258 



KNOWLEDGE. 



[No^•EMBER 1, 1894. 



speed constitutes in itself a formidable problem. Tliat 

 part of it alone which lies across the line of sight amounts 

 to thirty-five miles a second, or nearly twice that of the 

 earth in its orbit. This, according to Prof. Newcomb, 

 is beyond what can be swayed into a closed path by the 

 gravitative force of the entire sidereal system ; in other 

 words, 61 Cygni must be reckoned a " runaway pair." Un- 

 less, indeed, a single mass comparatively close at hand be 

 efl'ective where the innumerable scattered bodies of stellar 

 space are impotent. It may be worth whUe to take a 

 glance at the conditions involved in this hypothesis. 



Assuming Gl Cygni to revolve round a local centre in a 

 period of ten thousand years, it follows, from the elemen- 

 tary principles of celestial mechanics, that the radius of 

 its orbit, supposed to be circular, would be about 1752 

 thousands of millions of miles, while the mass occupying the 

 centre would exceed that of our sun two and a third million 

 times. The presence, however, of such an immense 

 gravitative power, even if lodged in an obscure body, would 

 assm-edly betray itself otherwise than by the proper motion 

 of one luminous couple. The sphere of its pre-eminent 

 influence might well be twenty billions of miles in dia- 

 meter, within which incomprehensibly vast region tides of 

 movement should be raised, neither inconspicuous nor 

 difficult of interpretation. But no such effects are trace- 

 able. The inference is irresistible that no obscure centre 

 of overwhelming power exists in that neighbourhood ; con- 

 sequently, that the motions of 61 Cygni must be otherwise 

 accounted for. 



Now, however, a further complexity has been added to 

 this already complex problem. 



In the middle of 1889, as the upshot of a keen scrutiny 

 with powers up to 1000 on the Lick thirty-six inch, Mr. 

 Burnham recorded both the stars of 61 Cygni to be 

 undoubtedly single. It is unlikely that they will ever 

 appear otherwise. Nevertheless, one at least of them 

 owns an attendant by no means insignificant in mass. 

 That is to say, the evidence of this being the case is very 

 strong ; it would be premature to assert it to be conclusive. 



The thirteen-inch International photographic refractor 

 of Potsdam was placed by Dr. Vogel, in the autumn of 

 1890, at the disposal of Dr. Wilsing for the purpose of 

 some experimental researches into stellar parallax ; and, 

 almost as a matter of course, their chosen object was 

 61 Cygni. Yet, the proved accuracy of the measures 

 notwithstanding, only anomalous results were obtained. 

 Now an anomaly often gives the clue to a discovery, and 

 Dr. Wilsing soon found himself on the track of a curiously 

 interesting one. His measurements were executed from 

 the middle point of the line joining the stars under examina- 

 tion to two comparison stars, one situated nearly in the 

 prolongation of the same line, the other at right angles to 

 it. The parallaxes derived from both ought, of course, 

 within certain narrow limits of error, to have been the 

 same ; yet they differed consistently and notably. After 

 other tests had been applied in vain, direct determinations 

 were carried out of the distances separating Glj from 61, 

 Cygni at various successive intervals of time. And here 

 at last the secret of the observed perplexing inconsistencies 

 was found to lurk. The stars, it seems, alternately 

 approach and recede from each other, in a period of 

 twenty-two months, by the total angular amount of three- 

 tenths of a second — a gross quantity in an investigation 

 of so delicate a character. The fashion of this change, so 

 far as it has yet been made known, is shown in the accom- 

 panying drawing by Dr. Wilsing. It will be observed that 

 the swing backwards of the stars is much more quickly 

 performed than their swing forwards, and this obviously 

 through no accidental errors of measurement, but as a 



distinctive feature of the systemic movements unexpectedly 

 betrayed. These, as Dr. Wilsing pointed out on the 26th 

 of October, 1893," must be due to the presence of one or 

 more obscure companions to the bright stars. In due 

 time we shall doubtless learn which suffers disturbance, 

 or whether both alike are composed of a shining and an 

 obscure body. In the latter case, the argument in favour 

 of their permanent physical union will be strengthened 

 beyond contravention. 



The spectrograph will afford the surest criterion of the 

 genuineness of Dr. Wilsing's discovery. Up to this, nothing 

 has been ascertained regarding the motions of 61 Cygni in 

 the line of sight. Its rays are too scanty, unless concen- 

 trated by a very powerful telescope, to afford satisfactory 

 information on the point. None of the Potsdam instruments 

 are adequate to the purpose ; but with the Pulkowa thirty- 

 inch, perfectly definite results ought to be attainable. A 

 complete set of spectral photographs of both stars, with 

 iron-lines for comparison simultaneously imprinted on the 

 plates, seems a primary requisite for the pursuit of this 

 important inquiry. The line-shiftings legible on them 

 would possess a particular significance. It is, indeed, 

 probable that the multiplicity of 61 Cygni might have been 

 detected by the spectroscope alone. For the plane of the 

 minor orbits traversed by some of its components appears 

 to lie along the line of sight from the earth ; and twice in 

 every revolution, accordingly, nearly the whole of their 

 velocity in those orbits — amounting probably to six or 

 eight miles a second — must be directed towards or away 

 from the earth. It is thus permissible to hope that many 

 of the intricate questions relating to our classic double 

 star are on the eve of receiving definitive replies. 



THE KONISCOPE. 



By De. .J. G. McPherson, F.R.S.E., Lecturer on, 

 Meteorohiyij in the University of St. Andrews. 



w 



E. JOHN AITKEN, F.R.S., has just given us the 

 results of some careful observations on colour 

 phenomena connected with cloudy condensation, 

 and an account of his new instrument, for 

 detecting the impure state of the air in rooms 

 by means of colour alone, may be interesting to readers of 

 Knowledge. No more painstaking or persevering physi- 

 cist lives than the discoverer of the now acknowledged 

 theory of the formation of dew. He has elucidated the 

 formation of fog particles by the attraction of dust for 

 water vapour, and has enumerated the particles of dust in 

 a cubic inch of air, and this is another example of his 

 assiduity and success. 



If steam be blown into the air inside a glass vessel, the 

 cloudy condensation will in time undergo a change. Of 

 course, the dust particles in the air have seized hold of the 

 water vapour of the steam to form visible steam particles, 

 each dust atom forming a free surface for the adherence of 

 the moisture. Particles fall and leave the upper part 

 clearer, and particles fall to the bottom also. Yet the 

 principal cause of the thinning change is in the smaller 

 particles becoming absorbed by the larger ones. The 

 smaller drops begin to lose their accumulated moisture, 

 while the larger ones are still increasing in size, growing 

 at the expense of the gradually diminishing smaller ones. 

 In the end a comparatively small number of drops have 

 absorbed the moisture which was previously distributed 



* Rilzungslerichte der Berliner .Ikademie der M'issenschaften, 

 xl., 879. 



