210 



♦ KNOWLKDGE ♦ 



[July 2, 1888. 



aceous, such other substances only being combined with the 

 carbon as will give it a hold on the paper, without which of 

 course the ink might be simph' washed off. This hold on 

 the paper should be such in some cases (as for cheapness, 

 &c.) that the written matter cannot be removed by any 

 mechanical means without destroying the paper itself. I 

 find that among the Kosmian inks the requirements of in- 

 delibility and suitable hold on the paper to prevent 

 mechanical removal are admirably provided for, and, from 

 experiments 1 have made on this ink, I infer that it is 

 essentially a carbon ink. 



SIR WM. HERSCHEL'S SURVEYS OF THE 

 HEAVENS. 



UltIN(T the interesting discussions respecting 

 the temporary star in the Andromeda 

 nebula, the misapprehensions which ]n-e- 

 vail in regard to the magnificent work 

 of the Herschels, and especially of the 

 elder Herschel, were very prominent. 

 Darwui has been wildly and widely mis- 

 understood ; insomuch that most men imagine his theory 

 (and Wallace's) merely another form of Lamarck's, that he 

 believed in evolution towards perfection, that according to 

 him each race has descended from a jaair of its own kind, 

 and that he regarded apes closely resembling some now 

 existent anthropoids as the ancestors of the human race. 

 And those who thus misunderstood Darwiir are hard to con- 

 vince : they will not believe, when science assures them that 

 Darwin's theory was accepted by men who had utterly 

 rejected Lamarck's, that he recognised evolution as often 

 tending to deterioration rather than to improvement, that 

 the descent of any race from a pair is absurdly inconsistent 

 witli the doctrine of natural selection, and lastly, that, as 

 regards the period when man and the present anthropoid 

 apes diverged, the Darwinian theory would indicate ancestors 

 at least as like man as like any existent ape, and probably 

 much more like — for the simple reason that man has 

 descended directly from that ancestry, while the existent 

 anthropoids have shared descent from it with descent from 

 contemporaries of lower race. But, preposterous as have 

 been the mistakes made about Darwin's doctrines, they can 

 hardly be compared with those into which even many men 

 of science and astronomers have fallen respecting the Labours 

 of Sir William Herschel. He was an observer of the 

 heavens for more than forty years, during all which time 

 his active mind was busily engaged applying various methods 

 of iuquu-y, inventing new ones, and testing old ones ; yet 

 many seem to imagine that at the end of his long series of 

 labours he stood in the precise position which he had occu- 

 pied at the beginning. They quote an opinion expressed during 

 the last ten years of his work, and an idea suggested thirty 

 years before, "in one and the same chapter, nay, in the same 

 paragraph, and even in the same sentence. Or, which is 

 not less incorrect, though not so obviously absurd, they 

 point to a result which he had obtained from one series of 

 observations and had interpreted on one paiticular prin- 

 ciple, as establishing such and such a conclusion when inter- 

 preted on another assumption — this assumption having been 

 adopted indeed by Herschel at one part of his labours, but 

 long given up when he had adopted the principle by which 

 he had explained that result. 



It is not commonly known, even though on this point the 

 whole value of Herschel's labours in reality depends, that 

 he employed two entirely different methods for gauging the 

 star depths. One would imagine that they must have been 



so similar as to be easily confounded together, seeing that 

 (except the present writer) only one student of astronomy, 

 the late W. Struve, has ever insisted on the distinction 

 between them. Even W. Struve, in his justly celebrated 

 (but little read) " Etudes d'Astronomie Stellaire," failed to 

 notice how inadecpiately Herschel had been able to test the 

 second method, his career as an observer having come to a 

 close le.-is than four years after he had devised it. In reality, 

 however, these methods, which Arago, Humboldt, and other 

 men of note, as well as hundreds of the mere astronomical 

 compilers, have confounded together, presented the following 

 points of rather strong contrast : — • 



Herschel's first gauging method depended on turning 

 one powerful telescope to different parts of the stellar 

 heavens in succession ; his second method depended on 

 turning many telescopes (ranging in power from his smallest 

 to his largest) successively to one and the same star-group- 

 ings. The first depended on the assumed power of penetra- 

 ting in all directions to the outermost limits of the galaxy ; 

 the other depended on the observed fact that in many direc- 

 tions the galaxy is unfathomable. In applying the first 

 method Herschel counted the number of stars in each field 

 of view, and judged the depth of the sj'stem to be propor- 

 tional to the number so counted ; in applying the other he 

 did not count the stai-s at all, but noticed only how much of 

 each field was cleared by the space- delving power of each of 

 the telescopes successively employed. (One may say that in 

 one method he counted stars, in the other he counted tele- 

 scopes.) The former method depended on the assumption 

 that the wealth of stars in the Blilky Way resulted from the 

 enormous length of the range of view through regions 

 occupied by stars uniformly strewn ; the second had been 

 devised because, in his own words, he had " found after a long 

 inspection and examination that the stars in the Milky Way 

 are arranged very diflerentlj' from those in our neighbour- 

 hood." Lastly, the first method was subjected to more than 

 thirty years' testing before its fundamental assumption was 

 thus recognised as erroneous ; the second method was tried 

 only for about three years, and the i-esults accumulated 

 during that time were never employed to test the validity of 

 the primary assumption, but presented only as indicating 

 such and such conclusions (/' the initial assumption were 

 trustworthy. 



That two methods so utterly unlike, nay, so opposite, in 

 plan, purpose, and principle, requiring such different 

 methods of observation, and so ditierent in their history, 

 should have been confounded together, would be amazing 

 were it not that men are so much readier to accept plain 

 statements as to facts than to analyse either preliminary 

 explanations or subsequent reflections. If Herschel, inter- 

 preting his earlier gauges by his first assumption, said that 

 the galaxy extends ten times farther towards the Milky Way 

 than towards the regions with black background — there is a 

 simple fact to be quoted ; let it be entered in our note-book. 

 If he found, interjjreting his later gauges on his second 

 assumption, that the JMilky Way is in many places un- 

 fathomable — that is not only a simple but an impressive 

 statement ; by all means put that down too. It may not 

 seem consistent with the former ; but both statements were 

 made by Herschel ; both appear (with only a trifling 

 interval of thirty years or so between them) in the " Pro- 

 ceedings of the Royal Society " ; there can be no objection 

 to their being used in the same paragraph, or even the same 

 sentence. Herschel could not analyse the ontei'most parts 

 of our galaxy, therefore they are hundreds of years' light- 

 journey away ; it would be a pity to omit that. And he 

 considered the nebula in Andromeda very near (on the 

 principle of his second gauging method), because his 

 smallest telescope, nay, even the naked eye, shows its milky 



