ASTRONOMY: H. D. CURTIS 
68] 
e. About twenty-five spectroscopic binaries are known in which the 
H and K lines of calcium do not partake at all of the periodic shift 
shown by the other spectral lines, or give a markedly smaller range of 
radial velocities. This phenomenon is well explained by the interposi- 
tion of a cloud of invisible calcium vapor between us and the binary. 
All but one of these stars are located in or near the Milky Way, and 
several are in or near dark rifts of the Milky Way. 
/. There exists a sidereal arrangement for which no adequate explana- 
tion has yet been found- — the peculiar grouping of the spiral nebulae 
about the galactic poles, and the entire absence of these bodies in the 
Milky Way structure. It has been suggested that occulting matter in 
the peripheral regions of our Galaxy cuts off from our view all spirals 
lying in or near our galactic plane, and presumably far outside of our 
stellar system. 
g. There is strong evidence that much absorbing material exists in 
the outer strata of planetary nebulae. 
In the case of the edgewise and nearly edgewise spirals, the evidence 
for occulting material seems unquestionable. Vacant spaces dividing 
these objects into two similar and parallel nebular forms would appear 
to be mechanically inconceivable. For nebulae whose planes make 
larger angles with the line of sight, the alternative hypothesis that these 
appearances are mere open lanes encounters several difficulties. Such 
lanes should show most clearly at the ends of the major axes of the 
elongated nebulae, for there the foreshortening effect due to the inclina- 
tion of the nebular planes would be least. But the photographs leave no 
doubt that these dark lanes are invariably parallel to the major axis, and 
are most apparent near the ends of the minor axis. 
A possible phase effect might be proposed as an explanation of the 
differences of intensity on opposite sides of the major axis. But such a 
theory demands that the light of the spirals be due to reflection. Against 
this theory may be urged: 
a. All attempts to detect evidences of polarization in the spiral nebulae 
have given negative results. 
5. The reflection hypothesis demands that the light of the outer por- 
tions of the nebulae should come ultimately from a central nucleus, or 
star, or aggregation of stars of sufficient brightness to illuminate the 
outer portions, and that these outer portions should in general be fainter 
than we find them. Many nebulae have no such central condensations, 
and in others the nucleus is so faint as to be entirely inadequate. Many 
others show patches of nebular matter in the outer regions of the nebu- 
lae which are as bright as or brighter than the central part, — an im- 
probable result on the reflection hypothesis. 
