[6] Mean of the positions given in Vols. III and IV. N.G.C. 7621 is 5ˢ.4 preceding, and 1′ 49″ south of 7623. There is a double star in the position published in the _Strassburg Annals_. TABLE III DISTRIBUTION OF VARIETIES OR CLASSES OF NEBULAE IN THE SEVEN CLUSTERS OR FIELDS ======+=========================================== | FIELD CLASS +----+----+-----+----+----+----+-----+------ | I | II | III | IV | V | VI | VII | Total ------+----+----+-----+----+----+----+-----+------ a | | | 1 | | | | | 1 c | | 1 | | | 1 | | 1 | 3 d | | 1 | 33 | 1 | | | 6 | 41 e | 27 | 39 | 109 | 41 | 27 | 21 | 20 | 284 f | 33 | 35 | 17 | 19 | 8 | 12 | 21 | 145 g | 7 | 3 | 4 | | 12 | 5 | 7 | 38 g₀ | 5 | | 8 | | | 3 | 1 | 17 h | 2 | 2 | 2 | 10 | 1 | 1 | 4 | 22 h₀ | 3 | 2 | 4 | | 1 | 2 | 2 | 14 i | | | | | | 1 | 2 | 3 k | 1 | | 2 | | | | | 3 n | | | | | | | 1 | 1 q | | 1 | | | | | 2 | 3 r | | | | | | 2 | | 2 v | | | | | | 1 | | 1 w | | 1 | 4 | | 2 | | | 7 irreg | | | 1 | 2 | | | | 3 ------+----+----+-----+----+----+----+-----+------ Messier 60 has a spiral as a very near neighbor--H III 44. The contrast in the two classes is well shown; so also in the case of N.G.C. 3379 mentioned above. Here there is a group of three fairly bright nebulae: N.G.C. 3379, a globular nebula of class e; 3389, an open spiral; and 3384, which might be called a spindle, except that the wings flare out from the nucleus. The three irregular forms are N.G.C. nebulae and are commented upon in the descriptions accompanying the positions. Very little can be said concerning the surface-brightness of these objects. It is independent of the distance so long as the angular diameter is sensibly greater than that of a faint star. The photographic plate therefore records the absolute surface-brightness of the nebulae. High luminosity is a comparatively rare attribute and there is some relation between luminosity and absolute size; that is to say, the brighter usually have the greater angular diameter. Since it is hard to conceive of a relation between distance and absolute brightness, the fact that the faint nebulae are usually the smaller can be interpreted only in the light of a relation between luminosity and absolute size.