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. LECTURE II. ORIGIN OF PHYSICAL THEORIES SCOPE OF THE IMAGINATION NEWTON AND THE EMISSION THEORY VERIFICATION OF PHYSICAL THEORIES THE LUMINIFEROUS ETHER WAVE THEORY OF LIGHT THOMAS YOUNG FRESNEL AND ARAGO CONCEPTION OF WAVE-MOTION INTERFERENCE OF WAVES CONSTITUTION OF SOUND-WAVES ANALOGIES OF SOUND AND LIGHT ILLUSTRATIONS OF WAVE-MOTION INTERFERENCE OF SOUND-WAVES OPTICAL ILLUSTRATIONS PITCH AND COLOUR LENGTHS OF THE WAVES OF LIGHT AND RATES OF VIBRATION OF THE ETHER-PARTICLES INTERFERENCE OF LIGHT PHENOMENA WHICH FIRST SUGGESTED THE UNDULATORY THEORY BOYLE AND HOOKE THE COLOURS OF THIN PLATES THE SOAP-BUBBLE NEWTON'S RINGS THEORY OF 'FITS' ITS EXPLANATION OF THE RINGS OVER-THROW OF THE THEORY DIFFRACTION OF LIGHT COLOURS PRODUCED BY DIFFRACTION COLOURS OF MOTHER-OF-PEARL. § 1. _Origin and Scope of Physical Theories_.