' PRESIDENT’S ADDRESS. 17 
confirmed in a remarkable way by Eddington,' using independent material 
discussed by a new and elegant method. Both results showed that, at 
least for extensive parts of space, there are a nearly equal number of stars 
moving in exactly opposite directions. The assumption, then, that the 
mean of the peculiar motions is zero may, at least for these parts of space, 
be still regarded as a good working hypothesis. 
Adopting an approximate position of the apex of the solar motion, 
Kapteyn resolved the observed proper motions of the Bradley stars into 
two components, viz., one in the plane of the great circle passing through 
the star and the apex, the other at right angles to that plane.? The former 
component obviously includes the whole of the parallactic motion ; the 
latter is independent of it, and is due entirely to the real motions of the 
stars themselves. From the former the mean parallactic motion of the 
group is derived, and from the combination of the two components, the 
relation of velocity of the Sun’s motion to that of the mean velocity of 
the stars of the group. 
As the distance of any group of stars found by the parallactic 
motion is expressed as a unit in terms of the Sun’s yearly motion through 
space, the velocity of this motion is one of the fundamental quantities to 
be determined. If the mean parallax of any sufficiently extensive group 
or class of stars was known we should have at once means for a direct 
determination of the velocity of the Sun’s motion in space ; or if, on the 
other hand, we can by independent methods determine the Sun’s velocity, 
then the mean parallax of any group of stars can be determined. 
Determination of Stellar Motion in the Line of Sight. 
Science owes to Sir William Huggins the application of Doppler’s 
principle to the determination of the velocity of star-motion in the line of 
sight. The method is now so well known, and such an admirable account 
of its theory and practical development was given by its distinguished 
inventor from this Chair at the Cardiff meeting in 1891, that further 
mention of that part of the matter seems unnecessary. 
The Velocity of the Sun’s Motion in Space. 
If by this method the velocities in the line of sight of a sufficient 
number of stars situated near the apex and antapex of the solar motion 
could be determined, so that in the mean it could be assumed that their 
peculiar motions would disappear, we have at once a direct determination 
of the required velocity of the Sun’s motion. 
The material for this determination is gradually accumulating, and 
indeed much of it, already accumulated, is not yet published. But even with 
1 Monthly Notices R.A.S., vol. \xvii. p. 34. 
2 Publications Astron. Laboratory Groningen, Nos. 7 and 9, 
1907. Fi 
