[[Categoria:Pagine che usano RigaIntestazione|Scientia - Vol. VIII.djvu{{padleft:41|3|0]]astronomers had long been dissatisfied with this hypothesis. It may seem strange that comparatively little effort was made to examine directly how far the truth differed from it. Insufficient data and other difficulties prevented a satisfactory study of the question; but probably the chief cause of neglect was the not unnatural opinion that any deviations from a haphazard distribution would be found to be complicated and differ from one part of the sky to another. It was not suspected that, over and above the local deviations, there might prevail everywhere a simple law in the distribution of the motions.

Of local irregularities various examples have been pointed out, where evidently associated groups of stars share a common motion more or less exactly. As might be expected the closely congregating stars of the Pleiades move together in this way; in the same constellation Taurus there is another more extensive but similar association of stars. Still more curious is the group formed by five out of the seven stars of the Plough (Ursa Major) together with Sirius, in which there is a remarkable community of motion, although the stars are by no means close together. It would seem that these six stars are in some way associated in their origin, and retain their original common velocity almost undisturbed by the rest of the stars, which not only surround but thoroughly interpenetrate the system. In dealing with statistics of stars, it is necessary to bear in mind the existence of these and other local streams. Any general law of distribution will evidently be subject to deviations due to this cause. If however a sufficiently large number of stars is considered, the disturbing effect of a local stream is generally very slight, and does not mask the more general law of distribution.

The examination of the motions of the stars, which first showed in their distribution an important systematic feature prevailing in all parts of the sky, was made by Prof. J. O. Kapteyn of Groningen. He arrived at the surprising result that the stars form two great streams moving through one another. As the result of his researches and those of subsequent investigators, the motions and characters of these streams are now known with some degree of definiteness. We shall presently indicate how it has been possible to recognise the presence of these streams from a study of the observed stellar motions; but in the first place, it may be well to state the