August, 1902.] 



KNOWLEDGE. 



177 



seen that for the immense majority of the small stars and 

 for those relatively brilliant, the condensations are found 

 to be sensibly in the same rei;;ions of the Milky Way. 



Again, a uniform distribution of stars that are sensibly 

 equal (Herschel's first hypothesis) is manifestly op[)osed 

 to what we have found, for in this case we should have to 

 admit that a series of condensations extends, by chance, 

 very far exactly in the line of sight. 



Mori-over, we may doubt whether we should have here to 

 do with real condensations. For we must not forget that 

 if the stars are really of very different brightness, a fairly 

 considerable extension of our system along the line of 

 sight in the direction A, compared with the depth of the 

 system in another direction B, should, iiccording to the 

 laws of probability, produce an apparent excess in the 

 number of stars of all classes in A (even for fairly bright 

 stars, some of which would thus be distant, but in reality 

 exceptionally brilliant or voluuiiaous). And from what 

 we know of the nearest stars, and from the researches of 

 Prof. Kapteyn. it is evident that the differences of actual 

 brightness of the stars are enormous. 



But, on the other hand, there is evidence of more than 

 one kind that there really exist in our system condensa- 

 tions ; that the " clustering power " already recognised by 

 William Herschel has produced its effect. We have first 

 the theoretical considerations of Celoria, Seeliger and 

 others, bearing on the fact that the Milky Way (actual) 

 is placed in a part of the universe where the stars ap- 

 proach more nearly to each other : then we know there 

 exist a great number of clusters, and that, moreover, they 

 abound in the Milky Way ; and, lastly, we have evidence of 

 another order, pcrhajis less conclusive than the evidence 

 based upon numbers, but not to be des])ised. — that fur- 

 nished by photographs. We see there a structure, such that 

 its accumulations must be admitted to be real, with dark 

 lines, the "dark lanes" which have all the appearance of 

 true fissures, as Mr. Maunder has already pointed out. 



Thus we may start with two well-based suppositions: — 

 (1) The considerable inequality of the stars in real bright- 

 ness ; and (2) the existence of veritable accumulations of 

 stars. I have wished to exclude at first all suppositions 

 relative to distance. 



With the aid of these two suppositions we can explain 

 in effect the principal phenomena of the apparent distribu- 

 tion of the stars. Nevertheless — and this is a fact which 

 seems to me important — these two suppositions are not 

 sufficient to offer a plausible explanation of certain facts 

 brought into evidence in the researches mentioned above. 

 Thus the ;omposition of the Milky Way, its proportional 

 richness in brilliant or faint stars, is very different in 

 different parts of the zone. Whilst the great stellar con- 

 centration in Cygnus is already shown in stars relatively 

 bright, another region as dense in Auriga and Moiioceros 

 is only evident in stars of the 8th or t»th magnitude, 

 whilst the region between Sagitta and Sobieski's Shield^ 

 so brilliant, however, in the Milky Way — only begins to 

 appear in Argelander's last class (95), and does not become 

 conspicuous, as is demonstrated in the foregoing researches, 

 until we come to the much feebler stars considered liy 

 Celona(llth magnitude). On the other hand, the strui-ture 

 of the condensations in Auriga and Monoceros is much 

 more homogeneous than that in Cassiopeia and Aquila. 

 Those differences cannot be attributed with any show of 

 plausibilit}' to irregularities of the composition or the con- 

 densation oC these regions of the Galactic zone ; here it is 

 that differences in distance come into play. 



The appearance of tie northern Galactic zone in draw- 

 ings of the Milky Way show besides that tlie whole 

 region between Cassiopeia, Ophiuchus and Aquila is 

 separated fairly clearly by a region in Perseus, thinly 



star so^vn, from the other Galactic region which extends 

 over Auriga, Monoceros and C'anis Major. In the first 

 region, as we have said, the branch which passes through 

 Aquila and Sobieski's Shield seems more distant than the 

 remainder ; but, on the other hand, it has an excess of 

 stars fairly bright or even brilliant neai- Cepheus and 

 between this constellation and the very crowded region in 

 Cygnus. It is as if this region, extraordinarily rich 

 in stars, brilliant ones as well as feeble, in Cygnus 

 forms a nucleus, from which come off streams which first 

 — in Cepheus — aj)proach the sun, to curve round again, 

 receding more and more when in Cassiopeia, Cygnus and 

 Aquila ; to traverse the whole southern hemisphere, 

 ending in Cauis Major and Auriga ; from this great arm 

 there detach branches into Ophiuchus, Scorpio, etc. The 

 sun is situated inside this gigantic spiral, in a com- 

 paratively sparse region of the system between the central 

 nucleus and Orion. Relying on other considerations, 

 developed among other places in Knowledge (March, 

 1898), I am disposed to think that such is indeed the 

 structure of the Galactic system; but this view does not 

 allow of a rigorous demonstration. 



It may be objected that one of the results of Prof. Kap- 

 teyn's, in conformity with the conclusions of B. A. Gould 

 and Schiaparelli, seems not to be in accord with such a 

 disposition of the stars of our Galactic system, for, 

 following these conclusions our sun would form part of a 

 secondary cluster. But we must observe that there may 

 well exist little clusters (Gould computes the number of 

 stars in the solar cluster at seven hundred) in the sparse 

 region of the system, and, besides. Prof. Kapteyn has lately 

 abandoned this idea of a "solar cluster" to which he 

 had been led by considerations, recognised as erroneous, 

 of other astronomers on which he had based it ; and as 

 for the conclusions of Gould and others, as these are 

 based exclusively on the apparent brightness of a small 

 number of stars, they have not any great weight. 



Prof. J. C. Kapteyn (the well-known collaborator of 

 Sir David Gill, founder of the Astronomical Laboratory 

 at Groningen, and medallist of the Royal Astronomical 

 Society) has continued in the last few years the important 

 researches which Mr. Gore and Miss Clerke have pub- 

 lished in their lucid papers in Knowledge. We know 

 that Kapteyn sees in the distribution of the proper 

 motions of stars a moans of increasing very materially 

 our knowledge of the Galactic system. He takes extreme 

 care to eliminate every inadmissible hypothesis and every 

 source of error. And lately Prof. Kajjteyu has been 

 occupied with a new method of determining a great 

 number of stellar parallaxes in the same region of the 

 sky, and it goes without saying that a more extended 

 knowledge of stellar parallaxes may lead ua to decisive 

 results of the highest importance. 



Unhappily, the results obtained so far by Prof. Kapteyn 

 in his last researches scarcely lot themselves be extricated 

 from the technical gear of formula3 ; and, besides, the 

 researches are not yet finished, and we may not anticipate 

 the result. Thus I will confine myself to mentioning that 

 Kapteyn has succeeded in establishing a formula to express 

 the mean parallax of stars of a given ])roper motion and 

 brightness, and that he, from this, finds the relative fre- 

 quency of stars of a determine 1 absolute brightness. In 

 his last publications,* he arrives at the conclusion that the 

 density, the richness in stars of stellar space is much 

 greater than is ordinarily represented ; that there exists 

 an excessive number of stars of feeble brilliancy. Kapteyn 

 gives, for example, the following values: — If we conceive 



* J. C. Kapteyn: Public. Aslron. Lahoratorif, Groningen, VIII., 

 1901; Proceedings Soyal Academy, Amsterdam, 1901. 



