The instrument used in the experiments forming the subject of this paper was a spectrum microscope, to which a polarising apparatus is attached, consisting of two Nicol's prisms, each of which is capable of being turned through any required number of degrees.

The first part of the paper relates to the spectra obtained when one or more thin films of mica or selenite are interposed between the polariser and the dispersion prisms, the light being subsequently analysed.

The method employed was, having first determined the axes of the films, to place them on the stage of the instrument which is rotatory, and to adjust them at various angles to the plane of polarisation.