August 20, 1891] 



NATURE 



379 



unexpected power of advance along the lines of the older 

 astronomy. In the future, a higher value may, indeed, be placed 

 upon this indirect use of the spectroscope than upon its chemical 

 revelations. 



By no direct astronomical methods could motions of approach 

 or of recession of the stars be even detected, much less could 

 they be measured. A body coming directly towards us or going 

 directly from us appears to stand still. In the case of the stars 

 we can receive no assistance from change of size or of brightness. 

 The stars show no true disks in our instruments, and the nearest 

 of them is so far off that if it were approaching us at the rate of 

 a hundred miles in a second of time, a whole century of such 

 rapid approach would not do more than increase its brightness 

 by the one-fortieth part. 



Still it was only too clear that, so long as we were unable to 

 ascertain directly those components of the stars' motions which 

 lie in the lire of sight, the speed and direction of the solar 

 motion in space, and many of the great problems of the consti- 

 tution of the heavens, must remain more or less imperfectly 

 known. Now the spectroscope has placed in our hands this 

 power, which, though so essential, appeared almost in the nature 

 of things to lie for ever beyond our grasp; it enables us to 

 measure directly, and under favourable circumstances to within 

 a mile per second, or even less, the speed of approach or of 

 recession of a heavenly body. This method of observation has 

 the great advantage for the astronomer of being independent of 

 the distance of the moving body, and is therefore as applicable 

 and as certain in the case of a body on the extreme confines of 

 the visible universe, so long as it is bright enough, as in the case 

 of a neighbouring planet. 



Doppler had suggested as far back as 1841 that the same 

 principle, on which he had shown that a sound should become 

 sharper or flatter if there were an approach or a recession 

 between the ear and the source of the sound, would apply 

 equally to light ; and he went on to say that the difference of 

 colour of some of the binary stars might be produced in this 

 w.-iy by their motions. Doppler was right in that the principle 

 is true in the case of light, but he was wrong in the particular 

 conclusion which he drew from it. Even if we suppose a star 

 to b : moving with a sufficiently enormous velocity to alter 

 sensibly its colour to the eye, no such change would actually be 

 seen, for the reason that the store of invisible light beyond both 

 1 mits of the visible spectrum, the blue and the red, would be 

 drawn upon, and light-waves invi-ible to us would be exalted or 

 degraded so as to take the place of those raised or lowered in 

 the visible region, and the colour of the star would remain 

 unchanged. About eight years later Fizeau pointed out the 

 importance of considering the individual wave-lengths of which 

 white light is composed. As soon, however, as we had learned 

 to recognize the lines of known substances in the spectra of the 

 heavenly bodies, Doppler's principle became applicable as the 

 basis of a new and most fruitful method of investigation. The 

 measurement of the small shift of the celestial lines from their 

 true positions, as shown by the same lines in the spectrum 

 of a terrestrial substance, gives to us the means of ascertaining 

 directly in miles per second the speed of approach or of reces- 

 sion of the heavenly body from which the light has come. 



An account of the first application of this method of research 

 to the stars, which was made in my observatory in 1868, was 

 given by Sir Gabriel Stokes from this chair at the meeting at 

 ICxeter in 1869. The stellar motions determined by me were 

 shortly after confirmed by Prof. Vogel in the case of Sirius, 

 and in the case of other stars by Mr. Christie, now Astronomer- 

 Koyal, at Greenwich ; but, necessarily, in consequence of the 

 inade(iuacy of the instruments then in use for so delicate an 

 inquiry, the amounts of these motions were but approximate. 



The method was shortly afterwards taken up systematically at 

 Greenwich and at the Rugby Observatory. It is to be greatly 

 regretted that, for some reasons, the results have not been suffi- 

 ciently accordant and accurate for a research of such exceptional 

 delicacy. On this account probably, as well as that the spectro- 

 scope at that early time had scarcely become a familiar instru- 

 ment in the observatory, astronomers were slow in availing 

 themselves of this new and remarkable power of investigation. 

 That this comparative neglect of so truly wonderful a method of 

 ascertaining what was otherwise outside our powers of observa- 

 tion has greatly retarded the progress of astronomy during the 

 last fifteen years, is but too clearly shown by the brilliant results 

 which within the last couple of years have followed fast upon 

 the recent masterly application of this method by photography 



NO. 1138, VOL. 44] 



at Potsdam, and by eye with the needful accuracy at ihe Lick 

 Observatory. At last this use of the spectroscope has taken its 

 true place as one of the most potent methods of astronomical 

 research. It gives us the motions of approach and of recession, 

 not in angular measures, which depend for their translation into 

 actual velocities upon separate determinations of parallactic dis- 

 placements, but at once in terrestrial units of distance. 



This method of work will doubtless be very prominent in the 

 astronomy of the near future, and to it probably we shall have 

 to look for the more important discoveries in sidereal astronomy 

 which will be made during the coming century. 



In his recent application of photography to this method of 

 determining celestial motions. Prof. Vogel, assisted by Dr. 

 Scheiner, considering the importance of obtaining the spectrum 

 of as many stars as possible on an extended scale without an 

 exposure inconveniently long, wisely determined to limit the 

 part of the spectrum on the plate to the region for which the 

 ordinary silver-bromide gelatine plates are most sensitive — 

 namely, to a small distance on each side of G — and to employ as 

 the line of comparison 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 spectrum rela- 

 tively 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 number 

 of lines, no fe^ver than 250 in the case of Capella, within the 

 small region of the spectrum on the plate. Already the motions 

 of about fifty stars have been measured with an accuracy, in 

 the case of the larger number of them, of about an English mile 

 per second. 



At the Lick Observatory it has been shown that observations 

 can be made directly by eye with 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 grating 14,438 lines 

 to the inch. The marvellous accuracy attainable in his hands 

 on a suitable star is shown by observations on three nights of the 

 star Arcturus, the largest divergence 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 

 determinations 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 parallax 

 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 200 years upon their journey. 



Mr. Keeler, with his magnificent means, has accomplished a 

 task which I attempted in vain in 1874, with the comparatively 

 poor appliances at my disposal, of measuring the motions in the 

 line of sight of some of the planetary nebulre. As the stars have 

 considerable motions in space, it was to be expected that nebula: 

 should possess similar motions, for the stellar motions must have 

 belonged to the nebulae out of which they have been evolved. 

 My instrumental means, limiting my power of detection to 

 motions greater than twenty-five miles per second, were in- 

 sufficient. Mr. Keeler has found in the examination of ten 

 nebulae motions varying from two miles to twenty-seven mile.-, 

 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 probably 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 i ■ 

 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 oi)tician could construct, and so enabled us to 

 penetrate into mysteries hidden in stars apparently single, and 

 altogether unsuspected of being binary systems. The spectro- 

 scope 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 clas 



