This conception of an ether was advocated, and successfully applied to various phenomena of optics, by the illustrious astronomer, Huyghens.  He deduced from it the laws of reflection and refraction, and applied it to explain the double refraction of Iceland spar.  The theory was espoused and defended by the celebrated mathematician, Euler.  They were, however, opposed by Newton, whose authority at the time bore them down.  Or shall we say it was authority merely?  Not quite so.  Newton’s preponderance was in some degree due to the fact that, though Huyghens and Euler were right in the main, they did not possess sufficient data to prove themselves right.  No human authority, however high, can maintain itself against the voice of Nature speaking through experiment.  But the voice of Nature may be an uncertain voice, through the scantiness of data.  This was the case at the period now referred to, and at such a period, by the authority of Newton, all antagonists were naturally overborne.

The march of mind is rhythmic, not uniform, and this great Emission Theory, which held its ground so long, resembled one of those circles which, according to your countryman Emerson, the intermittent force of genius periodically draws round the operations of the intellect, but which are eventually broken through by pressure from behind.  In the year 1773 was born, at Milverton, in Somersetshire, a circle-breaker of this kind.  He was educated for the profession of a physician, but was too strong to be tied down to professional routine.  He devoted himself to the study of natural philosophy, and became in all its departments a master.  He was also a master of letters.  Languages, ancient and modern, were housed within his brain, and, to use the words of his epitaph, ’he first penetrated the obscurity which had veiled for ages the hieroglyphics of Egypt.’  It fell to the lot of this man to discover facts in optics which Newton’s theory was incompetent to explain, and his mind roamed in search of a sufficient theory.  He had made himself acquainted with all the phenomena of wave-motion; with all the phenomena of sound; working successfully in this domain as an original discoverer.  Thus informed and disciplined, he was prepared to detect any resemblance which might reveal itself between the phenomena of light and those of wave-motion.  Such resemblances he did detect; and, spurred on by the discovery, he pursued his speculations and experiments, until he finally succeeded in placing on an immovable basis the Undulatory Theory of Light.

The founder of this great theory was Thomas Young, a name, perhaps, unfamiliar to many of you, but which ought to be familiar to you all.  Permit me, therefore, by a kind of geometrical construction which I once ventured to employ in London, to give you a notion of the magnitude of this man.  Let Newton stand erect in his age, and Young in his.  Draw a straight line from Newton to Young, tangent to the heads of