82 



♦ KNOWLEDGE ♦ 



[Jan. 1, 1886. 



every accession of powei' is followed by the resolution of 

 more and more of the spot, but that the highest powers 

 fail to resolve some parts. Also the spot contains stars 

 of the seventh magnitude. If then we are to look ou 

 irresolvability >vith given powers as a test of distance, 

 the trne figure of the system which apjiears to us as a 

 spot is a frustum of a cone — as A B D C, Pig. - — haviiig 

 the sun, S, at its vertex. And from the size of the largest 

 telescope used by your father (irresolvability under which 

 implied, he considered, a distance which light would take 

 20,000 j-eiirs in travelling) we should have A B at least 

 100 times greater than C D. This seems utterly contrary 

 to all reasonable probability : and I think we have pre- 

 cisely the same sort of argument here which you have 

 drawn from the Magellanic Clouds — the conclusion being, 

 that within limits of distance which are as about seventy 

 to seventy-one (remembering the small area of the spot in 

 Perseus) there may coexist stellar arrangements resulting 

 in all degrees of resolvability, from star groups almost 

 resolvable by the naked eye down to absolute irresolvability 

 in the largest telescopes man has yet constructed. Irre- 

 solvability being thus shown to be no test of distance, it 

 seems to follow, e converso, that absence of nebulous light 

 tinder given powers is no proof of relative nearness. 



If the gap in Ophiuchus is really due to distance, 

 however, there are certainly some indications we might 

 look for along the fading extremities of the two arms 

 which here extend towards each other. One is as you 

 point out the diminution of star-magnitude ; but it is to 

 be noticed that all we could look for is the absence of 

 stars of the brighter telescopic orders. The background 

 of this part of the Milky Way would not differ from the 

 background elsewhere. There would be stars of all 

 orders from those just visible up to a certain magnitude 

 — elsewhere there would be stars of all orders from those 

 just visible up to a certain higher order of magnitude. 

 The difference could only be determined by careful and 

 systematic observation directed to that special end. If 

 there is no such diSerence my notion about the Milky 

 Way would have to be abandoned, or at least looked on 

 as not probably correct. 



You will notice that my theory- indicates the possi- 

 bility that an infiuite variety of constitution may exist 

 in different parts of the Milky Way. 



I am not sure that one could expect the Milky Way 

 near Crux and thence to Argo to present any obvious 

 signs of the structure I suggest — supposing my sug- 

 gestion correct. The apparent difference between the 

 nearer and farther streams ivould be very slight. Even 

 in our own neighbourhood questions of relative distance 

 are so difficult to determine, that one might expect this 

 one to defeat any save the most systematic inquiry, 

 directed to the special purpose of determining whether 

 there are any signs to guide us here. I am perplexed by 

 the appearance given to the Milky Way in this neighbour- 

 hood in the large maps of the S. D. U. K. The rest of the 

 Milky Way is shaded to a uniform tint, and is obviousl}- 

 incorrect in many places. But in the south-polar map 

 the artist has suddenly indulged in the strangest variations 

 of shade. AVhat authority he had, if any, is not stated. 

 One cannot help feeling he must have had some — perhaps 

 notes by Lacaille or others. Inter alia, the Milkj' Way 

 is shaded mttch more darkly along one side of the 

 Coal-sack than along the other ; i',nd jn-esents in an 

 exaggerated form all the appearances my theory would 

 lead one to expect. Whether some southern traveller 

 noticed a slight difference of brilliancy, which has some- 

 how been expanded into the singular arrangement I have 

 mentioned, or whether the whole matter is the resitlt of 



carelessness I cannot say : but one actually seems to see 

 one stream crossing the other. This I noticed after my 

 theory was formed, so that I \\as natttrally rather 

 interested in the iieeuliarity : but if there is any such 

 difference of brilliancy it must be very slight. Would 

 it be possible that .such a feature could have escaped 

 your notice .' You remember Faraday used to say when 

 an experiment was to be shown him " AVhat am I to look 

 for ! And it seems just possible that not having the 

 thought of looking for such a feature or of iittaching anj- 

 importance to it if seen, it might have escaped even your 

 unrivalled powers of observation. I put forward this 

 view with extreme diffidence, and am quite prepared to 

 accept your opinion as decisive as to the appearance of 

 the Milky AVay here, if j-ou feel clear that the delicate 

 difference in question could not have escaped you. 



According to my theory, the proper motions of the 

 stars in Crux should be larger towards the a end of the 

 cross than towards the other. I try the experiment, 

 though without putting very much faith in it — as the 

 proper motions assigned to southern stars seem to require 

 revision. 



/.!■■ 



Fig. o. — Proper motions (in ;!().000 Fig. 4. — IUu>.trating the effects 

 years) of stars in the ^Southern of our sun being in the midst 

 Cross. of a stream o£ stars. 



One cannot make much I fear from what is shown in 

 Fig. 3. Of course it is part of my theory that the bright 

 stars, such as those in Crux, sway the Milky Way wisps, 

 and are mixed up with them. 



I did not mean to stissest that the sun is altos'ether 

 out of the plane rich in stars within which the Milky 

 Way streams have (according to my view) been formed. 

 But I cannot find any other interpretation of a well- 

 defined edge anyivJtert along the Milkj- AVay,than this, that 

 the sun is out of that particular gathering to which the 

 edge belongs. He might be on one part of the stream and 

 we should still se;- such an edge when looking at other 

 parts. But there is this difficulty : — If the sun is 

 altogether out of the stream, but near its medial plane, we 

 should see precisely the sort of increase towards the 

 galactic plane that we actually observe, the stream only 

 being the nuclear liiie (if one may speak so incorrectly) 

 of a spiral region rich in small stars. But if the sun 

 were in a stream, we ought to see an aggregation of 

 small stars towards two opposite points on the heavens — 

 as towards A and B (Fig. 5) — and a zone wanting in small 

 stars towards C and D. 



I am not so fond of my twisted-stream theory as not fo 

 recognise its weak points. But it seems to me we are 

 forced to take some stream-theory, when we apply the 

 laws of probability to the appearances presented by the 



