1.8.C. 48  Procs. of the Asiatic Soc. of Bengal. [N.S., XVII, 
of the motion of individual stars, be taken as exclusively due 
to the motion of translation of the Solar System. As the latter 
is known accurately in amount and direction, the mean paral- 
lactic drift gives at once the distance of the group. The 
Astronomer Royal has investigated the proper motions of the 
Groombride and the Carrington Stars and has obtained an ex- 
pression for the mean Secular parallax of stars of different 
magnitudes in the region. Kapteyn has in this way made an 
extensive study of the distance of the B-type stars which are 
known to have very small individual motions and of groups of 
stars possessing other peculiarities in common. 
he radial velocities, when treated in intimate association 
with proper motions, enable us to make independent estimates 
of distances of groups of stars. From the doctrine of chances, 
are deduced the relations between the average speed in space, 
the average radial velocity and the average of all the trans- 
verse components. Thus when the average radial velocity of 
a group, cleared of the effect of the Solar motion, has been 
found, it is easy to calculate in linear measure, the average of 
all transverse components. The latter being already known in 
angular measure from the available proper motions, we have 
an indirect but effective method for inferring the distance 
of the group. 
a comprehensive study of the various problems con- 
nected with the Stellar system it is found necessary to con- 
sider how the several observed facts are connected with the 
star’s spectral type. In the:words of Prof. Kapteyn ! “ the mix- 
ing up of all the spectral classes must singularly diminish the 
effectiveness of any statistical treatment. It is as if we inves- 
tigated statistically the size of all the members of the animal 
ingdom from the biggest to the smallest. It must be evident 
how much more effective must be the treatment of smaller 
groups such as the genera or the species.” : 
A statistical study of the counts of stars according to their 
spectra reveals some interesting facts. The stars of different 
spectral types are not uniformly distributed in the sky. The 
B-stars, as far as can be made out from the star-ratios, are 
found to be thinning out very rapidly ; they appear to belong 
exclusively to a local cluster in the neighbourhood of the Sun, 
whose equatorial plane is inclined at an angle of about ten 
degrees to that of the general galactic system. Among stars 
brighter than 6-5 the rapid increase in the number of the A, F, 
stars with decreasing brightness is another important feature 
which shows that stars of these types occur in larger numbers 
beyond a certain distance from our system. The Milky Way 
Regarding the distribution of fainter stars according to 
ices 



1 Groningen Publications 29. 
