240 



SCIENCE. 



[Vol. XVIII. No. 456 



G, — and 10 employ as the line of compavison the hydrogen 

 line near G, and recently also certain lines of iron. The 

 most minute and complete mechanical arrangements were 

 provided for the purpose of securing the absolute rigidity of 

 the comparison spectram relatively to that of the star, and 

 for permitting temperature adjustments and other necessary 

 ones to be made. 



The perfection of these spectra is shown by the large num- 

 ber of lines, no fewee fVmv. 050 j^, i]-^^ case of Capella, within 

 _j i,fuui on the plate. Already the 

 . : iii • ii o 1; ' . Vy .stars have been measured with an ac- 

 cur icy, in the case of a large number of them, of about an 

 English mile per second. 



At the Li<sk Observatory it has '. m shown that observa- 

 tijNS can be made directly by eye .. ;.h an accuracy equally 

 great. Mr. Keeler's brilliant success has followed in great 

 measure from the use of the third and fourth spectra of a 

 grf.'.i: ' ..,.-^-8 lieiri to the inch. The marvellous accuracy 

 a!i .. ij'i? '''.-^ ht- hr.;.,",:: on a suitable star is shown by obser- 

 vations on three nights of the star Arcturus, the largest di- 

 vergence of his measures being not greater than six-tenths 

 of a mile per second, while the mean of the three nights' 

 work agreed with the mean of five photographic determina- 

 tions of the same star at Potsdam to within one-tenth of an 

 English mile. These are determinations of the motions of 

 a sun so stupendously remote that even the method of paral- 

 lax practically fails to fathom the depth of intervening space, 

 and by means of light-waves which have been, according to 

 Elkin's nominal parallax, nearly two hundred years vipon 

 their journey. 



Mr. Keeler, with his magnificent means, has accomplished 

 a task which I attempted in vain in 1874, with the compara- 

 -tively poor appliances at my disposal, of measuring the mo- 

 tions in the line of sight of some of the planetary nebulse. 

 As the stars have considerable motions in space, it was to be 

 expected that nebulas should possess similar motions, for the 

 stellar motions must have belonged to the nebulas out of 

 which they have been evolved. My instrumental means, lim- 

 iting my power of detection to motions greater than twenty- 

 ■S.ve miles per second, were insufficient. Mr. Keeler has 

 ifound, in the examination of ten nebulae, motions varying 

 ifrom two miles to twenty-seven miles, with one exceptional 

 motion of nearly forty miles. 



For the nebula of Orion, Mr. Keeler finds a motion of re- 

 cession of about ten miles a second. Now this motion agrees 

 closely with what it should appear to have from the drift of 

 the solar system itself, so far as it has been possible at present 

 to ascertain the probable velocity of the sun in space. This 

 grand nebula, of vast extent and of extreme tenuity, is proba- 

 bly more nearly at rest relatively to the stars of our system 

 than any other celestial object we know; still it would seem 

 more likely that even here we have some motion, small 

 though it may be, than that the motions of the matter of 

 which it is formed were so absolutely balanced as to leave 

 this nebula in the unique position of absolute immobility 

 in the midst of whirling and drifting suns and systems of 

 suns. 



The spectroscopic method of determining celestial motions 

 in the line of sight has recently become fruitful in a new but 

 not altogether unforeseen direction, for it has, so to speak, 

 given us a separating power far beyond that of any telescope 

 the glass-maker and the optician could construct, and so en- 

 abled us to penetrate into mysteries hidden in stars appar- 

 ently single, and altogether unsuspected of being binary 

 systems. The spectroscope has not simply added to the list 



of the known binary stars, but has given to us for the first 

 time a knowledge of a new class of stellar systems, in which 

 the components are in some cases of nearly equal magni- 

 tude, and in close proximity, and are revolving with veloci- 

 ties greatly exceeding the planetary velocities of our sys- 

 tem. 



The K line in the photographs of Mizar, taken at the Har- 

 vard College Observatory, was found to be double at intervals 

 of fifty-two days. The spectrum was therefore not due to a 

 single source of light, but to the combined eiTect of two stars 

 moving periodically in opposite directions in the line of sight. 

 It is obvious that if two stars revolve round their common 

 centre of gravity in a plane not perpendicalar to the line of 

 sight, all the lines in a spectrum common to the two stars 

 will appear alternately single or double. 



In the case of Mizar and the other stars to be mentioned, 

 the spectroscopic observations are not as yet extended enough 

 to furnish more than an approximate determination of the 

 elements of their orbits. 



Mizar especially, on account of its relatively long period, 

 — about 105 days, — needs further observations. The two 

 stars are moving each with a velocity of about fifty miles a 

 second, probably in elliptical orbits, and are about a hundred 

 and forty-three millions of miles apa-rt. The stars, of about 

 equal brightness, have together a mass about forty times as 

 great as that of our sun. 



A similar doubling of the lines showed itself in the Har- 

 vard photographs of /3 Aurigae at the remarkably close in- 

 terval of almost exactly two days, indicating a period of 

 revolution of about four days. According toi Vogel's later 

 observations, each star has a velocity of nearly seventy miles 

 a second, the distance between the stars being little more 

 than seven and a half millions of miles, and the mass of the 

 system 4.7 times that of the sun. The system is approaehing 

 us at the speed of about sixteen miles a second. 



The telescope could never have revealed to us doahie stars 

 of this order. In the case of /3 Aurigss', combining Kegel's 

 distance with Pritchard's recent determination of the star's 

 parallax, the greatest angular separation of the stars its seen 

 from the earth would be one two-hundredth part of a second 

 of arc, and therefore very far too small for the htgbest pow- 

 ers of the largest telescopes. If we take the illation of 

 aperture to separating power usually accepted, an object-glass 

 of about eighty feet in diameter would be needed to resolve 

 this binary star. The spectroscopev which takes, no note of 

 distance, magnifies, so to speak, this minute angular separa- 

 tion four thousand times; in other words, the doubling of 

 the lines, which is the phenomeaon that we have to observe, 

 amounts to the easily measurable quantity of twenty seconds 

 of arc. 



There were known, indeed, variable stars of short period, 

 which it had been suggested might be explained on the hy- 

 pothesis of a dark body revolving about a bright sun in a 

 few days, but this theory was met by the objection that no 

 such systems of closely revolving suns were known to exist. 

 The Harvard photographs of vrhich we have been speak- 

 ing were taken with a slitless form of spectroscope, the prisms 

 being placed, as originally by Fraunhofer. before the object- 

 glass of the telescope. This method, though it possesses some 

 advantages, has the serious drawback of not permitting a 

 direct comparison of the star's spectrum with terrestrial 

 spectra. It is obviously unsuited to a variable star like Algol, 

 where one star only is bright, for in such a case there would 

 be no doubling of the lines, but only a small shift to and fro 

 of the lines of the bright star as it moved in its orbit alter- 



