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In assigning relative weights to the observations secured with the 3-pr. 16-in. and the 3-pr. 32-in. allowance is made for the facts that the lines on the plates of the higher dispersion show more irregularities of structure and are not so well exposed.
The high dispersion spectrograms, March 16, 1916, to April 3, 1917, inclusive, taken for the purpose of detecting effects of internal motion in the nebula, record the nebulium lines in the central region and in the bright ring (outer diameter about 26"X16"). Even the longer exposures show little more than traces of the lines for a few seconds of arc beyond the limits quoted above, and give us.no information concerning the fainter outer oval (40" X 35"). The appearance of the nebular lines obtained when the slit is placed along the major axis of the bright ring (in position angle 145°) is as in the drawings of N„ and N, made from the spectrogram of March 16, 1916, and reproduced in plate XXXII, figure 38, a and b, respectively. In order to show the small relative displacements in the lines, the horizontal scale is made four times that of the vertical. The widths of the lines, however, are not correspondingly increased. It will be seen that the central section of N2 consists of two components separated by a very narrow dark line. This separation is completely blotted out by over-exposure in the N, line. Its existence, however, is clearly shown on the spectrograms of April 1 and 3, 1917, taken with the 3-pr. 32-in. and using a narrow slit. The phenomenon appears to be of the same nature as that described in the case of N.G.C. 2392, except that here the separation of the components amounts to only about 0.5 A., and we seem also to have more definite indications of rotational effects. The lines on the spectrogram of March 26, 1916, taken with the slit on the minor axis, show no inclination of the lines as a whole. They show some broadening of the central portion, with a suspicion of separation into two components.
Below are collected Mr. Moore's measures of the radial velocities at different points along the lines on the spectrograms secured with slit on the major axis of the nebula. The values of the first three plates are from the N, and N2 lines, while for the last three they refer to N, alone.
On the plate of April 1, 1917, measures of the two components of the central section of the line gave radial velocities differing by 21 km./sec. At the points a and / the lines are faint and ill defined, and the measures at these points are entitled to little or no weight. Our results for the other portions of the line are in fair agreement and indicate that the inner edge of the ring on the major axis, some 5.5 seconds of arc southeast of the nucleus, is receding with respect to the corresponding part of the ring northwest of the center with a relative speed of 9.8 +2.5 = 12.3 km./sec. The general slope of the lines crossing the ring indicates a somewhat slower rotational speed for the outer edges of the ring than that corresponding to the inner edge.
In 1891 Professor Keeler22 made an attempt by the visual method (referred to above, p. 83) to determine whether this nebula had any sensible motion of rotation. Of his observations he says: "No change of position of the chief line could be detected on passing from one limb of the nebula to the opposite one, with the slit in different position angles. The bright inner ring was also observed in the same way without result. It is doubtful whether a smaller difference of motion than 7 or 8 miles a second could have been detected, and it is perhaps improbable that a nebula should have so large a motion of rotation as this." It is of interest to note that the relative velocity which we have found above, 12 km./sec., is not out of harmony with Keeler's observation.
22 Publ. Lick Obs., 3, 203, 1894.