Nov. ], 1885.] 



♦ KNOWLEDGE ♦ 



19 



tlie effect due to the stars' <i\vii motions and the 

 effect due to the sun's motions, assuming, — 



1. Sun's proper motion = stars' average proper 



motion. 



2. Motions equallj^ distributed in all directions. 



3. Stars equally distributed through space. 

 The evidence (obtained by Mr. Dunkin) that the 



sun's motion does not cori'ect the stars' motions in 



this propoi'tion — i.e., by one half, establishes the 



probability that the distances of stars of smaller 



mags, have been underrated — and that therefore 



the extent of the effect of the sun's motions upon 



such stars has also been underrated. 



20°. The stars of smaller mags, are not so numerous 



as they should be according to the theory of uniform 



distribution. 



[Professor Nichol speaks of their number as "accord- 

 ing ^yith their theoretic distances," but Struve's 

 hypothesis of the extinction of light was devised 

 to account for a marked want of accordance in this 

 respect.] 

 21°. The general appearance of the Milky Way is not 

 that which either the theory of a cloven disc, or that of a 

 flattened ring, requires. 



[Sir W. Herschel was gradually modifying the d^sc 

 theory — necessarily the first to be adopted after 

 his gauging processes ; and you have spoken in 

 many places of the evidence which appears in 

 form of a modified view.] 

 22°. The well-defined edges of parts of Milky Way 

 prove we are outside its streams ; for no cluster of objects 

 within which (cluster) the observer is situated can any- 

 where appear with well-defined edges. 



23°. The great gap in Argo is not explicable on either 

 the disc or the flat-ring theory. 



2i°. The " Coalsack," though more readily explicable 

 on the latter than on the former, remains a difficulty 

 with either. 



25°. The singular want of lucid stars in the " Coal- 

 sack " and between the branches in Scorpio, ifcc, is 

 another difficulty. 



26°. The break in the second stream of the Milky Way 

 is a great difficulty. 



27°. The knots and clustering aggregations along the 

 Milk}- Way seem to require a different theory. 



[The view I would suggest as roughly explaining 

 observed phenomena is the following* : — This ex- 







PROJECTION I 

 CIPHIUS 



Fig. 1. A perspective view of Milky Waj- supposed to be depicted 

 on a crystal globe. 



Fis 



i;eiieral figure of Milky 

 Way spiral in space. 



* The figures are copied directly from the sketches in the rough 

 draft of my letter. 



planation accounts for the singular diminution of 

 brightness from Cygnus to Ophiuchus, and the 

 equally singular increase from Cygnus to Aquila. 



28°. The lucid stars near the Milky Way seem to be 

 intimately associated with it — especially in certain 

 regions. 



29°. Along that part of the Milky Way which I have ptit 

 on the nearest jsart of the spiral the stars have in many 

 instances singularly large proper motions — also 61 C3"gni, 

 and a Centauri, the nearest known stars are on this 

 branch. 



30°. The proper motion of the Sun is such as the dyna- 

 mical conditions suggested by my spiral seem to require. 



31°. There are in many parts of the Milky Way out- 

 lying streamers. On either the flat disc or the flat ring 

 theory we cannot readily conceive these to be " columnar 

 excrescences bristling up from the general level." Tet 

 were it not for the strong antecedent improbability thus 

 indicated, this would be the natural conclusion, because 

 it is clearly unlikely that several sheets of stars would be 

 seen edgewise, even though one might be. 



32°. In places, a projection of the Milky Way has its 

 apex coincident with a bright star. 



Many other points might be dwelt upon — as 



33°. The fact that certain forms of aggregation are 

 singularly common in certain regions (double stars are 

 less common in Smithern heavens, you have noted). 



34°. — Variable stars are more common in some places 

 than others — and sometimes variables in certain dis- 

 tricts are characterised by the same law and mode of 

 variation, &c. 



3.5°. — The " access to the Nubeculse* on all sides is 

 through a desert " (Why 1 tmless those regions have been 

 drained to form the Nubectila^). 



36°. — The fact that manj- regular nebuliv are singu- 

 larly associated with single, double, or multiple stars. 



37°. — The existence of those faint " mottlings " (form- 

 ing streams) which you detected in the southern 

 heavens. 



38°. — The fact that in such clusters as Pra?sepe and 

 Perseus we find nebttlar tracts (in the former two dis- 

 tinct nebulie), and that the probability is enormously 

 against this being accidental. 



39°. The breaking otit of new stars on the borders of 

 the Milky Way. 



40°. The indicated existence of dark orbs (causing the 

 irregular proper motions of Sirius and Procyon). 



41°. The occurrence of such a change in a star as 

 would lead to the appearance of a so-called "new star" 

 — since it seems far more reasonable to believe that a 

 really minute star has thus suffered a fire change than 

 that a sun resembling ours has suddenly blazed out to 

 many times its u.sual brilliancy. 



42°. The variability and disappearance of many nebula?. 



But for the present the facts before cited seem sufficient 

 for my ptirpose. 



Per contra I ktiow of very few definite facts. There is 

 the general feeling of experienced observers that the 

 nebulas are far beyond our sidereal system. But on such 

 a point experience avails nothing — or rather no one has 

 yet obtained any experience enabling him to form an 

 opinion, /to »t observation, on this point. Experience can 

 tell an observer whether a star apparently single is likely 

 to be doubled with a moderate increase of power ; whether 

 an unresolved nebula is or not resolvable and so on : 

 because on these points an observer can have experience. 

 But the opinion an observer may form as to the distance 



* Another name for the Magellanic Clouds. 



