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The lines are not sharply defined, nor monochromatic. With the high-dispersion spectrograms the red edges of the nebular lines are the sharper and their intensities in general fade away to the violet edge. For this reason the velocity is not accurately determinable. The plate of May 17, 1916, under-exposed, naturally gives a smaller negative velocity than any of the others. The 32-inch camera, for the same reason, does not yield more accurate results than the 16-inch.
On the direct photographs obtained with the Crossley reflector, the image of this nebula shows a strong stellar nucleus surrounded by a bright disk about 8" in diameter. This appears to be but the brighter central section of an irregular oblong of fairly bright matter about 20 seconds by 13 seconds. From the north and south sides of the oblong extend long faint ansae in position angle about 165°.
Nine spectrograms with high dispeision were secured for a study of the forms of the lines given by this object: three with the slit in the direction of the long ansae, four with the slit in the direction perpendicular to the preceding one, and two in a position midway between these. Ouly the light from the small central disk and the bright surrounding oblong is recorded on our spectrograms. As noted above, the lines obtained with the slit in the three positions are complex and show marked variations in intensity in different parts of the same line. The general character of the two nebulium lines is represented in the accompanying illustration (pl. XXXIII, fig. 52), which is a reproduction of a drawing of the N, line, obtained with the 3-pr. 32-in. spectrograph, with the slit placed across the central portion of the nebula, in positions respectively at right angles to and along the line joining the ansae. Inasmuch as the nebular image appears on the slit as an irregular oval, brighter in the central region, some uncertainty exists as to the portion of the line corresponding to the exact center of the nebula.
The central point of the scale on the drawing has been adjusted to the probable center of the nebula, on the assumption that the midpoint of the nebular image was displaced an equal amount from the center of the slit on the two spectrograms of April 30 and August 8, 1916, taken with the slit in position angles respectively 345° and 165°. The same correction was applied to the other observations and, in particular, in fixing the scale in the drawing of the spectrogram of May 1, 1916, with the slit in position angle 255°. This uncertainty should be borne in mind in comparing the relative intensities at corresponding points in the three drawings.
The structure of the lines, somewhat masked by over-exposure and the coarser grain of the Seed 30 plates, is shown more clearly on the spectrogram of August 8 (fig. 52c i, which was secured with a narrow slit on a Seed 23 plate. The brighter red side of the line is seen to be the brighter component of a double line, and the complex line appears to be an example of the phenomenon observed in the spectrum N.G.C. 2392.
In the measurement of spectral lines showing such marked changes in relative intensities within the lines, our practice has been in general to bisect the width of the line, except of course where the line is plainly separated into two components. A close agreement between the radial velocities from the lines on different plates is therefore not to be expected. Our measures of radial velocity at the points a and b near the ends of the line, and at c near the center (with subscripts 1, 2, 3, 4), are contained in the following tables, which give the results when the slit was placed in the direction of the long ansae, perpendicular to this direction, and at an angle of 45° to it, respectively.
The values given above are the means of the measures of N, and N, on the plate of April 30.84, and are for the N, line alone on the other two plates. On the spectrogram of August 8, the measures at the points v„ r,, and c, are for the violet and red components and the dark line, respectively.
For the first two plates the measures refer to N, and N2, while on the last one only the N, line was measured. The values from this plate were given half weight in taking the means.
The N, and N„ lines were measured on the spectrogram of May 26, and the N, alone on the plate of May 29.
It will be seen that a general inclination of the line is indicated by the measures, for both the direction of the long ansae and the one perpendicular to it, in the sense that points south and east of the center are approaching us more rapidly than the corresponding point north and west of the center. When the slit was placed along the line intermediate between the two positions, i.e., on a southeast-northwest line, the line shows inclination in the same sense as before, but apparently of smaller amount. So far as they go, our results do not seem to conform to a simple rotational hypothesis. This is perhaps to be expected in view of the very complex form of the nebula as indicated by Dr. Curtis's photographs.
The plates are all good, and the large deviation of the second and third results from the mean value is surprising. The next to the last plate showed that the northern edge of the nebula is approaching 10 or 11 km. per sec. more rapidly than the southern edge; and if the slit during the second and third exposures drifted off the center of the nebula and toward the northern edge, a part of the discrepancy would find explanation.
The nebular image is an oval disk (10" X 7") with faint ansae or extensions in position angle about 165°. On the first spectrogram of June 18, 1916, taken with the slit along the major axis of the oval, the nebular lines are inclined about 1.5 degrees to the direction of the comparison lines. The measures of the N, line by Messrs. Campbell and Moore, expressed as the radial velocity of a point on the southeast edge of the nebula minus that of the corresponding point on the northwest edge, are given in the following table. Two methods of measurement were employed: (a) the angular measurement of the inclination and (6) the direct measurement of the relative velocities in the usual manner.
The nebular lines on the second spectrogram of June 18, corresponding to the minor axis of the nebula, are not inclined. Our results are in harmony with the hypothesis that the planetary is rotating about an axis in position angle approximately 75°.
N.G.C. 6369. a = 17h 23m2; 8 = —23° 41'; A = 261° 35'; /3 = —0° 30' The slit was directed upon the northern section of the ring, as that is the brightest part of the nebula. Longer exposures would have given more accurate results. N- is under-exposed on all of the plates, and H/3 is not recorded.
The image of the nebula is a minute round disk about 5 seconds of arc in diameter. On Curtis's photographs with the Crossley reflector a faint extension or wing is seen on the northwest edge of disk in position angle 290°. For the last spectrogram the slit was placed across the nebula in this position angle, but no definite evidence of inclination is shown by the N, line on this plate.