An easily constructed apparatus, giving illumination by light of any chosen colour, and of adequate purity in a narrow spectral region, has been made and found satisfactory in the determination of the optical properties of crystals.

Light of high intensity is focused on an adjustable slit, rendered approximately parallel by means of a lens, and passed through two hollow prisms containing alpha-monochloronaphthalene. In the widely divergent spectral beams the microscope moves along a calibrated arc and any desired wave-band of reasonable purity may be selected. The apparatus is illustrated in the accompanying diagram (fig. 1) and photograph (fig. 2).

A small carbon arc (A) using direct or alternating current gives adequate intensity, allowing for a narrow setting of the slit, with corresponding purity of the monochromatic beam.