Wavelength dependence of interstellar linear polarization has been observed for about 180 stars, mostly southern, in the UBVR spectral regions. A multichannel polarimeter-photometer, in which spectral regions are separated by dichroic filters, was used. Normalized wavelength dependence of interstellar linear polarization p follows closely a single empirical curve p(X)lp ma , x = exp [-1.15 ln 2 (A max /A)], where the wavelength A max at which the maximum interstellar linear polarization /? raax occurs takes values from 0.45 /x to 0.8 p.Wavelength A max is well correlated with the ratios of color excesses Eand Ev-^Ev-r. These correlations indicate that the ratio R of total to selective interstellar extinction can be found for any individual star from the relationship R = 5.5 A max . Polarimetry seems to be the most practical method of estimating R. A map of distribution of A max on the sky, based on values for about 350 stars, indicates several well defined regions with A max , and hence R, clearly larger (or smaller) than the median value A max = 0.545 /x, corresponding to i? = 3.0.The predominance of larger than average values of A max among stars nearer than 0.4 kpc and the negative correlation between A max and E B -v are explained by selection effects. There is evidence of negative correlation between A max and p m eLxlE Bv suggested by Kruszewski. The lower limits for color excess of Praesepe, M67, and several globular clusters are set by their linear polarization. The largest known values of interstellar circular polarization, \q\ ^ 0.06 percent, were found in near-infrared for two stars with exceptionally small A max : star No. 12 in association VI Cygni and HD 204827. Subject headings: instruments -interstellar extinction -interstellar matter -open clustersphotometry -polarization

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The polarization of the stars in open clusters, explained on the basis of the Davis-Greenstein theory, gives some information on the microstructure of the galactic magnetic field.The polarization is most conveniently described by the parameters Q, U, proportional to the Stokes parameters and defined by where p is the amount of polarization, θ is the position angle of the electric vector, and θ̄ is the mean value of θ for the region under consideration.

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