The same principle of interference afforded explanation of the colours of striated surfaces, such as mother-of-pearl, which vary with the direction in which they are seen. Viewed at one angle light of a particular colour reflected from different ridges will be in a condition to interfere, and this colour will be absent from the reflected light. At a different inclination, the light reaching the eye from all the ridges (within a certain angle) will be in precisely the same phase, and only then will light of that colour be reflected in its full intensity. With a micrometer scale engraved on glass by Coventry, and containing five hundred lines to the inch, Young obtained interference spectra. Modern gratings, with several thousand lines to the inch, afford the purest spectra that can be obtained, and enable the wave-length of any particular kind of light to be measured with the greatest accuracy. Young's dislike of mathematical analysis prevented him from applying exact calculation to the interference phenomena which he observed, such as subsequently enabled Fresnel to overcome the prejudice of the French Academy and to establish the principle on an incontrovertible footing. Young's papers attracted very little attention, and Fresnel made for himself many of Young's earlier discoveries, but at once gave Young the full credit of the work when his priority was pointed out. The phenomena of polarization, however, still remained unexplained. Both Young and Fresnel had regarded the vibrations of light as similar to those of sound, and taking place in the direction in which the wave is propagated. The fact that light which had passed through a crystal of Iceland-spar, was differently affected by a second crystal, according to the direction of that crystal with respect to the former, showed that light which had been so transmitted was not like common light, symmetrical in all azimuths, but had acquired sides or poles. Such want of symmetry could not be accounted for on the hypothesis that the vibrations of light took place at right angles to the wave-front, that is, in the direction of propagation of the light. The polarization of light by reflection was discovered by Malus, in 1809. In a letter written to Arago, in 1817, Young hinted at the possibility of the existence of a component vibration at right angles to the direction of propagation, in light which had passed through Iceland-spar. In the following year Fresnel arrived independently at the hypothesis of transverse vibrations, not as constituting a small component of polarized light, but as representing completely the mode of vibration of all light, and in the hands of Fresnel this hypothesis of transverse vibrations led to a theory of polarization and double refraction both in uniaxal and biaxal crystals which, though it can hardly be regarded as complete from a mechanical point of view, is nevertheless one of the most beautiful and successful applications of