ASTRONOMY: H. D. CURTIS 
679 
A first division of the evidence is supplied by those spirals which are 
seen edgewise, or nearly so, and show indubitable evidence of an ab- 
sorbing lane (cf . fig. 1 for three typical examples of this class) . Twenty- 
nine such objects have been photographed, and the published descrip- 
tions of nine additional edgewise spirals, not yet photographed, clearly 
indicate that they also should be included. 
That thirty-eight of the larger edgewise spirals should show clear evi- 
dence of bands of obstructing matter must be regarded as establishing 
that this phenomenon is a very common one, and is probably the rule 
rather than the exception. 
A second division of the evidence is afforded by that large group of 
spiral nebulae whose principal planes make a slight but appreciable 
angle with our line of sight. Any actual irregularities due solely to 
FIG. 1 
asymmetry of form in the spiral nebulae should, taking a sufficiently 
large number of cases, show asymmetrical effects oriented at random with 
regard^to the position of the major axis of the projected (elliptical) 
images. This, however, is not the case. In the relatively frequent 
cases where the greatly elongated spirals show any lack of symmetry, 
such asymmetry is almost invariably with reference to the major axis 
of the ellipse. This asymmetry manifests itself frequently in 'lanes' 
prominent on one side of the major axis and faint or invisible on the 
other, in a fan-shaped nuclear portion, in an apparent displacement of 
the nebular material on one side of the major axis, or in various combi- 
nations of these effects. Fifteen elongated nebulae show prominent 
dark lanes on one side of the major axis. Contributory evidence is 
afforded by thirty-one spirals in which the nebular matter is markedly 
