The system of luminosity classes introduced by van den Bergh (1960a, b, c) is a major advance in galaxy classification. The principal observational materials available to van den Bergh were the paper prints of the National Geographic-Palomar Sky Survey, and his illustrations of the system were taken from them. Although satisfactory to define the principles of the system, these materials are sometimes inadequate owing to small scale and insufficient photographic latitude in the centers of high-surface-brightness spirals.
With this in mind, we have adopted the precepts of van den Bergh but have set up the standards anew. The system of the RSA is defined by these new type examples and is illustrated in the set of 15 panels that follow.
Each of the first 13 panels contains galaxies of the same luminosity class. The galaxies within each panel are arranged in order of their absolute magnitudes, so as to visualize at a glance how the form changes (if at all) with luminosity. The forms within any given luminosity class are similar, despite differences of at least a factor of 10 in absolute luminosity. From this, and from a detailed analysis (Sandage, Tammann, and Yahil, 1979; Tammann, Yahil, and Sandage, 1979), it is clear that although the luminosity-class designation is very useful as a descriptive device, the original hope that it is a good indicator of absolute luminosity has not been realized.
The great dispersion in absolute magnitude is illustrated explicitly in the last two panels, where extreme examples of the dispersion are shown.
Technical details and descriptions of the galaxies are given on the page facing each of the 15 sets of reproductions. The symbols preceding the plate number identify the telescope; the letters that follow identify the observer. The telescopes are coded as follows: P200 for the Palomar Hale 5-meter reflector, P48 for the 1.2-meter Palomar-Schmidt, W100 for the 2.5-meter Mount Wilson Hooker, W60 for the 1.5-meter Mount Wilson 60-inch, C100 for the Las Campanas 2.5-meter du Pont, and W10 for the Mount Wilson 10-inch refractor (used by Henize in South Africa for the SMC plate). Of the 84 galaxies illustrated (of which six are shown twice), 51 were photographed with the P200, 17 with the C100, 11 with the W100, 2 with the W60, 2 with the P48, and 1 with the W10.
The observers are coded as follows: B = Baade, Bm = Baum, Dr = Dressler, He = Henize, H = Hubble, MH = Humason, Ro = Rose, and S = Sandage.
Other information such as absolute magnitude, redshift, and orientation of the photograph are given in the legends. The scale of each photograph is indicated by the horizontal line marked on each reproduction. Most of these marks are 120 arcsec long, but a few are one-tenth of this. Note the differences in enlargement. Those prints having the longest horizontal lines are, of course, of galaxies with the smallest angular diameter, and hence the images contain fewer resolution elements. Note therefore that these generally show the most grain and the largest star images owing to the finite seeing.