20 



KNOWLEDGE 



[Not. 1, 1885. 



of a nebula can never be confirmed or disproved, and so 

 he can never acquire real experience on this subject. 



Many view the careful study and analysis of observa- 

 tions, work which they call "paper astronomy," with 

 contempt. You are not likely to share that feeling, re- 

 membering that Copei-nicus, Kepler, and Newton — to 

 name no more — were pajier astronomers. I feel that 

 there is room for a good deal of paper astronomy 

 on the subject of this letter ; and that it would be 

 good service to enlist fellow -labourers in paper-worlc. 

 It is not that I undervalue observation — rather that 

 others do. A large mass of observations is available 

 to modern thinkers — but most men prefer to make new 

 observations. In this way the real worth of observation 

 is lost ; and further, many who take part in it would do 

 better work in other directions. 



My object in writing is very much as follows : — I have 

 been trying to show that useful results could be obtained 

 if a few who have time would overhaul the accumu- 

 lated mass of observations now lying at the disposal of 

 astronomers. I have myself very little time to work at 

 this subject. If I can save an hour or two a week for 

 such work, it is as much as I can do without injustice to 

 my family. I should like to see many of those now 

 engaged in accumulating preposterously useless observa- 

 tions at work in the field over the fence of which I have 

 looked. A hundred modes of inquiry should be followed 

 up ; observed facts should be co-ordinated and rendered 

 significant ; new modes of observation should be devised. 

 Thus may clear views, perhaps, be obtained respecting 

 that which it has hitherto seemed hopeless to inquire 

 about — the arrangement and configuration of the star- 

 groups in our own neighbourood, the architecture of the 

 nearer portions of the sidereal heavens. — Tours very trulj', 



R. A. Peoctor. 



THE UNIVERSE, 



LETTER TO RI(H.\K1I k. PROC'TrtR. 



Bv Siu John Heeschel. 



CiiUingwood, Amj. 1, 1869. 

 HOPE you will not regard my delay in reply- 

 ing to your letter of the 2-tth [ult.] as indi- 

 cating any absence of interest on my j)art in 

 its .subject matter, or in the views you have 

 been led to base on the constitution of our 

 sidei'eal system. On the contrary, it has 

 been precisely by reason of the number and 

 variety of the striking facts you have brotight together, 

 and the evident bearing of a great proportion of them 

 on the great problem it offers to human speculation, that 

 I have been unwilling to make a hasty reply. 



I have never participated in that feeling which you 

 designate as "a contempt for astronomy on paper " in 

 the sense of repudiating such speculations as aim at 

 grouping together known but apparently disconnected 

 facts, within one general view, in consonance with known 

 laws, — holding that in the midst of so much darkness 

 we ought to open our eyes as wide as possible to any 

 glimpse of light, and utilise whatever twilight may be 

 accorded us, to make out, though but indistinctly, the 

 forms that surround us. 



Under this impression, I should not feel so bigotted to 

 the " ring " or the " disc theory " of the Milky Way, as 

 to reject your proposed sj'stem of convolutions, which 

 certainly seems to give a plausible, if not a fully satis- 

 factory account of the gap in Argo, and the break in the 



following branch of the double stream about Ophiuchus ; 

 though I confess that the " Coal-Sack " does not at all 

 give me the idea of the perspective view of a loop, formed 

 by the visual ray passing over one portion, and under 

 another,* of one and the same continuous belt : and the 

 disappearance bj^ the simple effect of remoteness of the 

 interrupted stream in Ophiuchus, would involve, in con- 

 sequence, a regular pn'ogressive diminution in magnitude 

 of the component stars, in approaching the brenk from 

 either side, down to unresolvable nebulosity and thence to 

 evanescence. t 



I do not see the absolute necessity for placing the sun 

 fairly out of the Belt. Distant portions of the Belt from 

 which the sun's position is quite remote may still appear 

 sharply terminated, — and it is a fact that abundance of 

 stars of all magnitudes are seen in situations inclined at 

 all angles to the galactic plane, their density increasing 

 gradually and regularly down to that plane. J 



The considerations you adduce relative to the proper 

 motions of the stars are exceedingly curious and interest- 

 ing. § Of late years catalogues have gone into so much 

 detail, and with such accuracy, that these motions are, of 

 course, much bettir known to ns than some twenty or 

 thirty years ago. The community of proper motions 

 over large areas, of which you give a picture in Gemini 

 and Cancer is most remarkable, and the coincidence of 

 proper motion in ft, y, i, c, and i^ Ursa; Majoris is most 

 striking. Tour promised paper on this subject cannot 

 fail to be highly interesting. 



I cannot say that I am at all surprised at its being 

 found that the average proper motions of stars of small 

 magnitude is not less than of large, considering (as I have 

 always done) that the range of individual magnitude (i.e., 

 lustre) must be so numerous that multitudes of very 

 minute stars may in fact be our very near neighbours. 

 But your remark (in your No. 19) on the conclusion I 

 have been led to draw relative to the small effect of the 

 correction due to the sun's proper motion, will require to 

 be very carefully considered, and I shall, of course, give 

 it ever}- attention. 



In my Isographic Projection of the Nebulne, I made no 

 distinction or separation into classes, Ac. Ton appear to 

 have done so (in your second proposition) ; and the con- 

 nection of the great mass of globular clusters with the 

 Milky Way in the region about Sagittarius and Scorpio, 

 has led me to conclude, as you seem to have done, a 

 decided connection with, and, in fact, inclusion, in that 

 system, of the clusters in question. 



A remark which this connection, and the structure of 

 the Magellaine Clouds, has often suggested to me, has 

 been stnjngly recalled by what you say of the inclusion 



* I am quite with Sir John Herschel on this point, since I have 

 seen the Southern Heavens. — R. A. P. 



t By no means necessarily. It is easy to conceive a star-cloud or 

 stream in which the arrangement of the stars of various orders 

 would be such that no order would ever present unresolvable 

 nebulosity at any distance, while the several orders would present 

 the same appearance at all distances, the brighter orders taking the 

 place of the fainter (apparently) as the distance increased. — R. A. P. 



J On trying Sir John Herschel's gauges in the Southern heavens 

 on a plan proposed by me, and accepted as trustworthy by Sir J. 

 Herschel, Mr. Sydney Walters found the increase of density to be — 

 as I had anticipated — by no means gradual and regular, but rapid, 

 and as it were sudden, in the neighbourhood of the Milky Way, and 

 irregular over the visible cloud-like gatherings in the galaxy. — R. A. P. 



§ Sir George Airy, in a letter addressed to me in February, 1873, 

 confounded my discovery of Star Drift with what Pond, his pre- 

 decessor as Astronomer-Royal, had already noticed. But Pond's 

 observation applied solely to the general drift of all the stars in the 

 heavens, on account of the sun's proper motion through space. This 

 is a very different matter from the drift of star groups, which I had 

 indicated in my maps of stellar proper motions.- R. A. P. 



