That philosopher obtained a perfectly pure and complete coloured spectrum, with all its dark and bright lines, by the interference of light alone, from a sunbeam passing through a series of fine parallel wires covering the object glass of a telescope. In this spectrum, formed independently of prismatic refraction, the positions of the coloured rays depend only on the lengths of their waves, and M. Fraunhofer found that the intervals between them are precisely proportional to the differences of these lengths. He measured the lengths of the waves of the different colours at seven fixed points, determined by seven of the principal dark and bright lines. Professor Powell, availing himself of these measures, has made the requisite computations, and has found that the coincidence of theory with observation is perfect for ten substances whose refrangibility had been previously determined by the direct measurements of M. Fraunhofer, and for ten others whose refrangibility has more recently been ascertained by M. Rudberg. Thus, in the case of seven rays in each of twenty different substances, solid and fluid, the dispersion of light takes place according to the laws of the undulatory theory: and there can hardly be a doubt that dispersion in all other bodies will be found to follow the same law. It is, however, an express condition of the connexion between the velocity of light and the length of its undulations, that the intervals between the vibrating molecules of the ethereal fluid should bear a sensible relation to the length of an undulation. The coincidence of the computed with the observed refractions shows that this condition is fulfilled within the refracting media; but the aberration of the fixed stars leads to the inference that it does not hold in the ethereal regions, where the velocities of the rays of all colours are the same. Strong as all that precedes is in favour of the undulatory theory, the relative velocity of light in air and water is the final and decisive proof. By the Newtonian theory the velocity is greater in water than in air, by the undulatory theory it is less; hence if a comparison could be made it would decide which is the law of nature. The difficulty consisted in comparing the velocity of light passing through a small extent of water with the velocity of light in air, which is 10,000 times greater than the velocity of the earth in its orbit. This delicate and difficult experiment was made by means of an instrument invented by Professor Wheatstone for measuring the velocity of electricity. It consists of a small mirror which revolves in its own plane like a coin spinning on its edge. When it revolves very rapidly the reflected image of an object changes its place perceptibly in an inconceivably small fraction of a second. The mirrors used in the experiment were made to revolve more than 1000 times in a second, by which means the places of the two images—one from light passing through