THE INTERSTELLAR EXTINCTION LAW

A discussion is presented, from the point of view of an empiricist, of a number of topics which are important to our current understanding o f interstellar extinction. These include the misuse of mean reddening laws, the origins of differential reddening, and the question of varia tions in R, the ratio of total-to-selective extinction. The effectiven ess of detailed reddening studies -including reddening line determinat ions, dereddening of individual stars, and variable-extinction analyse s- of open clusters is demonstrated, with particular reference to R-de terminations for Tr 37 (R = 2.83 +/- 0.12), Sco-Oph (R = 3.34 +/- 0.08 ), and NGC 6611 (R = 2.99 +/- 0.08, with some evidence that R = 4.42 /- 0.10 in the dusty northern region) and to the discovery of new evid ence for circumstellar extinction. It is demonstrated that the observa ble variations in R from 2.8 and 3.0 to 3.3 in these cluster fields ar e tied to comparable variations in the reddening line slope E(U - B)/E (B - V) for these lines of sight (X = 0.80, 0.74, and 0.63, respective ly), but that the evidence does not support the arguments from IR, stu dies for anomalous extinction with values of R approximately 4 or more . The question is raised as to whether or not the highly-reddened star s in Sco-Oph serve as proper standards for studies of extinction law v ariations.