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 of interstellar extinction. These include the misuse of mean reddening laws, the origins of differential reddening, and the question of variations in P, the ratio of tot al-to- selective extinction. The effectiveness of detailed reddening studies including reddening line determinations, dereddening of individual stars, and variable- extinction analyses of open clusters is demonstrated, with particular reference to P-determinations for Tr 37 (P = 2.83 0.12), Sco-Oph (P = 3.34 1 0.08), and NGC 6611 (P = 2.99 1 0.08, with some evidence that P = 4.42 1 0.10 in the dusty northern region) and to the discovery of new evidence for circumstellar extinction. It is deinonstrated that the observable variations in P from 2.8 and 3.0 to 3.3 in these cluster fields are 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, respectively), but that the evidence does not support the arguments from IR studies for anomalous extinction with values of P 4 or more. The question is raised as to whether or not the highly-reddened stars in Sco-Oph serve as proper standards for studies of extinction law variations.