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. But Newton committed a graver error than this. Science, as I sought to make clear to you in our second lecture, is only in part a thing of the senses. The roots of phenomena are embedded in a region beyond the reach of the senses, and less than the root of the matter will never satisfy the scientific mind. We find, accordingly, in this career of optics the greatest minds constantly yearning to break the bounds of the senses, and to trace phenomena to their subsensible foundation. Thus impelled, they entered the region of theory, and here Newton, though drawn from time to time towards truth, was drawn still more strongly towards error; and he made error his substantial choice. His experiments are imperishable, but his theory has passed away. For a century it stood like a dam across the course of discovery; but, as with all barriers that rest upon authority, and not upon truth, the pressure from behind increased, and eventually swept the barrier away.