When anything takes time to travel from one place to another, it can only be one of two things. It must either be an actual object which is transferred bodily from place to place, like a letter sent by post or a bullet fired from a gun, or else it must be a wave-motion created in a medium of some kind which fills all space. The illustrious Newton suggested an hypothesis or supposition as to the nature of light, viz. that it consists of small _corpuscles_ shot out violently from every luminous body. It is a wonderful testimony to Newton’s exalted powers of thought, that the most recent investigations show that hot and luminous bodies, such as the sun and a lamp, are in fact projecting small bodies called corpuscles into space, but there is abundant proof that these are not the cause of light. Subsequently to the date of Newton’s speculations on the nature of light, the alternative hypothesis was developed, viz. that it consists in a wave-motion in a universally diffused medium called the æther. A great gulf, however, separates mere conjecture and speculation from that accumulation of rigid proof which scientific investigation demands, and hence, although this conception of an æther had arisen as an hypothesis in the minds of Huyghens, Descartes, and many other philosophers, it was not accepted by Newton, and the general assent of scientific investigators to the hypothesis of a universal æther was long deferred. The philosopher to whom we owe the crucial demonstration of the validity of, and indeed necessity for, this assumption was Dr. Thomas Young, the first Professor of Natural Philosophy in the Royal Institution of London. Young was a man whose exalted intellectual powers were not properly appreciated by the world until after his decease. His researches in physical optics alone are, however, epoch-making in character. He it was who first gave a proof that under some circumstances it is possible for two rays of light to destroy each other, and thus produce darkness. Briefly described, the experiment is as follows: If a beam of light of one colour, say red, proceeding from a single source of light, falls upon a screen in which are two small holes very near together, we shall obtain from these holes two streams of light originating, as it were, from closely contiguous sources. If we then hold a white screen not far from these holes, and receive on it the light proceeding from them, we shall find that the screen is marked with alternate bands of red light and black bands. If we cover up one of the small holes, the black bands vanish and the screen is uniformly illuminated. Young pointed out that this effect was due to _interference_, and that the difference of the distances from any black band to the two holes was an exact odd multiple of a certain small distance called the wave-length of the light. If light is a substance, no possible explanation can be given which will enable us to account for the combination of two rays