Astronomy. — “Further Remarks on the Dark Nebulae in Taurus”. 
By Dr. A. PANNEKOEK. (Communicated by Prof. J. C. Kapreyn). 
(Communicated at the meeting of October 30, 1920). 
§ 1. In a previous communication, assuming that the star-voids 
in Taurus are caused by absorbing nebulae, we have determined the 
distance of those nebulae at about 140 parsees. The light-absorption of a 
region with moderate absorption, for which data were availablealso 
for the 12% magnitude, proved to be 1 a 2 magnitudes; for the 
darkest regions A and 5 the average must then amount to about 
2 magnitudes, which is not in conflict with the logarithmic defect 
for 15,9; the blackest kernels therein have a far stronger absorption 
still. The existence of such extensive regions (the dimensions of A 
are 9° by 3°, that is to say 20 by 7 parsees; B is most irregular, 
but about equal in area) of which the absorption is known, allows 
us to draw some conclusions regarding the density and mass of these 
gas-clouds. 
We assume, therefore, the existence of such a gas-cloud in space, 
the molecules of which absorb the light through scattering. Lord 
Ray.eieH in his investigations on the cause of the blue colour of 
the sky, has deduced a formula for the absorption of the light through 
a medium containing small particles in suspension in which the 
suspended particles scatter the light to all sides *). Scuusrer pointed 
out, in 1909, that the extinction of the light in our atmosphere is 
to be- attributed almost exclusively to such scattering, where the 
molecules of air themselves play the part of scattering particles, 
whilst the selective absorption constitutes but a minor factor’). As 
the absorption in magnitudes is proportional to the density > thickness, 
and therefore to the number of molecules the ray of light meets, 
the density and mass of a cosmic gas-cloud can be determined 
through comparison with the data of the atmospheric extinction. 
ABBorr gives for Mount Wilson in the zenith a transmission-coeffi- 
cient 0.95, an absorption therefore of 0,056 magnitude, valid for a 
column of air of 6 km., in height, and a density of 0,0013. If for 
the thickness of the gas-cloud in Taurus (after the linear dimensions 
') Philosophical Magazine, 1899, page 379. 
*) Nature, 1909, page 97. 
