42 



KNOWLEDGE 



[Mabch 2, 1891. 



very closely with the bright stars seen in the same field 

 of view, then must its structure be somewhat as shown in 

 the figure. ... It will be seen at once how, to an 

 observer placed at S, the various features of the Milky 

 Way can be accounted for by this figure. Towards 1 

 would Ho the gap in Argo ; towards 3, 4, 5, two 

 branches, one faint, and in part evanescent through 

 vastness of distance, the other forming the brightest 

 part of the spiral ; towards G the projection in 

 Cepheus ; towards 7 the faint part of the IMilky Way 

 in Gemini and Monoceros. The Coal-sacks would 

 be simply accounted for by conceiving that branches 

 seen towards the same general direction, but at 

 different distances, do not lie in the same general 

 plane, and so may appear to interlace upon the 

 heavens. We are not only justified in supposing 

 this, but forced to do so by the way in which the 

 stream of milky light is observed to meander on 

 its course athwart the heavens. The branching 

 extensions serve very well to account for the appear- 

 ance of the Milky Way between Cepheus and 

 Ophiuchus, where the interlacing branches and the 

 strange convolutions and clustering aggregations 

 described by Hir -John Herschel are chiefly 

 gathered."— Otfin- Worlds, p. 2.50. 



Proctor's affection for this spiral is remarkable. 

 It was advanced first of all with some caution as a 

 conformation which seemed to account for all the 

 known peculiarities of structure in the galaxy — in 

 short, it supplied a useful working theory. For 

 example, he makes the following admission : " I 

 would not have it understood, however, that I at 

 all insist on the general shape of the spiral shown 

 in the figure. On the contrary, that curve is only 

 one out of several which might fairly account for 

 the observed appearance of the Milky Way ; and I fig. 

 have often felt inclined to doiibt whether a single 

 spiral of this sort is in reality the best way of account- 

 ing for the observed appearance of the galactic zone. 

 What I do insist upon as obviously forced upon us by 

 the evidence is that (1) the apparent streams formed by 

 the Milky Way upon the heavens indicate the existence 

 of real streams in space ; and (2) that the lucid stars seen 

 on the stream are really associated with the telescopic 

 stars which form, so to speak, the body of the stream. 

 Whether that stream form a smgle spiral or several, 

 or whether, instead of spii-als, there may not be a number 

 of streams of small stars, placed at dili'erent distances 

 from us, and lying in all directions round the medial 

 plane of the galaxy, but more or less tilted to that plane 

 (the sun not lying within any one of the streams), are 

 questions which can only be resolved by the systematic 

 scrutiny of this wonderful zone." 



But just as Sir Wilham Herschel's working-theories of 

 star-distribution came eventually to be regarded as esta- 

 blished facts, so in course of time, without any more 

 apparent grounds. Proctor came to regard his theory with 

 increasing confidence. Scattered throughout his writings 

 will be found very trustful remarks on the conformation 

 of " that strange spiral." 



Without doubt, if the star-groups, such as the Pleiades, 

 are real, and not merely caused by stars far apart, though 

 near the same visual-line, being seen projected into a 

 small area of the celestial sphere (and Michell's, not to 

 mention later results, seem to make it certain enough) ; 

 if the star-streams are real, and not mere optical coinci- 

 dences ; if the manifold signs of association and dissocia- 

 tion among the stars are what common-sense would 

 teach ; then it is as certain as the doctrine of chances can 



make it that the Milky Way is a stream, or collection of 

 streams, of stars. So much, but no more. Proctor has 

 proved. What he has not proved, nor even made pos- 

 sible, is that the Milky Way partakes in any way of a 

 spiral form. 



Fig. 1. — The Milky Way as seen in the Heavens. 

 -The Spir.\l Stream which Mr. Proctor assumed represented the actual 

 form of the MiUiy Way. 



Let US see what the brightness of the Milky Way can 

 tell us. Proctor assumed the brightness of its various 

 parts would be a good rough test of their relative dis- 

 tances — a curious mistake for one so conversant with the 

 laws of brightness. [It may be noted that Sir .lohn 

 Herschel erred in the same direction.] Now it may be 

 proved that brightness alone is no test of distance. For 

 let a be the apparent area of any small portion of the 

 Milky Way at some distance taken as unity, and contain- 

 ing n stars, X the average amount of light received from 

 each star, /S the average brightness of the area. If this 

 area be removed to distance d, the light received from 



each star will be reduced to -j^ ' ^lut the apparent area 



containing the n stars will be reduced to ,.,, and hence 



the stars are apparently compressed into a smaller area in 

 the same proportion as their light is reduced. /3, there- 

 fore, remains unaltered. The apparent breadth of the 

 galaxy would be a much safer test of its distance than the 

 brightness, although its actual breadth may vary between 

 very wide Umits. As a matter of fact, the broadest parts 

 of the Milky Way are, on the whole, the faintest ; 

 whereas in its narrowest part, at the " isthmus " leading 

 into the Southern Cross, it is almost at its maximum 

 brilliancy. 



During the course of his long review of the elder 

 Herschel's cloven-flat-disc theory, Proctor pointed out 

 again and again the remarkable tendency of the brighter 

 stars to congregate along the course of the Milky N\'ay 

 and its branches. This feature is so noticeable in the 



