680 
ASTRONOMY: H. D. CURTIS 
fainter on one side of the major axis of the projected elHpse, or in which 
it appears to extend farther from the apparent center on one side of 
the major axis. A fan-shaped nuclear portion is prominent in thirteen 
elongated nebulae. Figure 2 shows a typical example of this group. 
It is not impossible that these effects may be due to the same general 
cause which produces analogous effects in our own Galaxy, though there 
is manifestly a good deal of assumption in postulating the same char- 
acter of occulting material in the vicinity of objects so different in 
spectrum, in space distribution, and in space velocity, as the spirals and, 
for example, the great diffuse nebulosities. It will be of interest, how- 
ever, to mention some probable intra-galactic manifestations of occulting 
material. 
a. Many diffuse nebulosities show a marked falHng off in the number 
of faint stars in their immediate vicinity. Very faintly luminous or 
non-luminous matter in the peripheral regions of these nebulae seems to 
offer the only possible explanation. 
FIG. 2 
h. The 'Coal Sacks' and other starless regions in or near the Milky 
Way seem to be best explained as due to the interposition of great ex- 
panses of occulting material between these areas and our own position 
in space. 
c. Professor Barnard has described many small starless regions, which 
he believes to be 'dark nebulae,' and a number of these are available 
for study on negatives taken with the Crossley Reflector. It is impos- 
sible to believe that these are actual 'holes' in the Milky Way. As 
Campbell has pointed out, the age of these must be of the order of 
hundreds of millons of years, and the random motions of the stars would 
long since have obliterated the clear-cut edges, if not the entire phenome- 
non, if they ever had the character of 'holes.' 
d. Similar dark patches are seen projected on the luminous back- 
ground of many of the giant diffuse nebulosities. It is impossible to 
conceive that these clear-cut spots are 'holes' extending through a mass 
of nebular matter for distances measured in Hght years. 
