the less refrangible light. A yellow solution of
picric acid also allows the green to pass, but quenches all the more
refrangible light. What must occur when we send a beam through both
liquids? The experimental answer to this question is now before you:
the green band of the spectrum alone remains upon the screen.

The impurity of natural colours is strikingly illustrated by an
observation recently communicated to me by Mr. Woodbury. On looking
through a blue glass at green leaves in sunshine, he saw the
superficially reflected light blue. The light, on the contrary, which
came from the body of the leaves was crimson. On examination, I found
that the glass employed in this observation transmitted both ends of
the spectrum, the red as well as the blue, and that it quenched the
middle. This furnished an easy explanation of the effect. In the
delicate spring foliage the blue of the solar light is for the most
part absorbed, and a light, mainly yellowish green, but containing a
considerable quantity of red, escapes from the leaf to the eye. On
looking at such foliage through the violet glass, the green and the
yellow are stopped, and the red alone reaches the eye. Thus regarded,
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when the violet glass is alternately introduced and withdrawn, is very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