712 
more remote stars of the magnitude m. For increasing & the 
logarithmic defect approaches here to a limit-value (calculated from 
Se 
the unobscured stars before the screen only), as represented in the 
drawing by the heavy line (e = oo). 
d. For the bright magnitudes the value of the logarithmic defect 
depends mainly on the distance @9,, for the faint magnitudes it depends 
in the first place on the absorption-coefficient ¢ of the dark nebula. 
For increasing 9, the effect of one and the same absorption on the 
logarithmic defect decreases.. 
From this follows in the first place, tbat it will be difficult to 
apply this method in general. In the case of small black spots (like 
the trifid hole near y Aquilae) the defect can be ascertained over 
some magnitudes (e.g. from the 11'* to the 16 magnitude), 
but this range is too small to separate the two unknowns g, ande 
and to find both; the number of brighter stars is too small to allow 
of any deductions. As we require data over the most divergent 
magnitudes, this method can only be profitably applied to regions 
of such extent, that it gives us the disposal also over a sufficient 
material of bright stars. This is the case with the dark nebulae in 
Taurus. 
Big. d. 
§ 3. For the star-density NV’, the following sources have been used: 
a. The “Bonner Durchmusterung” up to the star-magnitudes 6,5, 
8,0 and 9,0 incl. (the total number up to 9,5 could not be used, on 
