My desire in these lectures has been to show you, with as little breach of continuity as possible, 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 of the composition of white light, Newton had been everywhere triumphant—­triumphant in the heavens, triumphant on the earth, and his subsequent experimental work is, for the most part, of immortal value.  But infallibility is not an attribute of man, and, soon after his discovery of the nature of white light, Newton proved himself human.  He supposed that refraction and chromatic dispersion went hand in hand, and that you could not abolish the one without at the same time abolishing the other.  Here Dollond corrected him.