N.G.C. 6572. a= Observations of this brilliant nebula on fourteen nights by Keeler gave a radial velocity —9.7 ±1.9 km. per sec. The mean from 8 3-pr. 16-in. spectrograms is -8.8 km. From three one-prism spectrograms obtained in 1901, Wright determined the velocity of this 29 Lick Obs. Bull., 1, 153, 1902. Hartmann obtained a radial velocity of -10.5 km. in 1901, from two 1-prism and two 3-prism plates.30 From one spectrogram, in 1915, Pease and Adams obtained an observed velocity of -12 km.31 The image of N.G.C. 6572 is elliptical in form, the bright central portion being 9" X 14", with the major axis of figure in position angle 0°. The series of fifteen spectrograms (November 30, 1915, to May 21, 1916, inclusive) were secured with high dispersion, with the slit of the spectrograph placed, as described in the table, along the major axis, minor axis and the two diagonals of the nebular image, in order to test for rotational effects. All of the spectrograms, except those taken with the slit on the minor axis, show the nebular lines to be of the familiar S-shaped form. Plate XXXIV, figure 54, is a reproduction of a drawing of the N, line made from the plate of May 7.83 obtained with the slit placed along the major axis. This spectrogram was secured with the 3-prism 32-inch spectrograph on a Seed 23 plate, using a slit width of 0.037 mm. The scale on the left of the drawing gives the distance north and south from the center (nucleus) of the nebula in seconds of arc. It will be noted that only the light from the nebular image whose major axis is about 8 seconds of arc is recorded, and that even with this short exposure the bright central portion of the line corresponding to the central 3" of the nebula is considerably broadened. This effect is still more noticeable on the spectrograms of longer effective exposure and is in harmony with the visual observations by Professor Keeler, 32 who found that the lines from the nucleus of this nebula are not monochromatic. This effect is not wholly one of visual or photographic irradiation. Measures of the relative velocities indicated by different parts of the line will evidently be affected by this broadening of the line, and, for this reason, only the results obtained with the highest dispersion on fine-grained plates are given. Table I, (a), contains the relative velocities for the four points a, b, d, e (indicated in the drawing) on the N, line of the two spectrograms, May 3.94 and May 7.83, taken with slit set upon the longer diameter of the nebula, with reference to the center of the line bd. In the last two columns are measured distances between the points in millimeters, and the corresponding quantities in seconds of are for the image on the slit. The results for the corresponding points of the N, and N, lines on two spectrograms taken with the slit placed on the two diagonal axes in position angles, respectively, 315° and 225° are given in Table I (b). Since the outer ends of the lines are rather faint, the results for points a, e, a1, e1, a, e, are subject to more uncertainty than those for points b, d, b1, d1, b, d. The three spectrograms of March 27, May 13, and May 21, 1916, were taken with the slit placed along the minor axis of the nebular image, and show no inclination of the spectral lines. With the exception of point a,, our results are in harmony with the hypothesis that this nebula is rotating about an axis whose position angle is approximately 90°. 30 Astroph. Jour., 15, 294, 1902. 31 Publ. A. S. P., 27, 240, 1915. 32 Publ. Lick Obs., 3, 209, 1894. The last two spectrograms were taken with the slit placed along the major axis of the bright oval image of the nebula (8′′ x 5′′). On both plates the N1 and N2 lines are S-shaped, with very strong inclination of the bright central section whose length corresponds to about 3.5 seconds of arc. The inclinations of this central section from measures of N, and N2 on the two plates are as follows: 2 |