Some Problems Concerning the Structure and Dynamics of the Galactic System and the Elliptical Nebulae NGC 3115 and 4494.

After a general introduction, in which some features of the density distribution and velocity distribution are discussed in connection with the possibifity that the galaxy may have a spiral-like structure, the author attempts in the third section to elucidate the relation between density distribution and distribution of peculiar velocities. It is shown that if the large-scale unevenness in density found in a recent study is accepted, it is natural to expect a deviation of the stream vertex from the longitude of the galactic center of similar order as the deviation actually observed. The distribution of light in the nearly spherical nebula NGC 44~4 and in the strongly flattened elliptical nebula NGC 3115 has been investigated with the aid of plates taken by Dr. Oosterhoff with the 6o-inch reflector on Mount Wilson. The results, as shown in Table i and Figures 3 and 4, can be fairly well represented by Hubble's formula I = Io/(' + r/a)2, though some small but probably real deviations are indicated. The isophotes in NGC ~ i i5, of which a few typical ones are shown in Figure 5, deviate markedly from effipses and show remarkably sharp points at the equator. The ratio of the axes varies with the distance from the center; it has a minimum for a semimajor axis of i' (see Fig. 6). The space distribution of light in NGC ~ii5 has been calculated; a section with isophotal surfaces is shown in Figure 7, while the run of space density in the equatorial plane may be found in the last column of Table i and in the dotted curve in Figure 4. The light density near the center is of the same order as that in the center of the globular cluster M ~. In the last section these data are considered in relation to Humason's measures of the rotation of NGC 3115. It is found that beyond about io" from the center in the equatorial plane the mass density must be about constant. The actual density depends on the unknown flattening of the attracting mass, but it should be at least of the order of 140 solar masses per cubic parsec (io~ gm/cm3), or 2000 times that near the sun. In this region of constant mass density the light-density diminishes with a factor of a