30 



NATURE 



[November ><, iqco 



both in regard to their proper motion and their 

 distribution. 



Now the spectroscopic similarity between the gaseous 

 stars and the "bright-line" and "new" stars, and the 

 planetary nebulas, justifies our assuming provisionally 

 that they exist under some similar conditions, and, as 

 they are all confined to the Milky Way, we are further 

 justified in assuming that they lie at the same distance 

 from us. 



The smaller proper motion of the hottest stars, in 

 which I include the bright-line stars, proves that the 

 region which gives rise to them as well as the new stars, 

 and the planetary nebulae, is far away on all sides. If 

 it were not so we should get a very small proper motion 

 in one direction and a very large proper motion in 

 another. 



But the stars in question in the Milky Way, which is a 

 great circle, are all equally remote ; and the only place 

 whence such a state of things can be observed must be 

 a point equally distant from all, that .is, in the centre 

 of the system under observation. 



It is worth while to repeat that it is because we are in 

 the centre, because the solar system is in the centre, that 

 the observed effect arises, and if we imagine the solar 

 system very far from the centre we should get very different 

 proper motion conditions on this side and on that ; but 

 seeing that we have found that we get the smallest proper 

 motion with regard to all the hottest phenomena that we 

 know of in space, we have to consider that the still 

 truly nebulous region is far away from us in every 

 direction, and that it practically is limited to the plane of 

 the Milky Way. 



Photographs of some drawings made by Herschel, 

 when he was first brought into the presence of the 

 wonderful nebulae with which the heavens are peopled, 

 will give an idea of what possibly may be the condition 

 of- things touching our own system. We have amongst 

 them drawings of " globular " nebulse, possibly not globes, 

 but systems looked down upon from their poles, and the 

 possibility of that arises from the fact that many nebulcC 

 are looked at edge-ways, and are very thin. Hence we 

 do not know that the apparently globular clusters are not 

 really those things looked at from the poles of their move- 

 ment. We have not only those globular and elliptic 

 nebulae, but we have double elliptic nebulae, which 

 might be considered as explaining how the Milky Way 

 happens somehow or another to be doubled. In addition 

 to these we have well-defined ring nebulae, the best 

 example of which is in the constellation Lyra. It has 

 been often imagined, up to now, by those who have con- 

 sidered this subject, that the Milky Way owes its appear- 

 ance to the fact that there is really a spiral nebula in 

 question, and that the stars which form the stellar system 

 and form the companions of the sun exist at the centre of 

 a spiral nebula. One of these spiral nebulae, which we 

 observe looking down on the whole system from 

 the pole is the spiral nebula in Canes Venatici. 

 The wonderful nebula in Andromeda, also a spiral 

 nebula, we look at side-ways, and so it appears 

 elliptical, and in this we notice that the greatest 

 condensation is in the centre. But we know, from 

 what I have stated, that our greatest condensation is not 

 in the centre ; in our case the greatest vacuity is in the 

 centre. We are in the quiet, in the centre ; so that cer- 

 tainly if we take our choice of these different forms, we 

 must say that our system is much more like that of the 

 ring nebula in Lyra than it is to such systems as those 

 in Canes Venatici and Andromeda. We, according to 

 Gould's work, have in the centre of our system, repre- 

 sented by the Milky Way, a small number of cooling 

 stars all congregating together; outside that at an infinite 

 distance from these relatively cool bodies, we have the 

 Milky Way stretching with all its concomitants of 

 gaseous stars, planetary nebula;, bright-line stars, new 

 NO. 1619 VOL. 63I 



stars, and so on. We must therefore consider that in our 

 present knowledge such a condition of things as is repre- 

 sented by the ring nebula in Lyra fits our facts very much 

 better than the condition which is represented by such a 

 spiral structure as Andromeda, in which the greatest 

 heat — I say that because there is obviously the greatest 

 luminosity — is located at the centre. I have already 

 referred to the proper motion evidence. It is obvious 

 that in the case of the nebula of Andromeda, if we 

 imagine an observer at the centre, large, medium, and 

 small proper motions would be observed in every direction 

 in the plane of the system, for the reason that the spirals 

 lie in some cases near to, and in others far from, the 

 centre, and that there are many spirals. We practically 

 know that in our system the centre is the region of least 

 disturbance, and therefore cooler conditions. 



I now come to another point which must be considered 

 in the next place. 



Let us assume for the moment that the average bright- 

 ness of stars depends upon their distance ; then the 



Fig. I. — Spiral nebula of Canes Venatici, from a photograph by Dr. 

 Roberts. 



number of stars of a given magnitude indicates the stellar 

 density at a corresponding distance. Gould from actual 

 enumeration has given a formula which shows us that if 

 the stars were uniformly scattered in space, and the light 

 from them suffered no extinction in coming to us — if it 

 did not meet anything that it could not get through — 

 then the number of stars visible to us through a telescope, 

 such as we have at Kensington or at Mount Hamilton, 

 should be about 12,000,000,000. But the number actually 

 visible, so far as counts are concerned, is certainly very 

 much less, and, in fact, it has been estimated that the 

 countable number, instead of being 12,000,000,000, is 

 bnly about 100,000,000. This estimate seems to me very 

 low, but I am bound to give it. When we co ne to consider 

 the stars of different magnitudes in different parts of 



