SCIENCE 



NEW YORK, OCTOBER 30, 1891. 



CELESTIAL PHYSICS.' 



(Continued from p. 234.) 



Besides its more direct use in the chemical analysis of the 

 heavenly bodies, the spectroscope has given to us a great and 

 unexpected power of advance along the lines of the older 

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

 placed upon the indirect use of the spectroscope than upon 

 its chemical relations. 



By no direct astronomical methods could motions of ap- 

 proach or of recession of the stars be even detected, much 

 less could they be measured. A body coming directly to- 

 wards 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. Tbe 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 ap- 

 proach 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 line of sight, the speed and direction of the 

 solar motion in space, and many of the great problems of 

 the constitution 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 forever beyond our grasp : it 

 enables us to measure directly, and under favoi'able circum- 

 stances 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 mov- 

 ing body, and is therefore as applicable and as certain in the 

 case of a body on the extreme confines of the visible uni- 

 verse, so long as it is bright enough, as in the case of a 

 neighboring 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 color of some of the binary stars might be produced in 

 this way 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 be moving with a sutBciently enormous 

 velocity to alter sensibly its color to the eye, no such change 

 would actually be seen, for the reason that the store of in- 

 visible light beyond both limits of the visible spectrum, the 

 blue and the red, would be drawn upon, and light-waves in- 

 visible to us would be exalted or degraded so as to take the 

 place of those raised or lowered in the visible region, and 

 the color of the star would remain unchanged. About eight 



1 Inaugural address at the meeting of the British Association for the Ad- 

 vancement of Science, at Cardiff, August, 1891, by William Hugglns, president 

 of the association (Nature, Aug. 30). 



years later Pizeau poin.r^d oui tl;e impiuiuiicj o" ■ '>"('»:.!,.;.' 

 the individual wavelengths of v/hich white light is com- 

 posed. As soon, however, a.s we had learned to recognize 

 the lines of known substances in the spectra^.f the heavenly 

 bodies, Doppler's principle became applicable as the basis of 

 a new and most fruiUul method of investif-ation. The 

 measurement of the sr^all shift of the celestial bodies from 

 their true positions, as shown by tbe same lines in the spec- 

 trum of a terrestrial substance, gives to us the means of as- 

 certaining directly in miles per second the speed of approach 

 or of recession of the ueavenly body from which the lijilit. 

 has come. 



An account of the first application of this method of re- 

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

 1868, was given by Sir Gabriel Stokes from this chair at the 

 meeting at Exeter in 1869. The stellar motions determined 

 by me were shortly after confirmed by Professor Vogel in 

 the case of Sirius, and in the case of other stars by Mr. 

 Christie, now astronomer-royal, at Greenwich; but, neces- 

 sarily, in consequence of the inadequacy 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 sufBciently accordant and accurate for a research of 

 such exceptional delicacy. On this account probably, as 

 well as that the spectroscope at that early time had scarcely 

 become a familiar instrument in the observatory, astrono- 

 mers were slow in availing themselves of this new and re- 

 markable power of investigation. That this comparative 

 neglect of so truly wonderful a method of ascertaining what 

 was otherwise outside our powers of observation 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 

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

 Lick Observatory. At last this use of the spectroscope has 

 taken its true place as one of the most potent methods of as- 

 tronomical 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 deter- 

 minations of parallactic displacements, but at once in terres.-- 

 trial 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. Professor "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 



