What is Light? The ancients supposed it to be something emitted by the eyes, and for ages no notion was entertained that it required time to pass through space. In the year 1676 Roemer first proved that the light from Jupiter's satellites required a certain time to cross the earth's orbit. Bradley afterwards found that, owing to the velocity with which the earth flies through space, the rays of the stars are slightly inclined, just as rain-drops which descend vertically appear to meet us when we move swiftly through the shower. In Kew Gardens there is a sun-dial commemorative of this discovery, which is called the _aberration of light_. Knowing the velocity of the earth, and the inclination of the stellar rays, Bradley was able to calculate the velocity of light; and his result agrees closely with that of Roemer. Celestial distances were here involved, but a few years ago M. Fizeau, by an extremely ingenious contrivance, determined the time required by light to pass over a distance of about 9000 yards; and his experiment is quite in accordance with the results of his predecessors. But what is it which thus moves? Some, and among the number Newton, imagined light to consist of particles darted out from luminous bodies. This is the so-called Emission-Theory, which was held by some of the greatest men: Laplace, for example, accepted it; and M. Biot has developed it with a lucidity and power peculiar to himself. It was first opposed by the astronomer Huyghens, and afterwards by Euler, both of whom supposed light to be a kind of undulatory motion; but they were borne down by their great antagonists, and the emission-theory held its ground until the commencement of the present century, when Thomas Young, Professor of Natural Philosophy in the Royal Institution, reversed the scientific creed by placing the Theory of Undulation on firm foundations. He was followed by a young Frenchman of extraordinary genius, who, by the force of his logic and the conclusiveness of his experiments, left the Wave-Theory without a competitor. The name of this young Frenchman was Augustin Fresnel. Since his time some of the ablest minds in Europe have been applied to the investigation of this subject; and thus a mastery, almost miraculous, has been attained over the grandest and most subtle of natural phenomena. True knowledge is always fruitful, and a clear conception regarding any one natural agent leads infallibly to better notions regarding others. Thus it is that our knowledge of light has corrected and expanded our knowledge of _heat_, while the latter, in its turn, will assuredly lead us to clearer conceptions regarding the other forces of Nature.