22 PRESIDENT’S ADDRESS. 
of motion in the line of sight, appears to be sensibly smaller than that 
derived from fainter stars. The explanation appears to be that certain 
of the brighter stars form part of a cluster or group of which the Sun is 
a member, and these stars tend to some extent to travel together. For 
these researches the existing material, especially that of the determination 
of velocities in the line of sight, is far too scanty. 
Kapteyn has found that stars whose proper motions exceed 0’-05 are 
not more numerous in the Milky Way than in other parts of the sky ;! in 
other words, if only the stars having proper motions of 0-05 or upwards 
were mapped there would be no aggregation of stars showing the existence 
of a Milky Way. 
The proper motions of stars of the second spectral type are, as a rule, 
considerably larger than those of the first type ; but Kapteyn comes to the 
conclusion that this difference does not mean a real difference of velocity, 
but only that the second-type stars have a smaller luminosity, the mean 
difference between the two types amounting to 2} magnitudes.” 
The Future Course of Research. 
In the last Address delivered from this Chair on an astronomical sub- 
ject, Sir William Huggins, in 1891, dealt*so fully with the chemistry of 
the stars that it seemed fitting on the present occasion to consider 
more especially the problem of their motion and distribution in space, 
as it is in this direction that the most striking advances in our 
knowledge have recently been made. It is true that since 1891 great 
advances have also been made in our detailed knowledge of the chemistry 
of the Sun and stars. The methods of astro-spectrography have been 
greatly improved, the precision of the determination of motion in the line 
of sight greatly enhanced, and many discoveries made of those close 
double stars, ordinarily termed spectroscopic doubles, the study of which 
seems destined to throw illustrative light upon the probable history of the 
development of systems from the original nebular condition to that of 
more permanent systems. 
But the limitations of available time prevent me from entering more 
fully into this tempting field, more especially as it seems desirable, in the 
light of what has been said, to indicate the directions in which some of 
the astronomical work of the future may be most properly systematised. 
There are two aspects from which this question may be viewed. The first 
is the more or less immediate extension of knowledge or discovery ; the 
second the fulfilment of our duty, as astronomers, to future generations. 
These two aspects should never be entirely separated. The first, as it 
opens out new vistas of research and improved methods of work, must often 
serve asa guide to the objects of the second. But the second is to the 
astronomer the supreme duty, viz., to secure for future generations 
those data the value of which grows by time. 
) Verl. Kn. Ahad. Amsterdam, January 1893. 
* Ibid., April 1892. 
