The range in the stars involved is about 2.5 mag., and in the total luminosities of the nebulae, about 3.8 mag. This latter is consistent with the scatter in the diagram exhibiting the relation between total luminosities and diameters. The associated objects, M 31 and 32, represent the extreme limits among the known systems, and the mean of these two is very close to the mean of them all. LUMINOSITY OF STARS INVOLVED IN NEBULAE The number of nebulae of known distance is too small to serve as a basis for estimates of the range in absolute magnitude among nebulae in general. Further information, however, can be derived from a comparison of total apparent magnitudes with apparent magnitudes of the brightest stars involved, on the reasonable assumption, supported by such evidence as is available, that the brightest stars in isolated systems are of about the same intrinsic luminosity. The most convenient procedure is to test the constancy of the differences in apparent magnitude between the brightest stars involved and the nebulae themselves, over as wide a range as possible in the latter quantities. An examination of the photographs in the Mount Wilson collection has revealed no stars in the very faint objects or in the bright elliptical nebulae and early-type spirals. This was to be expected from the conclusions previously derived. Observations were therefore confined to intermediate- and late-type spirals and the irregular nebulae to the limiting visual magnitude 10.5. The Magellanic Clouds and N.G.C. 6822 were added to the nebulae in Holetschek’s list. Altogether, data were available for 32 objects, or about 60 per cent of the total number in the sky to the adopted limit. For this reason it is believed that the results are thoroughly representative. TABLE XVI Difference in Magnitude between Nebulae and Their Brightest Stars ==========+=============+============+=============== N.G.C. | m_{s} | m_{T} | m_{s} – m_{T} ----------+-------------+------------+--------------- Sb ----------+-------------+------------+--------------- 224 | 15.5 | 5.0 | 10.5 1068 | 17.5 | 9.1 | 8.4 2841 | >19.5 | 9.4 | >10.1 3031 | 18.5 | 8.3 | 10.2 3310 | >19.0 | 10.4 | > 8.6 3623 | >20.0 | 9.9 | >10.1 3627 | 18.5 | 9.1 | 9.4 4438 | >19.0 | 10.3 | > 8.7 4450 | 19.5 | 10.0 | 9.5 4736 | 17.3 | 8.4 | 8.9 4826 | >19.5 | 9.2 | >10.3 5055 | >19.0 | 9.6 | > 9.4 5746 | >19.5 | 10.4 | > 9.1 7331 | 19.0 | 10.4 | 8.6 ----------+-------------+------------+---------------