394 ASTRONOMY: A. VAN MAANEN 
THE DISTANCES OF SIX PLANETARY NEBULAE 
By Adriaan van Maanen 
Mount Wilson Solar Observatory, Carnegie Institution of Washington 
Communicated by W. S. Adams, November 7, 1918 
In a previous note to these Proceedings 1 the parallax was published for the 
planetary nebula N.G.C. 7662. Attention was called to the scarcity of ma- 
terial then existing for the distances of the nebulae in general. The work has 
since been taken up with the 60-inch reflector of the Mount Wilson Observa- 
tory (the equivalent focal length of 80 feet being used), and in all, the paral- 
laxes of eight nebulae have now been determined; six of these are planetaries, 
while the other two are spirals; the full details for the latter have been pub- 
lished in Mount Wilson Contributions, No. 158. 
The images of the central stars were even better adapted to accurate meas- 
urement in the case of the five other planetaries measured after N.G.C. 7662 
than on the plates of this object; here the nebulous ring was close to the cen- 
tral star and of considerable density; in all others, however, the nebula is 
either so much fainter than the central star, as in N.G.C. 2392, 6804, 6905, 
and 7008, that it is hardly visible on the plates, or it is at such a distance from 
the central star, as in N.G.C. 6720, that the nebula is not likely to interfere 
with measuring the central images. For this reason the existence of a large 
systematic error in the parallaxes is very improbable; other conditions also 
favor the belief that any systematic error will be small. As all the fields are 
near the Milky Way we have an abundance of comparison stars close to the 
central stars; in no case was the distance greater than seven minutes of arc; 
the central stars have in all cases the appearance of normal stars and are 
measurable with great accuracy. 
N.G.C. 2392 is relatively bright; on that account a rotating sector with 
an opening of 36 degrees was used in order to make the central star compar- 
able in magnitude with the faint stars which are preferably to be used for 
comparable purposes. 
In table 1 the observational material is collected for the fields of all six 
planetaries. 
The central star of N.G.C. 6720 is so faint that exposures of 35 or 40 
minutes might have been better; fortunately, for this nebula another reliable 
parallax determination is available — that by Newkirk, who has derived from 
74 plates taken with the Crossley reflector of the Lick Observatory a parallax 
of +0".015 =•= 0".007. 2 In the following discussion the weighted mean, viz., 
+0".008, has been used for the absolute parallax. 
To derive a homogeneous system for the photographic magnitudes of the 
central stars, counts were made of the number of stars of equal and brighter 
magnitudes in as large an area as the plates would allow; then with the help 
