March 2, 1896.] 



KNOWLEDGE. 



61 



that of our sun, tbe mean distance corresponding to a 

 period of thirty-sis years would be roughly one thousand 

 three hundred million miles. But, since stellar orbits 

 are mostly very elongated, their separation would, by a 

 safe presumption, greatly transcend and fall short of 

 this measure every eighteen years. Now the visible 

 companion sweeps through periastron at a distance of i 

 fourteen hundred milhon miles ; an actual coUision, then, 1 

 with its inconspicuous colleague (usmg the word in its 

 strict sense) would be no improbable event ; and approaches 1 

 so close as to involve a complete subversion of the system 

 could scarcely be averted. The conclusion is thus fully 

 warranted that in 70 Ophiuchi the companion is the 

 duplex star. 



The combined spectrum of the pair was registered by | 

 Vogel as of the solar type, for the Arcturian variety of 

 which it is claimed in the Draper Catalogue. Their rays 

 have not yet been separately analyzed ; indeed, so delicate 

 an operation is scarcely yet feasible. 



The colour of the smaller object of 6'o magnitude appears 

 subject to change. White, with " an inclination to red," 

 in the elder Herschel's time, it showed to his son and to 

 Sir James South as "livid"; Struve found it purplish: 

 Admiral Smyth, violet ; Flammarion, in 1879, termed it 

 rose-coloured ; while some intermediate observers recorded 

 it as yellow; and yellow it now indisputably is. 'There 

 is, in fact, no perceptible difl'erence between its hue and 

 that of its 4-5 magnitude primary. It will be curious to 

 notice whether chromatic divergence sets in as they retire 

 from periastron. 



The mechanism of the system must be exceedingly 

 intricate, but can scarcely, for many decades to come, be 

 submitted to detailed investigation. Dr. See barely 

 indicates the arduous nature of the task ; we trust that 

 his own powers may one day be employed in grappling 

 with it. By that time, possibly, mathematical resources 

 may have developed to meet the urgency of the demands 

 made upon them by sidereal science. It is certain that 

 the very highest powers of the human mind will be called 

 into play for the solution of the multitudinous problems 

 unfolded by the stars. 



I^There is just one addition I should like to make to 

 Miss Gierke's graphic and striking account of a fact which 

 must soon bulk largely in astronomical literature. Dr. 

 See has drawn public attention to an interesting feature 

 of the system of 70 Ophiuchi, and he has been the first to 

 actually compute the movements of the companion on the 

 assumption of an unseen third star. But this work can 

 hardly be entitled a " discovery." The presence of the 

 third member has long been postulated amongst those who 

 have devoted themselves to the close study of double-star 

 orbits. Nor does 70 Ophiuchi stand alone in this respect, 

 and herein lies the exceeding importance of the subject. 

 Fully ten per cent, of the best known binary systems show 

 irregularities as truly periodic in character ; many of them 

 are even more pronounced and striking. Amongst others 

 I might refer to 30 Andromedse, ^ Herculis and a Ophiuchi, 

 and the beautiful pair referred to above by Miss Gierke, 

 T) Cassiopeia. The "dark companion" is not a rare 

 exception ; it has already been recognized in many 

 instances, and future observations will certainly add to the 

 number. 



I would also emphasize Miss Gierke's objection to Prof. 

 Schur's method of rejecting the measures of distance in 

 the computation of an orbit. With modern appliances 

 these are at least as trustworthy as the observations 

 of position-angle, and their rejection nowadays can only 

 be condemned as a discarding of important material. — 

 E. Walter Maunder. 1 



PHOTOGRAPHY OF INVISIBLE OBJECTS. 



By J. J. Stewart, B.A.Cantab., B.Sc.Lond. 



IN his presidential address to the Royal Society in the 

 year 1893, Lord Kelvin predicted that future dis- 

 covery with regard to the relations of the luminiferous 

 ether and ponderable matter would have as its basis 

 knowledge obtained from experiments on the dis- 

 charge of electricity through highly rarefied gases. What 

 has taken place in the two years which have elapsed since 

 that address was given has abundantly verified the 

 correctness of Lord Kelvin's view as to the line along which 

 future advance in physical science would take place. 



The latest discovery in this department of knowledge 

 has been of unusual interest, and has excited wide- 

 spread attention beyond the limited circle of those more 

 directly occupied with scientific work. Prof. Eontgen, of 

 Vienna, has just published an account of some remarkable 

 experiments he has been carrying out, and the results 

 already attained are in themselves of great importance, 

 whilst the possibilities of future insight into the nature of 

 the ether which they open out cause them to be of intense 

 interest to physicists. 



It may be of interest to the readers of Knowledge to 

 have put before them a short statement of the facts dis- 

 covered and the methods of experimenting employed by 

 Prof. Eontgen. He found that when he caused an 

 electric discharge from an induction coil to pass through 



riiotDgriipli of IV Leather Piii'sc, sliovviu^ mi-tal clasp, coin, iiiul key 

 iusidc. Bv Ur. Dawson Turiior. 



a lliltorf's vacuum tube, or a tube eontuining a high 

 vacuum like those employed by Crookcs and Lenard, he 

 got bright lluorcscence produced on a piece of paper 

 covered with platino-cyanide of barium, and this fluo- 

 rescence was excited even when the tube itself was 

 completely covered over with blackened paper so that 

 nothing within it was visible. Moreover, the tlnorescent 



