36 



KNOWLEDGE 



[Febeuary 1, 1895. 



the surface by means of convection-currents — by the bodily 

 transport, that is to say, of intensely hot matter upwards, 

 and of comparatively cool matter downwards ; and thirdly, 

 that the photosphere is a surface of condensation, forming 

 the limit set by the cold of space to this circulating 

 process." 



The following chapter, on recent solar eclipses, well 

 sums up the main results of observations, and gives an 

 impartial account of the various theories as to the nature 

 of the corona, none of which seem completely satisfactory. 

 Briefly to sum up the results, we know that the corona is 

 not a " solar atmosphere" : it is partly made up of self- 

 luminous gases — chiefly hydrogen, and the unknown 

 substance named coronium giving the green ray 1474 ; 

 partly of white hot solid or liquid particles shining with 

 continuous light. It is practically certain that its form 

 varies with the ebb and flow of solar acti^•ity, and it is 

 certain that it is of the extremesfc tenuity, as comets pass 

 through it without sensible perturbation. " Not even Mr. 

 Crookes's vacua can give an idea of the rarefaction which 

 this fact implies. Yet the observed luminous effects may 

 not Ln reality bear witness contradictory of it. One 

 solitary molecule in each cubic inch of space might, in 

 Prof. Young's opinion, produce them; while in the same 

 volume of ordinary air at the sea-level, the molecules 

 number (according to Mr. Johnstone Stoney) twenty 

 thousand trillions ! " 



The chapter on solar spectroscopy, into which we cannot 

 enter in detail, contains a needed caution against the hasty 

 assumption that a substance does not exist in a sun or 

 star if its distinctive lines are not seen. "It maybe 

 situated below the level where absorption occurs, or under 

 a pressure such as to efface lines by continuous lustre ; it 

 may be at a temperature so high that it gives out more 

 hght than it takes up, and yet its incandescence may be 

 masked by the absorption of other bodies ; finally, it may 

 just balance absorption by emission, with the result of 

 complete spectral neutrality." 



Impartiality is one of the highest virtues of a historian, 

 and it must be allowed that Miss Gierke has shown this 

 virtue in a high degree. Indeed, if we may permit our- 

 selves to criticize what is on the whole an admirable 

 performance, we should say that she sometimes carries the 

 virtue of impartiality to excess. Here and there she appears 

 unwilling to exercise her judgment in estimating the 

 relative values of conflicting theories. Occasionally, too, 

 her judgment seems to the writer to be somewhat at fault. 

 Thus, in referring to Prof. Holden's valuable monograph 

 on the nebula of Orion, Miss Gierke quotes, as almost an 

 ascertained fact, his conclusion that the brightness of 

 the various parts of the nebula has been and is in a state 

 of continual fluctuation. Now, without in the least wishing 

 to detract from the merit of Holden's excellent work, we 

 think that in view of the materials he had to deal with — 

 consisting exclusively of hand-drawings — the evidence in 

 favour of changes of brightness is really very slight. 

 Anyone who has attempted to delineate such objects will 

 admit that while it is very diflicult to attain accuracy in 

 regard to form, it is still more diflicult to be true to the 

 relative intensity of the various parts of a conjplicated 

 structure ; and a comparison of the most detailed drawing 

 of the Orion nebula (that of Lord Eosse) with one of the 

 photographs of Dr. Common or Dr. Roberts, will show at 

 once that it is here that the drawing most conspicuously 

 fails. In a matter of such difficulty, therefore, we should 

 prefer to consider the subject of changes of brightness as 

 entirely an open one, and trust for the future to the 

 evidence of photography alone. 



In an appendix. Miss Gierke gives a brief chronology of 



astronomical discoveries from 1774 to 1893, a list, with 

 particulars, of the forty largest telescopes in the world, and 

 one or two other useful tables ; and last, but not least, we 

 have a good and copious index, which adds much to the 

 value of the work.— W.H.W. 



BOOKS EECEIVED. 



Elliptical Orbits : their destructive mechanical characteristics 

 and their possible oriffin. By Henrv Larkin. (T. Fisher Unwin.) Is. 



Pen Pictures and How to Draic them. Bv Eric Meade. (L. 

 Upcott Gill.) 28. 6d. 



Text-Book of Hound, being Vol. I. of " The Tutorial Physics." By 

 Edmund Catclipool, B.So. L md. (University Correspondence College 

 Press. Strand, W.C.) 3s. 6d. 



Practical Forestry. By A. D. Webster. (W. Eider & Son.) Ss. 



A Few Chapters in Astronomy. Bv Claudius Kennedv, M.A- 

 (Taylor & Francis.) 



Animals, their Past and Future. By Bev. G-. H. Pembcr, M.A. 

 (Hodder & Stougliton.) Is. 



Travels with a Sunbeam: or Flenients of Astronomy. By Arthur 

 Z. Dade. (W. Gr. Moore & Co., Birniinghaiu.) 



The Journal of the Camera Club for December. (Harrison & Sons, 

 St. Martin's Lane, W.C.) Is. 



The ■Journal of the British Astronomical Association, Vol. V., 

 Ko. 2. (Eyre & Spottiswoode.) Is. 6d. 



The Journal and Transactions of the Moyal Photographic Society 

 of Great Britain for December 22nd. 



Journal of the Photographic Society of India for December. 

 (Thacker, Spink & Co., Calcutta.) 



The Review of Reviews for December. (125, Fleet Street, E.C.) 



Note. — The publishers would be glad to be informed of any books 

 sent in for review during the last few months, and not acknowledged 

 owing to the long illness of Jlr. Kanvard. 



DARK "LANES" OF THE MILKY WAY. 



By E. Walter Maunder, Hon. Sec, B.A.S. ; President, 

 British Astronomical Association ; Superintendent of the 

 Physical Department, Boyal Observatory, Greentvich. 



THAT the stars generally, and more particularly the 

 stars of the Milky Way, appear to show a definite 

 structure in their grouping is undeniable. The 

 question as to whether this appearance of structure 

 has any real significance is by no means so simple. 

 It is impossible to examine a rich field in the Milky Way, 

 or a photograph of such a field, without the eye at once 

 linking neighbouring stars together, and tracing out Unes, 

 curves, circles, ovals, and festoons of stars, not to speak 

 of more compact clusters ; but the reality of these forms 

 has been often called in question. 



It has been pointed out, first of aU, that a purely chance 

 arrangement of points and dots gives rise inevitably to 

 precisely similar groupings. Eaindrops on a pavement, 

 splashes of ink from a brush, are just as fertile in showing 

 such lines and curves. It is argued, therefore, that no 

 stress whatsoever can be laid upon such forms. They are, 

 so it is said, far more a result of our own mental processes 

 than of stellar distribution, and if of stellar distribution 

 at all, then only one of chance. 



It is further urged that though we see the stars as if 

 projected on a plane, we are really looking through an 

 infinite depth of space. Of, say, six stars apparently at 

 equal distances in a straight line — and this would be a 

 very definite and precise formation — one might be at a 

 distance of but twenty light-years, another at twenty times 

 that distance, and the rest irregularly distributed in 

 between. So far, then, from these stars forming a system, 

 we ourselves might positively be nearer to one of them 

 than any one of them was to any other. 



A third argument has been that even in the telescope 

 the apparent disc of a star is enormously greater in angular 



