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PROFESSOR A. S. EDDINGTON, P.K.S., ON 



proper motions, as well as some work on radial velocities, is to 

 give a fairly definite knowledge of these two streams. Stream 

 I is moving faster relative to us than Stream II, and it contains 

 rather more stars than the latter in the proportion of 3 to 2. 

 The two motions are inclined to one another at an angle 

 of about 100 degrees. But these motions are distorted by our 

 own arbitrary point of view, since we are looking at them from 

 the sun, or — what comes to the same thing — the earth. If we 

 clear out the solar motion we shall find that the two streams 

 are moving, not at an obtuse angle, but in exactly opposite 

 directions. (That sounds like a remarkable fact, but in reality 

 it is only a truism.) Further, the line along which they move 

 (in opposite directions) is exactly in the fundamental plane of 

 the stellar universe, viz., the plane of the Milky Way. The 

 stars are not scattered in a globular form, but flattened some- 

 thing like a bun. The plane of the bun is marked con- 

 spicuously in the sky by the Milky Way, and it is interesting 

 to find that the two great streams of stars move in that 

 plane. 



We are led then to a conception of the stellar universe in 

 which there are two vast streams of stars sweeping through 

 one another in opposite directions. They are thoroughly inter- 

 mixed and interpenetrating. Of course the stars do not move 

 exclusively in the two directions ; but they are two prepondera- 

 ting tendencies. I do not think that any objection can be taken 

 to this description ; it is, I believe, an inevitable conclusion from 

 the observations. But we need not jump to the conclusion that 

 these two streams — these two opposing tendencies — really 

 indicate that two great systems of stars have come together 

 and are rushing through one another. That may be the case, 

 and it is the most obvious and direct interpretation of what we 

 see. But I think most astronomers would rather cling to the 

 idea of some essential unity in the stellar system, believing 

 that the two streams arise in some natural way as parts of one 

 whole. However that may be, it is safe to describe what is 

 going on as a streaming of the stars in two directions ; we are 

 introducing some amount of speculation when we account for 

 these streams as two definite systems. 



There is yet another remarkable thing that we have 

 recently learnt about the movements of the stars. You know 

 that by the spectroscope a minute examination of the quality 

 of a star's light can be made, and important conclusions as 

 to its physical condition deduced. The spectroscope, like a 

 prism, spreads out the different constituents of the star's light 



