The system of luminosity classification based on the correlation between the absolute luminosity and degree of development of spiral arms, which has previously been developed for late-type galaxies, is extended to include spirals of type Sb. The luminosity-classification system enables one to estimate the absolute magnitudes of individual galaxies with an accuracy of about half a magnitude Evidence is presented which tends to indicate that some spiral galaxies in clusters differ in appearance from similar galaxies in the general field All Shapley-Ames galaxies north of = -27 have been classified on the present system An analysis of the spatial distribution of these galaxies indicates the existence of a large relatively nearby cluster of galaxies in Canes Venatici.
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Sidney van den Bergh (1960) studied this question.
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