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Published online by Cambridge University Press: 14 August 2015
One hundred and fifty six open clusters with modern photometry, and apparent diameter measures by Trumpler, have been used to measure the mean ratio of total to selective extinction, R̄ It is found that cluster diameter is related to both concentration class and richness class. Diameter also seems to depend on the background stellar density and to decrease with reddening. With such effects measured and removed R̄ = 3.15 ± 0.20. This result is not significantly dependent on the cluster's position in space.