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| 39 | STAR-STREAMS |
direction, and averaging about 28 kilometres per second. This is the fact that emerges when we have abstracted from the results and thrown aside whatever has peculiar reference to the Sun, and has not a general cosmical importance.
A somewhat different conception of the distribution of stellar motions has been put forward by Professor Schwarzschild. His theory is rather difficult to describe in non-mathematical language and we have therefore preferred to keep to the older point of view of the « two-drift theory » in this article; but it is clear from his researches that it is the existence of the axis of symmetry of the stellar motions, mentioned in the last paragraph, rather than the division into two distinct systems, that is the fact directly indicated by the observations. Professor Schwarzschild showed that a « spheroidal distribution of velocities » (a kind of haphazard distribution modified by the assumption of greater mobility in the direction of the axis of symmetry) would agree very satisfactorily with the observed results. Although his theory contemplates the universe as single, instead of as composed of two distinct systems, there is really but little difference between the actual distributions of the motions, which the two theories formulate, They express nearly the same law, but by the aid of different mathematical functions. Their relation has been well summed up by Professor Dyson. « The dual character of Kapteyn’s system should not be unduly emphasized. Division of the stars into two groups was incidental to the analysis employed, but the essential result was the increase in the peculiar velocities of stars towards one special direction and its opposite. It is this same feature and not the spheroidal character of the distribution, which is the essential of Schwarzschild’s representation. »
The position in the sky of the line of relative motion of the two streams is not without a deep significance, for it lies accurately in the plane of the Milky Way. The course of the Milky Way in the heavens marks out a plane towards which the stars show a strong tendency to crowd. It seems a little curious that, whereas the principal feature in the distribution of the stars themselves is the existence of this plane of symmetry, the principal feature in the distribution of their motions is the existence of an axis of symmetry. That the axis should lie in the plane might perhaps have been anticipated,
