Page:Popular Science Monthly Volume 87.djvu/239

Rh not follow that the more distant and fainter stars will show the same preferential motions as the brighter and nearer ones which led Kapteyn to his hypothesis, though it should be said that a fairly extensive study of stars fainter than the Kapteyn stars made by Comstock led to results in good agreement with Kapteyn's. May it not be possible that the preferential motions observed are in some way connected with rotational phenomena within our stellar system, especially as the line of preferential motions lies approximately in the plane of the Milky Way, or are local to what we may call our region of the system, and not be true of the system as a whole?

An alternative hypothesis of prevailing stellar motions, proposed by Schwarzschild, seems to have advantages from the point of view of probability, but it appears not to accord so well with the facts of observation. Schwarzschild suggests that if from a given point we draw vectors whose directions and lengths represent the directions and speeds of existing stellar motions, then the outer extremities of these vectors will define the surface of an ellipsoid (of preferential motions) having three unequal axes.