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PROFESSOR A. S. EDDINGTON, F.R.S., ON 



each one of them is a great system, equal to the great system 

 of hundreds of millions of stars that we have hitherto con- 

 sidered. The name " nehula " has been used to denote a 

 number of celestial objects having entirely different characters ; 

 the great gaseous irregular nebulae, such as that in Orion, are 

 undoubtedly within the stellar system ; but the spiral nebula? 

 are not at all of the same character, and we have as yet no 

 evidence as to whether they are within or without the 

 system. 



The spiral nebula? are a great puzzle ; if they are not other 

 universes, it is hard to say what they are. Many astrono- 

 mers consider that we should not let our imaginations run to 

 such grandiose ideas until we have some clear evidence that they 

 cannot be within our own system. The stellar system is vast 

 enough beyond conception. Can we not be content with it ? 

 Must we still run on 



" From star to star, from kindred sphere to sphere, 

 From system on to system without end " 1 



I do not agree with this prohibition ; it seems to me that in the 

 absence of definite information we may, nay we must, keep both 

 alternatives before us. And for the moment the idea that the 

 nebulae are stellar universes co-equal with our own seems to 

 present great advantages as a working hypothesis. It suggests a 

 model of our own system which we can try to follow out and 

 test. For instance, Mr. Easton has discussed how the Milky 

 Way works out in detail on the assumption that it is the outer 

 part of a spiral. Again, all the spiral nebula? known are double 

 spirals, that is to say, they have two arms. We do not under- 

 stand the dynamics of spiral nebula?, but I think it is clear that 

 matter must be flowing in along the two arms, or flowing out — 

 it does not matter which. Let us suppose it is flowing in. Now, 

 taking the stars in our own neighbourhood, will there not be 

 some'trace there of the two opposing currents which flow in 

 from opposite directions ? That gives us a possible interpreta- 

 tion of the two star-streams as due to the two arms of the 

 spiral. Moreover, the line of flow is — as it should be — exactly 

 in the plane of the spiral, i.e., of the Milky Way. That is at 

 least one point in favour of the island universe theory. 

 Marcus Aurelius wrote in his " Meditations " — 



" Now among them that were yet of a more excellent nature, as 

 the starres and planets, though by their nature farre distant from 

 one another, even among them beganne some mutual correspondencie 

 and unitie." (Casaubon's translation.) 



