256 REPORTS ON THE STATE OF SCIENCE. 



has shown that the hitherto measured radial velocities of these stars 

 are all subject to a systematic error of about 5 km. per second, which 

 may arise from a pressure-shift of the spectral lines. In the light 

 of this discovery some of the results will probably need revision. 

 However, the main fact is not disturbed, but rather emphasised ; Orion 

 stars have very small individual motions and do not share in the star- 

 stream motions to any appreciable extent. This latter fact is the more 

 curious, because it is the next type in order of evolution, Type A, that 

 shows the star-streaming in its purest form. Dyson 1{i found that 

 these stars diverge veiy little from the general directions of the drifts 

 to which they belong, a fact which accords with their low individual 

 velocities, which should leave the drift-motions predominant. On the 

 other hand, my results from the examination of Boss's catalogue, 

 and Halm's 17 more detailed investigation, indicate that there are some 

 other stars (of what type has not been ascertained) that share with the 

 Helium stars the peculiarity of being nearly at rest and not belonging 

 to the two streams. These stars, which Halm has called Drift O, 

 are, like the Helium stars, concentrated towards the galactic plane— 

 a fact which, as we have seen, usually indicates remoteness. The 

 vast distance of the Helium stars appears to me so fundamental a 

 fact in considering this subject, that I will quote in support Prof. 

 Boss's conclusion. 18 He finds that a space around the sun having a 

 radius corresponding to parallax 0"'015 is ' almost wholly devoid of 

 these stars. ' la It is not necessary to conclude that the part of space 

 round our sun is unusually bare of Helium stars ; I would rather 

 suggest that they are extremely rare everywhere (except in certain 

 moving clusters), but that owing to their brightness they are visible 

 at, say, ten times the distance of an ordinary star, and therefore 

 throughout a volume of space a thousand times as great. In any 

 case the exceptional behaviour of the Helium stars now becomes 

 explicable ; they do not conform to the two star-streams because they 

 lie beyond the region of space through which the star-streams prevail. 

 Similarly, I think it is likely that in the Drift stars we have to deal 

 with remote stars beyond the limits of the star-streams; but there is 

 no direct evidence on this point. 



I must pass briefly over another subject to which I alluded in my 

 preliminary survey — the Moving Clusters. A question of general inte- 

 rest is, Are they similar to the star-clusters ordinarily so called, the 

 distinction being that the moving clusters are much nearer to us? 

 Prof. Boss has shown that in the far future his Taurus cluster will 

 appear as a small globular cluster. On the other hand, Prof. Turner 

 has called attention to the remarkable fact that in the Ursa Major 

 stream, and probably also in the Perseus stream, the stars lie very 

 nearly in a plane. One would certainly have supposed that the 

 ordinary star-clusters must contain the stars much more closely 



16 p r0C- R y. g oc _ Edinburgh, xxix., p. 390. 



17 Monthly Notices, lxxi., p. 620. 



18 Astron. Journ., Nos. 623-24. 



19 This space would, arguing from the sample eighteen stars round the sun, con- 

 tain at least 40,000 stars. 



