A Wollaston photometer.

A photometer of the McDonald Observatory was converted for interchangeable use in either UB V photometry, or in six-color polarization measures. The polarization is analyzed with a Wollaston prism and two multiplier phototubes by simultaneous integration. A complete series on a fairly bright object, at six position angles and in six colors, takes about 60 minutes; this requires 144 integrations as at each angle the tubes are exposed twice, namely with and without a depolarizer in the incoming beam. Neither thin clouds nor poor seeing affect the polarization measures. The effe~tive wave lengths range from 3250 to 9900 A. Two types of tubes are used, namely RCA 7102, having S-I response, and RCA 7265, with multi-alkali cathode. Reductions of the observations are made with the IBM 650 of Indiana University. The internal probable errors are ~2o in position angle, larger when the polarization is very small, and ~0. I in the percentage polarization. Results on the main program of solar system studies are in a separate paper. As a check on this new equipment, eight strongly polarized stars were observed that occur in the lists of Hall and Mikesell and of W. A. Hiltner; in visual light the agreement is close. However, it was found that in the ultraviolet as well as in the infrared the interstellar polarization is less than in visual light. The sunlit blue sky shows approximately the same wavelength dependence. It was found that neither the 82-inch mirror nor the photometer cause depolarization in the ultraviolet; the observations and the depolarization test will be fully reported in another publication. Astronomy Department Indiana University Bloomington, Ind.