something of the past growth and
present aspect of a department of science, in which have laboured some
of the greatest intellects the world has ever seen. I have sought to
confer upon each experiment a distinct intellectual value, for
experiments ought to be the representatives and expositors of
thought--a language addressed to the eye as spoken words are to the
ear. In association with its context, nothing is more impressive or
instructive than a fit experiment; but, apart from its context, it
rather suits the conjurer's purpose of surprise, than the purpose of
education which ought to be the ruling motive of the scientific man.

And now a brief summary of our work will not be out of place. Our
present mastery over the laws and phenomena of light has its origin in
the desire of man to _know_. We have seen the ancients busy with this
problem, but, like a child who uses his arms aimlessly, for want of
the necessary muscular training, so these early men speculated vaguely
and confusedly regarding natural phenomena, not having had the
discipline needed to give clearness to their insight, and firmness to
their grasp of principles. They assured themselves of the rectilineal
propagation of light, and that the angle of incidence was equal to the
angle of reflection. For more than a thousand years--I might say,
indeed, for more than fifteen hundred years--the scientific intellect
appears as if smitten with paralysis, the fact being that, during this
time, the mental force, which might have run in the direction of
science, was diverted into other directions.

The course of investigation, as regards light, was resumed in 1100 by
an Arabian philosopher named Alhazen. Then it was taken up in
succession by Roger Bacon, Vitellio, and Kepler. These men, though
failing to detect the principles which ruled the facts, kept the fire
of investigation constantly burning. Then came the fundamental
discovery of Snell, that cornerstone of optics, as I have already
called it, and immediately afterwards we have the application, by
Descartes, of Snell's discovery to the explanation of the rainbow.
Following this we have the overthrow, by Roemer, of the notion of
Descartes, that light was transmitted instantaneously through space.
Then came Newton's crowning experiments on the analysis and synthesis
of white light, by which it was proved to be compounded of various
kinds of light of different degrees of refrangibility.

Up to his demonstration