After alluding to various methods which had been employed in investigating experimentally the nature of elliptically-polarized light, that is to say, the elements of the ellipse described, the author exhibited and described a new instrument which he had invented for the purpose. In its construction he had aimed at being in all important points independent of the instrument-maker, assuming nothing but the accuracy of the graduation.

The construction is as follows:—A brass rim, or thick annulus, is fixed on a stand, so as to have its plane vertical. A brass circle, graduated to degrees, turns round within the annulus, and the angle through which it is turned is read by verniers engraved on the face of the annulus. The brass circle is pierced at its centre, and carries on the side turned towards the incident light a plate of selenite, of such a thickness as to produce a difference of retardation in the oppositely polarized pencils amounting to about a quarter of an undulation for rays of mean refrangibility. On the side next the eye the brass circle carries a projecting collar, and round this collar there turns a moveable collar carrying verniers, and destined to receive a Nicol's prism.

The observation consists in extinguishing the light by a combination of the two movements. The retarding plate converts the elliptically-polarized light which has to be examined into plane-polarized, and this plane-polarized light is extinguished by the Nicol's prism.