464 ASTROPHYSICS 



tional data for their solution should cover the whole sky. Until one 

 year ago radial velocity measures were confined to the northern two 

 thirds of the celestial sphere. Further attempts to deduce the solar 

 motion from northern observation alone would not be justified. 

 Observations in the southern third of the sky were needed, not only 

 to represent that large region in the solution, but in order that the 

 unknown systematic errors which affect the northern observations, 

 as well as the southern, might be eliminated, through the symmetrical 

 balancing of the material. Fortunately the energetic and wise policy 

 of the Cape Observatory and the generosity of Mr. D. O. Mills have 

 provided two complete equipments, which are now busily engaged 

 in supplying the southern data required. The Mills spectrograph in 

 the northern hemisphere has secured about three thousand spectro- 

 grams of approximately five hundred stars, and the Mills spectro- 

 graph in the southern hemisphere has secured four hundred spectro- 

 grams of one hundred and twenty-five stars. The number of stars 

 not on the Mills list, and accurately observed with other high-dispersion 

 spectrographs, is not known, but it is probably between one hundred 

 and two hundred. We may reasonably expect that, in two or three 

 years, as many as eight hundred well-determined radial velocities 

 may be brought to bear upon pressing sidereal problems. 



It is a frequent question: Is the solar system moving in a simple 

 orbit, and will it eventually return to the part of its orbit where it is 

 now? The idea of an affirmative answer to this question is very pre- 

 valent in the human mind. It is natural to think that we must be 

 moving on a great curve, perhaps closed like an ellipse, or open like a 

 parabola, the centre of mass of the universe being at the curve's prin- 

 cipal focus. The attraction which any individual star is exerting 

 upon us is certainly very slight, owing to its enormous distance; and 

 the combined attractions of all the stars may not be very much 

 greater; for since we are somewhere near the centre of our stellar 

 system, the attractions of the stars in the various directions should 

 nearly neutralize one another. Even though we may be following a 

 definite curve at the present time, there is, in my opinion, little doubt 

 that we should be prevented from continuing upon it indefinitely. In 

 the course of our travels we should be carried, sooner or later, quite 

 close to some individual star whose attraction would be vastly more 

 powerful than that of all the other stars combined. This would draw 

 us from our present curve and cause us to follow a different one. At 

 a later date, our travels would carry us into the sphere of attraction 

 of some other great sun which would send us away in a still different 

 direction. Thus our path should in time be made up of a succession of 

 unrelated curves. 



. Spectroscopic binary systems, as by-products of radial velocity 

 measurements, are of exceedingly great interest from the light which 



