January 1, 1898.] 



KNOWLEDGE 



13 



crossing of the two contorted streams which really are at 

 different distances from the eye " — is possible but not very 

 probable, I think, after the evidence furnished lately by 

 Gould and Russell. I do not think it was necessary in 

 this case to turn aside from Sir John Herschcl's opinion 

 that the Mility Way in the neighbourhood of tlie Coal Sack 

 is just " a distant mass of comparatively moderate thick- 

 ness, simply perforated from side to side," or as an oval 

 vacuity which is seen " foreshortened in a distant fore- 

 shortened area." (" Outlines," ; 702.) 



There is doubtless much truth in Proctor's supposition 

 of the branches which are detached from the main stream 

 of the Mdky Way, and which penetrate into the neighbour- 

 hood of the sun, and of the less important ramifications 

 which spring up at different points of the galactic course. 

 But as regards the general reasoning followed by Proctor 

 in comparing bis figure with the aspect of the Milky Way, 

 he is fundamentally in error ; and this is an interesting 

 point, since Sir John Herschel had already made a similar 

 mistake. Herschel says that the brilliant and well-defined 

 part of the Jlilky Way about Argo and Crux " conveys 

 strongly the impression of a greater proximity"; and he 

 deduces from this that the sun occupies an excentric 

 position in the interior of the Milky Way, which is nearer 

 to the southern than to the northern part of its circuit. 

 Taking up this argument and amplifying it. Proctor admits 

 that the stream grows gradually fainter with increase of 

 distance towards Canis Jlinor and Monoceros ; and in 

 speaking of the brilliant portions of the Galaxy in Aquila 

 and Sagittarius he is satisfied that " this part, which is so 

 very bright, corresponds to the part which my spiral brings 

 so venj )b ar to the sun." 



But we should see precisely the opposite in this case. 

 Sir John Herschel and Proctor have been too much taken 

 up with the idea of a stream, of a " distant mass," which 

 they represent as continuous, like a band of cloth, whose 

 details are perceived with more clearness the nearer they 

 are. But the phenomenon of the Galaxy is of quite a 

 different nature. As long as the brightness of each indi- 

 vidual star is of great importance, and their mutual 

 distances which we see projected are insignificant, the 

 reasoning of Sir John Herschel and of Proctor holds good. 

 But it is, above all, the closeness. In projection, of the 

 small stars in the Milky Way which produces the optical 

 phenomenon of a galactic gleam. The individual brilliancy 

 matters little. This can be easily demonstrated. From 

 the gauges of Sir William Herschel and from the star- 

 counts of G. Celoria, it follows that the number of stars 

 sufficiently brilliant (seventh to eleventh magnitude) which 

 take part in the formation of the lacteal light, is much 

 more considerable in the region of Monoceros than of 

 Aquila, and in spite of the fact that the Milky Way is, 

 without gainsay, much more luminous in the latter portion 

 of the sky than in the former. Then, the abundance of 

 stars in certain lacteal regions (Scutum, Cygnus, etc.) is 

 so great that the relatively bright stars form but an insig- 

 nificant part of it distributed here and there among the 

 multitudes of small stars. But, whatever may be the 

 number of stars necessary for this, the stars are snfB- 

 ciently near each other in perspective for their collective 

 light to produce a strong enough impression on the 

 extremity of our optic nerve and give us the impression of 

 the lacteal gleam— an impression that could not be pro- 

 duced by stars each much more brilliant than the others 

 united, but their projections too distant for their images 

 to fall on the same nerve bundle in the retina. 



The same thing is seen when a celestial object is resolved 

 into stars which had until then appeared nebulous. Thus 

 the parts of the Milky Way which are nearest to ua would 



appear by the same rule vague, large, and rich in stars 

 relatively bright, whilst the distant portions of the zone 

 would appear more crowded and better defined — more 

 luminous in themselves, though numbering fewer brilliant 

 stars. One could easily represent this appearance by 



Qa.'i 3ti 



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^^' y' 





^"B:!"^^. 



b .■'■ 





1 <> 

 2 I ■■■'-.' .'=•:- * ■ ■■■■ I «> 



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The Milky Way according to Celoria. 



imagining oneself within a huge circle of trees, nearer to 

 one part of the circumference than to the rest. In the 

 near part the trees do not form a continuous band, whilst 

 they are confounded in one straight dark line in the 

 further portions of the circle. 



I have written at some length on this point because it 

 undermines the reasoning of Proctor in more than one 

 particular, and also demonstrates that one remark of Sir 

 John Herschel, often quoted, rests on an erroneous argu- 

 ment, and that to my knowledge these points have been 

 raised before. 



After what I have just said it would be superfluous to 

 criticise in detail the " spiral " of Proctor. For the rest, 

 even if they furnished a perfect explanation of all the 

 principal features which Proctor finds in the Milky Way, 

 they could no longer serve, now that the principal features 

 of the galactic zone in the two hemispheres — thanks to 

 the drawings and to the photographs,* and in despite even 

 of the dift'erences that may be perceived there — appear to 

 us under.quite a different form. Must we, then, return to 

 the theory of a cloven disc or to that of a galactic ring ? 

 Certainly not. Proctor has, without doubt, been right in 

 giving up these premisses : that the theory of a stellar 

 stratum in form could not be defended in these days, and 

 that the phenomenon of the Galaxy is due to a distribution 

 of the stars of a much more complicated character than 

 could be produced by a ring, however irregular. 



Without entering into details which would take too 

 much space here, I hope to give a summary of what has 

 led up to the result that the most modern researches (after 

 Proctor) have established with sufficient certainty. 



The visible universe, stars and nebulm (with the excep- 

 tion of nebuliE properly so called/, is extended in a flat 

 layer irregularly condensed. The stars differ extremely, 

 not only as regards their volume, but also as regards the 



* Drawings of Heis, Houzeau, Davis and Thome (G-ouId), Boed- 

 dicker, Easton, and otfier.-* ; photographs of Barnard, Wolf, ^nd 

 Russell. Tlie readers of Knowlkd&e hare often had prints of these 

 admirable photographs of the Milky Way. 



