Since last meeting of the Society I have found that Broch (in Dove's Repertorium) employed the combination of prism, Nicols, and quartz plate, for the purpose of measuring the rotatory power of quartz for different wave-lengths. I do not find, however, that he suggests its use for the determination of wave-lengths according to one definite standard. Nor does he seem to have used great thicknesses of quartz, which is essential to accuracy in the application I have proposed. In the Annales de Chimie, 1846, there is a translation of a part of Broch's paper, with the remark that the process, which he called a new one, was due to Fizeau and Foucault. Their paper, however, refers to quartz cut parallel to the axis; but it introduces the question, very important so far as my object is concerned, of the interference of two polarized rays after one has been retarded more than the other by very many wave-lengths.