54 Transactio7is of the Boyal Society of South Africa. 
of the solar apex, i.e., the point in the heavens towards which the sun's 
motion is directed. 
The discrepancies in these quantities found by different investigators < 
either starting with different data or utiUsing different methods for 
the combination and discussion of the same material, had long been a 
puzzle to astronomers. The key to the situation was at length furnished 
by Professor Kapteyn, of Groningen, who, in an epoch-making paper 
read before the British Association in Cape Town, first pointed out 
that the apparent motions of the stars indicated not merely the existence 
of a single solar apex, but that there were two separate regions of the 
sky towards which a preference was shown by the directions of motion 
of the Bradley stars. 
This was a phenomenon which could not be explained by a simple 
motion of translation of the sun, as evidently the sun's motion could not 
be directed to two different points simultaneously, and the only feasible 
explanation was that the stars consisted of two groups and that the 
motion of the sun relatively to one of these groups differed from its 
motion relatively to the other, or that, though the stars appeared inter- 
mingled in space, they possessed an independent relative motion, which 
might be regarded as located in one group or in the other, but which 
was shared by all the stars peculiar to the group. 
The theory of the existence of two streams or drifts of stars thus put 
forward by Kapteyn has since received full confirmation by other investi- 
gators, notably by Eddington, who based his examination on the early 
observations of Groombridge, and by Dyson, who limited his discussion to 
a selected list of stars possessing considerable proper motions. 
Eecent investigations at the Cape, to which I shall refer later, have led 
us to examine in somewhat more minute detail the proper motions of the 
Bradley stars, with the result that, though the phenomena first noticed by 
Kapteyn stand out as the most prominent feature, certain subsidiary 
features of no less importance have been brought to light. 
It will be clear that, if we fix our attention on stars situated in the 
region of the sky immediately surrounding the solar apex, the transverse 
motion of the stars due to the sun's own motion must vanish. The same 
will be true for stars situated near the antiapex or region from which 
the sun is receding. 
The transverse motion will be greatest for stars on a zone situated 
half-way between these two points. In the case of a double drift, as 
suggested by Kapteyn, it is clear that the only detectable systematic 
motions in the neighbourhood of the apex or antiapex of the first drift 
will be those of stars belonging to the second drift and vice versa. 
In all cases the actual drift motions are obscured by the peculiar 
motions of individual stars, and this, in combination with the facts that 
