350 
ASTRONOMY: H. SHAPLEY 
perhaps thus playing a large part in the conspicuous rifts in the Milky 
Way. 
Barnard^s dark markings, recently catalogued,!^ do not furnish direct 
evidence of this obscuration, for, singularly enough, more than half of 
his objects fall outside the region of avoidance, if we exclude one small 
region near Messier 11 for which 30 separate positions are listed. It 
appears that most of the markings may be affiliated with the local 
cluster, and at no great distance from the sun. Thus in the Taurus- 
Orion region, two-thirds of the dark markings have negative galactic 
latitudes, lying on the average more than 10° south of the galactic 
circle; in the opposite region the latitudes are largely positive, the dark 
markings in Ophiuchus and Scorpio lying intermingled with the seem- 
ingly unaffected globular clusters. Along the middle line of the region 
of avoidance relatively few markings are recorded. 
Indirectly, however, in Barnard's nebulae we have an argument 
favoring the hypothesis that globular clusters are concealed in mid- 
galactic regions, for, if a considerable amount of obscuration is asso- 
ciated with the relatively small local cluster, it suggests that such ma- 
terial may also be common in other stellar regions. Although the star 
counts in typical open and globular clusters fail to reveal as yet the 
presence of such obscuration, the distribution of stars in the Magellanic 
Clouds suggests the possibility of its presence there. 
Another point of considerable weight against a real absence of globular 
clusters from the region of avoidance is the difficulty and improbability 
of such a dynamical condition. The distribution of globular clusters in 
space shows their very close relationship to the Galaxy; the average 
velocity and probable mass both appear to be very great; the possi- 
bility, therefore, of repelling a globular cluster from the stellar stratum, 
or completely transforming it during a single passage, seems remote. 
From a gravitational standpoint we should naturally expect the fre- 
quency of clusters to be greatest at or near the galactic plane, and that 
many oscillations must occur before the hypothetical assimilation and 
transformation is completed for an average globular cluster. 
Of the several arguments favoring the reality of the empty zone, we 
recall that the most important are the completeness of the observed 
absence at all distances from the sun, and the various suggestions of 
immediate genetic relationship between the external globular clusters 
and the open clusters within, including the observation that the globular 
clusters nearest the plane appear to be the most open. 
