A photoelectric polarimeter.

A photoelectric polarimeter may be defined as a photoelectric photometer designed to measure the intensity and plane of that fraction of light from a given source that is polarized. The fundamental principle used in the device developed a~t Amherst is similar to that employed by Ohman.1 The principal differences lie in the nature of the analyzer used and in the method employed to make the observations. The analyzer in the Amherst polarimeter is a Glan Thompson prism. This prism permits measurements to be made over an extensive spectral region. The method employed to make the observations is such that both the plane of polarization and the intensity of the polarized light are indicated on an automatic recorder once every two minutes. Bench tests have indicated that 0.1 per cent of polarized light can be easily detected when a I P2 I is used as the photosensitive element. With a PbS cell this limit can be extended to one or two parts~in 10,000. The same limits can be achieved when the sun is used as a source and the seeing conditions are favorable. Tests with a I P2 I and the I 8-inch Amherst refractor indicate that it would be possible to detect ~` of one per cent of polarized light from stars of the seventh magnitude under average seeing conditions. Plans are being formulated for the observation of some eclipsing stars of special interest. This work has been supported in part by a grant from the Research Corporation of New York City. I.Ark. Mat. Astr. Fys. 29B, No. 12, 1943. A mherst College Observatory, Amherst, Mass.