ing chapter. Supposing, for the moment, _M_0_ = 10
we get for _D_0_ the value 22, corresponding to a number of 90 stars
within a distance of one siriometer from the sun. We should then know a
fifth part of these stars.

34. _Parallax stars._ In Sec.22 I have paid attention to the now available
catalogues of stars with known annual parallax. The most extensive of
these catalogues is that of WALKEY, containing measured parallaxes of
625 stars. For a great many of these stars the value of the parallax
measured must however be considered as rather uncertain, and I have
pointed out that only for such stars as have a parallax greater than
0".04 (or a distance smaller than 5 siriometers) may the measured
parallax be considered as reliable, as least generally speaking. The
effective number of parallax stars is therefore essentially reduced.
Indirectly it is nevertheless possible to get a relatively large
catalogue of parallax stars with the help of the ingenious spectroscopic
method of ADAMS, which permits us to determine the absolute magnitude,
and therefore also the distance, of even farther stars through an
examination of the relative intensity of certain lines in the stellar
spectra. It may be that the method is not yet as firmly based as it
should be,[15] but there is every reason to believe that the course
taken is the right one and that the catalogue published by ADAMS of 500
parallax stars in Contrib. from Mount Wilson, 142, already gives a more
complete material than the catalogues of directly measured parallaxes. I
give here a short resume of the attributes of the parallax stars in this
catalogue.

The catalogue of ADAMS embraces stars of the spectral types F, G, K and
M. In order to complete this material by parallaxes of blue stars I add
from the catalogue of WALKEY those stars in his catalogue that belong to
the spectral types B and A, confining myself to stars for which the
parallax may be considered as rather reliable. There are in all 61 such
stars, so that a sum of 561 stars with known distance is to be
discussed.

For all these stars we know _m_ and _M_ and for the great part of them
also the proper motion [mu]. We can therefore for each spectral type
compute the mean values and the dispersion of these attributes. We thus
get the following table, in which I confine myself to the mean values of
the attributes.

TABLE 6.

_MEAN VALUES OF _m_, _M_ AND THE PROPER MOTIONS ([mu]) OF PARALLAX STARS
OF DIFF