725 
further in tbe line of sight as perpendicularly to it, one unity in @ 
means a lateral dimension of 26°, and two unities in @ (a void 
therefore from 7, to 2,51 7,), a lateral extension over 50°. Hence, 
if we want to explain a clearly evident defect of stars (over 20 °/, 
for instance, log V'/y > 0,10) over a small area (below 10°) by real 
spatial voids in the star universe, we come to the hardly acceptable 
assumption of protracted, tubular cavities, all running in the direction 
of the line of sight. It is only in those places, where the stars do 
not extend equably alongside of the visual line, but are clustering 
into actual clouds and other objects, that real voids between them 
can play an important part in the aspect of the Galaxy. 
Thus, if we abide by the explanation through absorption, but 
without the enormous mass, the particles that cause the scattering 
must have a mass, smaller than hydrogen-molecules, thus they would 
have to be for the greater part free electrons. The question as to whether 
there really is absorption, could be settled by means of an investi- 
gation into the colours of the stars in the poor regions. The absorption 
through scattering is inversely proportional to 4‘, so that the stars 
behind the gas-cloud must be strongly reddened. For a number of 
nebulous stars, stars which are surrounded by visible nebulous halos, 
in Monoceros, Scorpio and Ophiuchus, Snares and HvBBrr have 
recently found’) that their colour is considerably more red than it 
should be according to their spectral type, that therefore their light 
is scattered and dimmed by the nebula through which they shine. 
On calculating what portion of the stars of each magnitude lies 
behind the gas-cloud, assuming for its distance once more 160 
parsecs (g, = 6,05), we get for 
ci whee) 6 7 8 9 10 11 12 
0,4°/, ee a hie a ole ele 31 “lle 50°/, 
It is only with stars fainter than the 12' magnitude, therefore, 
that the majority will show this reddening through absorption. As 
in the case of such faint stars a comparison with the spectral type 
is difficult to accomplish, it will not be feasible directly to determine 
the reddening with absolute certainty; it may be, however, that a 
statistic determination of the colour or the effective wave-length of 
the fainter classes will lead to a decision. 
Postscript. Professor pr Sitter has drawn my attention to the 
fact, that the absorption of a mass consisting of opaque particles 
') F. H. Seares and E P. Husste. The color of the nebulous stars. Astro- 
physical Journal, 52, 8 (July 1920). 
47* 
