RECENT PROGRESS IN ASTROPHYSICS. 
15 
Of more importance is the comparison with other cata¬ 
logues. The most noteworthy result is this, that the tele¬ 
scopic stars in the northern BD. are given fainter between 
the declinations 0° and + 20° than betwnen + 20° and 
4- 40°. This result follows from a comparison of the BD. 
with all the other catalogues. It only indicates, however, 
that the BD. scale, which has hitherto served as the standard 
scale, is not uniform in all parts of the sky, and should be 
systematically tested, not only upon the equatorial system, 
but also upon the galactic fundamental plane. 
As regards the systematic difference between the PD. and 
the other catalogues, it was found that in general the PD. 
magnitudes are one or two tenths of a magnitude fainter— i. e., 
that the PD. needs a reduction to the other systems, like this: 
PD.— other catalogues = +0 M .l or -f 0 M .2. 
A detailed examination with regard to color has disclosed a 
more important systematic difference, which is very instruct¬ 
ive and deserves further investigation. From a comparison 
with all other photometries it follows that the PD. magni¬ 
tudes need a color correction . We may express this fact by 
saying that the PD. gives the white stars too faint. Designat¬ 
ing white and yellow by the letters W and Y, we can put 
this correction into the formula : 
W—Y = +0 M .3 or +0 M .4. 
Thus the difference of estimation of brightness between the 
white and yellow stars amounts to at least one-third of a 
magnitude, a quantity that cannot be disregarded in photo¬ 
metric work. It would seem, then, that the general sys¬ 
tematic difference between the PD. and other catalogues is 
mainly due to the white stars— i. e., the PD. magnitudes are 
generally fainter because the white stars have been observed 
too faint. 
This result is in nowise prejudicial to the PD. Every as¬ 
tronomical work is affected by a systematic difference from 
other work of the same kind, and the problem is only to find 
