58 



KNOWLEDGE. 



[Maboh 1, 1898. 



the general naked-eye aspect, two minutes' study on a fine 

 evening in September is sufficient to establish the great 

 superiority in brightness of the Milky Way between 

 Sagittarius and Cepheus over that between Cepheus and the 

 Twins. As for the counts and stellar gauges, Sir William 

 Herschel found an average of IGTS stars in his gauges 

 about Aquila as compared with 82-5 about Monoceros. 

 Celoria found likewise for all stars down to the eleventh 

 magnitude in ah equatorial zone of about six degrees 

 breadth, ")8-8S3 stars in the region containing the Milky 

 Way about 18h., and 43 82ii in the part that the Milky 

 Way crosses about Gh.* This is a fact that it is quite easy 

 to establish, but whose consequences have not received the 

 attention that they merit. 



Unless we admit that we are situated in the centre of 

 the ring, but that in the body of this irregular ring the 

 stars increase systematically, so to speak, towards a point 

 (which is evidently most improbable), we must conclude, as 

 was said above, that the sun in the interior of this hypo- 

 thetical ring occupies an excentric position, fairly near the 

 side where is Monoceros, moderately distant from Aquila. 



But why, then, does the lirciuUh of the galactic zone in 

 Monoceros differ so little from that in Aquila ? Evidently 

 the Milky Way in general ought to appear larger to us 

 the nearer we approach the hypothetical ring, for we could 

 not presuppose (and before such an utterly improbable 

 thing has been proved independently) that the irregulari- 

 ties in the breadth of the zone (any more than the irregu- 

 larities of brightness) increase towards a given point in 

 the circumference. But at first sight the Milky Way 

 appears, on the contrary, larger in the region of the Eagle, 

 because of the two brilliant branches, and that is why 

 Kant has already placed the sun near to that part of 

 the Milky Way where this constellation is found. After 

 studying it, however, more attentively with the naked eye, 

 and including all the branches, it appears rather broader on 

 the majority of charts in Monoceros than in Aquila, but 

 the difference is far less than theory would indicate. Is 

 this circumstance due to the mode of formation of the 

 visual Milky Way itself '? (See my preceding paper.) No, 

 for in the paper of Prof. Celoria we find an easy way of 

 measuring the breadth of the zone where the stellar density 

 is greater than the "average" {" jihi/^icul dutaxy" fi; 

 and it follows from one of his tables — Tavola V — that 

 for the stars as far as the eleventh magnitude (and 

 also for the whole of the fainter stars that W. Herschel 

 saw in his great telescope), the Milky Way is consider- 

 ably larger in Aquila than in Monoceros, and even 

 (particularly for the relatively brilliant stars — 11) that 

 the principal branch in the Eagle alone has almost the 

 same breadth as the entire Milky Way in Monoceros, where 

 the galactic light is, moreover, so feeble. 



This evidently contradicts the hypothesis of a simple 

 and continuous ring whose parts are all situated at con- 

 siderable distances from the sun. ( Situated in the interior 

 of such a ring, we ought to be able to observe a correlation 

 between the narrow, brilliant, and well-defined portions on 

 one hand, and on the other between the feeble, diffused, and 

 broad portions.) The hypothesis that there is a real 

 duplication of the Milky Way into two branches at the 

 same distance from us, over almost exactly the half of its 



•Sir John Herschel, Outlines; F. G-. W. Stiure, Etudes ; J. T. 

 'Enckc, Astroa. Nac/iHchten, XXYL.lSiS.p. 3S6; Houzeau, Urano- 

 graphie ; Atlas, Mons, 1878; Easton, Voie Lactee, 1893; Astron. 

 Nachrichten, 3270; Plassmaun, Jahresberichte der I'.A.P., Berlin, 

 1898; Celoria, Fubbl. del Oss. di Srera, XIII. 



t " Le region! in cui le densita stellari sono piu grandi dcUa densita 

 media si possono chiamare regioni lattec." Celoria, ibid., p. 43. 



course, is obviously improbable ; but it is also incompatible 

 with tlie reality, for the classic representation of the 

 "simple " Milky Way* in Cygnus, Monoceros, and Crux, 

 as opposed to the double portion in Crux, Aquila, and 

 Cygnus, does not exist. + If we hold to an annular Milky 

 Way we are compelled to accept at least lu-n rings, which 

 both surround us but at very different distances. The 

 nearest ring easily explains the very remarkable circum- 

 stance that the fairly brilliant stars — those found in the 

 " Bonn Durchmusterung " of about — 0-5 magnitudes — 

 are, contrary to the others, more numerous in Monoceros 

 than in Aquila, a phenomenon that is repeated under 

 another form in the belt of bright stars of Sir John 

 Herschel and of (Jould.+ Celoria, moreover, does not 

 hesitate to admit " due uiiiJli distinti, ne inai intermtti nel 

 loro corso." The stars in the nearest ring are projected on 

 the sky following the circle ; Cassiopeia, Hyades, Orion, 

 Crux, Scorpius, Ophiuchus, Cepheus, those in the more 

 distant ring following Cassiopeia, Auriga, Monoceros, 

 Crux, Sagittarius, Scutum, Sagitta. The Italian astro- 

 nomer does not venture an opinion as to whether these 

 two rings really interlace or are only in projection. 



At the time when Celoria's researches were published 

 (in 1878), this theory of two distinct and uninterrupted rings, 

 that appeared to explain fairly well the general features 

 of the galactic phenomenon, did not so much clash as it 

 does to-day with the objection that, presented in this 

 form, it is unacceptable because of the structure of the 

 Milky Way revealed by drawings, and, above all, by photo- 

 graphs. For this reason a single ring (the principal ring, 

 for instance, in Sagittarius and Monoceros) cannot be 

 imagined but by straining probability ; as for two complete 

 rings, they are quite inadmissible. The phenomenon is 

 evidently much more complicated even in its principal 

 features. 



But is this a reason for throwing overboard the irhole 

 of this theory of Celoria's, which rests, moreover, on serious 

 observations and deductions '? By no means. It is not 

 admissible in its entirety, but may weD be true in part. 



Suppose, for example, that these "rings" of Celoria 

 are not "unbroken," nor even complete rings, but annular 

 detached segments roughly disposed in two planes — or, 

 rather, in a " broken plane " (Strnve) — the grave objection 

 that we have just raised ceases to exist, and the system is 

 in accord with the results that Celoria and other astro- 

 nomers have obtained. 



But, first, here are some considerations of a different 

 nature. 



If we imagine the Milky Way to be an assemblage of 

 stars and of clusters of stars distributed quite by chance, 

 we ought to find in all regions of the galactic zone the 

 same characteristics very nearly : these characteristics 

 depending on the chance of the projection which should 

 manifest itself sensibly in the same manner in all direc- 

 tions. The details of the distribution will differ greatly 

 in one direction from another, but the general character — 

 the type — will depend only on the general conditions of the 

 whole ; the limits between which vary the stellar density, 

 the volume aud brightness of the stars in different parts 

 of the system, the frequency of nebulosities and of opaque 

 bodies, etc. — this type will be constant. 



In reahty it is not so in the Milky Way. Those who 

 have studied it best, both in its aspect to the naked eye 



* " Theme ' (Cvgnus to Perseus, etc.) " the stream is single." 

 Proctor, ilont/ilt/ A'otices, XXX., p. 50. 



t Boeddicker, The Milky Wai/ ; Easton, La Voie Lactee, etc. 

 X Celoria, Hid.; Sir John Herschel, Outlines ; U. A. Gould, Urano- 

 meiria Argentina, 1. 



