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the sudden change from green to red, and from red back to green, when the violet glass is alternately introduced and withdrawn, is very surprising.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 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 take in the impressions thus generated, the human eye and brain, however we may simplify our conceptions of their action,[8] must be highly complex.

Whence this triple complexity?  If what are called material purposes were the only end to be served, a much simpler mechanism would be sufficient.  But, instead of simplicity, we have prodigality of relation and adaptation—­and this, apparently, for the sole purpose of enabling us to see things robed in the splendours of colour.  Would it not seem that Nature harboured the intention of educating us for other enjoyments than those derivable from meat and drink?  At all events, whatever Nature meant—­and it would be mere presumption to dogmatize as to what she meant—­we find ourselves here, as the upshot of her operations, endowed, not only with capacities to enjoy the materially useful, but endowed with others of indefinite scope and application, which deal alone with the beautiful and the true.