Astronomy. — “The Distance of the Dark Nebulae in Taurus’’. 
By Dr. A. PANNEKOEK. (Communicated by Prof. J. C. Kaprryn). 
(Communicated at the meeting of Sept. 25, 1920). 
§ 1. Various investigations made in recent years, have demonstrated 
ever more clearly the existence of dark cosmic nebulae, that aborb 
and weaken the light of the stars behind them. Between the luminous 
patches and streams in the Galaxy, dark spots and cavities are seen, 
which were originally considered as empty spaces in thestar-filled galactic 
system. The improbability of these empty spaces extending as conic 
tubes through HerscHel’s lenticular star-system, with our sun as 
vertex, constituted one of the main arguments for the conception of 
the Galaxy as a ring of no great extension in depth. For a long 
time the possibility that they should originate by means of absorption 
has played no part in the theories concerning the structure of the 
universe. 
It is through the photographs of Max Worr and BARNARD that we 
first have become acquainted with numerous details scarcely allowing 
of any other interpretation. Small dark spots are to be seen in the 
midst of the luminous star clouds; long, dark, fantastically shaped 
lanes intersect the luminous parts, and are evidently connected with 
faintly luminous nebulae. Max Worr has repeatedly pointed out the 
existence of extensive absorbing nebulous masses, as one of the 
main causes that determine the aspect of the Galaxy. The galactic 
system is then to be considered as a mixture of dense starclouds, 
luminous nebulae and dark nebulous masses. 
In an investigation of some star-photographs in Aquila '), comprising 
the densest parts of a starcloud and also a black spot therein, 
_ the author of the present article found that in the black spot the 
densities of the stars from the 11" to the 15" magnitude were all 
smaller in the same proportion, compared with the cloud besides it; 
if the spot were caused by absorption, the absorbing substance should 
therefore not lie in the far depths of the starcloud, but a great deal 
nearer by, so that it was only accidentally projected against this 
luminous background. 
1) A. PANNEKOEK. Investigation of a galactic cloud in Aquila. Proceedings R. A. 
of S. Amsterdam, Vol. XXI, Nr. 10. (March 1919). 
46 
Proceedings Royal Acad. Amsterdam. Vol. XXIII. 
