DEVELOPMENTS IN ASTRONOMY—PLASKETT. 265 
parent nebula which, while condensing, separates into two masses, 
and that these masses, as condensation proceeds, and by the influ- 
ence of tidal action, gradually increase their distance from one 
another, this being accompanied, of course, by an increase in the 
period and also, as the results show, by an increase in the eccen- 
tricity—a greater departure from the circular form—of their orbits. 
There is no sharp line of distinction between spectroscopic and visual 
binary orbits except that the latter have much longer periods and 
generally higher eccentricities. 
The information already obtained, and that which will in the near 
future be obtained, about these spectroscopic binary systems, has a 
most important bearing on the problem of the constitution of the 
sidereal universe, and we must now come to consider recent progress 
in our knowledge of the extent and form and motion of its parts. 
This is certainly the most important problem in astronomy, as practi- 
cally all observing data, whether astrometrical or astrophysical, 
whether dealing with the absolute positions, proper motions, and 
radial velocities of the stars, with their distances, dimensions, and 
densities, with their evolution and spectral type, or with the investi- 
gation of variables and binary systems, are all either directly or 
indirectly obtained with this end in view and all are, undoubtedly, 
directly of use in its solution. As I said in the early part of the 
paper, there has been no time when so many different investigations 
were converging toward this end, and I will try and give you some 
details of the principal results. 
One of the most striking of recent advances has been the discovery 
of star drifts and star streams in the sidereal universe. These have 
been discovered by statistical methods applied in the discussion of 
the absolute positions and proper motions of stars and also by the 
aid of their radial velocities. The one man to whom we owe more 
than any other the development of this work is Prof. Kapteyn, who 
is director of what is called the astronomical laboratory of Groningen, 
where the instruments of research are not telescopes and spectro- 
scopes but measuring machines and mathematical tables, where no 
observations are taken but photographs are measured and observa- 
tions discussed. I will try and give you a general idea of the present 
state of our knowledge in regard to the motions of thestars. Although 
we call them the fixed stars the term is a misnomer, for they are all 
in motion. We can measure this motion in two components. First, 
the motion at right angles to the line of sight, across the sky, deter- 
mined from successive observations of the star’s position in the sky 
and measured by the change of position in seconds of arc in a year or 
a century. The change of position varies between about 9’’ per 
year and 0; the average annual proper motion, as it is called, for 
first magnitude stars being 4’ and for sixth magnitude about 95” 
