Multicolor photometry, over the range of wavelength from 0 35 in the ultraviolet to nearly 10 in the infrared, has been interpreted in terms of interstellar extinction. The value of R = -v has been found to vary from 3.0 to more than 6. In addition to the high value that had previously been found for the Orion Nebula region, values of R in the neighborhood of 6 have been found for regions in Cepheus and Ara, and for the cluster NGC 2244. These variations in R plainly have serious implications regarding the distance scale and the structure of the Galaxy.
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Fleming et al. (1986) studied this question.