The work of Thomas Young was done in the very first years of the nineteenth century. He was for two years professor of Natural Philosophy in the Royal Institution just founded by Count Rumford, and he was the first to fill that chair. In 1801, in a paper presented to the Royal Society, he argued in favor of the undulatory theory, showing how the interference of waves would explain the color of thin plates. His papers were not, for several years, received favorably, and they were severely criticised by Lord Brougham. Augustus Fresnel followed Young, but quite independently, about ten years later, and by him the undulatory theory received elaborate experimental and mathematical treatment. In the mean time another Frenchman had made a capital discovery in optics, which seemed at first to be quite incompatible with the wave theory. This was the discovery of what is known as _polarization of light_ by Malus, a French engineer, who hit upon it while investigating double refraction of crystals, for a study of which the French Institute had offered a prize in 1808. Malus found that when light fell upon a surface of glass at a certain angle a portion of the reflected light appeared to have acquired entirely new properties in regard to further reflection, and the same was true of that part of the beam which was transmitted through the glass. The light thus affected was incapable of further reflection under certain conditions, and as the beam seemed to behave differently according to how it was presented to the reflecting surface, the term polarization was applied to the phenomenon. It was found that the two rays into which a single beam of light was split by a doubly refracting crystal (a phenomenon which had long been known) were affected in this way, and that light was polarized by refraction as well as by reflection. Malus was a believer in the corpuscular theory of light, but it was shortly proved, first by Thomas Young, that the phenomenon of polarization was not only not opposed to the wave theory, but that that theory furnished a rational explanation of it. This explanation, in brief, assumes that ordinary light is a wave produced by a vibratory motion confined to no particular plane, the direction of vibration being at right angles to the direction of the wave, and in any, or, in rapid succession, in _all_ azimuths. When light is polarized the vibratory motion in the ether is restricted to one particular form, a line if plane polarized, a circle or an ellipse if circularly or elliptically polarized. This simple hypothesis has been found quite adequate, and through its application to the various phenomena of polarization, together with the application of Young’s theory of the interference of waves to the production of color, the undulatory theory of light was firmly established before the middle of the century. There were many noted philosophers, however, who stood out long against it, notably Brewster,