The dependence of the extinction ratio on galactic longitude

Evidence is presented for a small but significant variation in the ratio of total to selective extinction, R, with galactic longitude. R is found to be systematically higher by ∼ 0.3 in the Southern Milky Way than in the North, a conclusion which is supported by a study of variations in the wavelength of maximum polarization. The polarization data suggest a dependence for R of the form R = R0 + R1 sin(l + θ) where R0 = 3.08 ± 0.03, R1 = 0.l7 ± 0.06 and θ = 175° ± 20°. The most probable explanation of this effect is a variation in the mean size of the dust grains in the local spiral arm.