ASTRONOMY: CAMPBELL AND MOORE 
567 
brighter planetary nebulae. We have employed 3-prism dispers'on 
and cameras either 16 or 32 inches in focal length. The linear dis- 
persions at are respectively 20 and 10 angstroms to the millimeter. 
Up to the present time thirty-three planetary nebulae have been exam- 
ined for evidences of rotational effects. These effects manifest them- 
selves by Dopper-Fizeau displacements of the spectral lines with refer- 
ence to the normal directions of these lines as defined by the lines of 
the comparison spectra of hydrogen and helium impressed upon the 
same photographic plates. The general results of this work are briefly 
summarized below. It is convenient to speak at first of the results as 
relative motions in the nebulae rather than as rotations, the former 
being the more general term. 
1. Of the 33 planetaries observed, 16 give definite evidence of rela- 
tive motions, and for 5 others we suspect that the nebular lines are 
slightly incHned in some cases and distorted in others. For 12 of the 
33 observed nebulae no indications of relative motion have been found. 
2. Attention should be called to the apparent effect of the size of 
the nebular images upon the results. For the so-called 'stellar' nebu- 
lae, or those whose images are essentially round and less than 5 seconds 
of arc in diameter, the chances of observing line inclinations or distor- 
tions are reduced by two causes: first, our experience with nebulae 
elliptical in outline has been that the relative motions are most pro- 
nounced along the major axes of figure, and in the stellar nebulae the 
positions of the major axes are unknown; and secondly, the spectral 
lines from the stellar nebulae are very short. Of the stellar objects 
only one has certainly shown rotational effects. Seven of the 12 
objects mentioned in the preceding paragraph as giving no evidence 
of relative motion belong to the stellar class. 
3. Of the nebulae whose diameters are between 5 and 20 seconds of 
arc, 7 give certain evidence of relative motion, 3 are suspected and 5 
seem to give no evidence. 
4. All of the 8 nebulae whose diameters exceed 20 seconds of arc give 
strong evidence of relative motion. 
5. With one or two exceptions, the images of the nebulae in which 
relative motion has been observed are approximately elliptic in form. 
The greatest observed effects occur in general with the elliptical forms 
of greatest eccentricity. 
6. The 5 nebulae with diameters between 5 and 20 seconds of arc 
which show no relative motions are sensibly round, which suggest that 
the axes of rotation may lie very nearly in the line of sight; in which 
cases the spectrographic test would not be applicable. 
