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 TH