Februaky 1, 1897.] 



KNOWLEDGE. 



37 



significance as regards the nature of the ultimate particles 

 of matter ; and their first detection in the rays of incon- 

 ceivably distant stars curiously illustrates the close mutual 

 dependence of cosmical and terrestrial physics. 



With the apparatus in use at Harvard College, star- 

 spectra can be photographed by the score together ; and 

 as a result of these expeditious proceedings, the " Draper 

 Catalogue ' ' of ten thousand three hundred rind fifty-one stars, 

 referred to fifteen spectral subdivisions, appeared in 1890. 

 At Potsdam, where the same kind of work was executed 

 with a slit spectroscope instead of with an objective prism, 

 a few bright stars were made the subjects of a stricter 

 examination. Moreover, the main purpose of the scrutiny 

 was not to ascertain their chemical and physical pecu- 

 harities, but to determine their motion. 



Christian Doppler, of Prague, enounced in 1842 the 

 principle of au alteration in the refrangibility of hght 

 emanating from a source retreating or advancing rela- 

 tively to au observer. It is, indeed, tolerably obvious that 

 the ethereal waves must be virtually lengthened hi the 

 first case, shortened in the second ; and the rate, no less 

 than the direction of radial movement, can be ascertained, 

 as Fizeau showed in 1818, by measuring the displacements 

 of lines, bright or dark, in the spectra of the moving 

 bodies. Twenty years, however, elapsed before any 

 attempt was made to realize the indicated method. It 

 was reserved for Dr. Huggins, in 18(58, to apply it prac- 

 tically to stars and nebabe. 



A fresh and incalculable impulse was thus given to 

 sidereal science. Telescopically, bodies travelling in the 

 line of sight appear stationary ; but the spectroscope, by 

 this exquisite mode of discernment, supplies the missing 

 component of motion, irrespective both of distance and of 

 time. Here, then, geometrical and physical inquii-ies 

 meet on common ground, each furnishing data of com- 

 parable value for the mathematical treatment of problems 

 in celestial mechanics. Without the aid of photography, 

 however, no sufficient accuracy in this matter was possible. 

 In estimating linttdisplacements the eye is continually 

 baffled by atmospheric agitations, which the chemical 

 retina, in the long run of, say, an hour's exposure, is 

 enabled to ignore. 



Dr. Vogel, Director of the Potsdam Observatory, ably 

 seconded by Dr. Scheiner, initiated, in 1888, the spectro- 

 graphic measurement of radial movements, and the detaUs 

 of his model investigation were pubUshed in 1892. An 

 average rate of ten and a half miles a second was derived 

 from fifty-one stars, of which Aldebaran, with a recession 

 of thirty mUes a second, was the most rapid. On the 

 completion of a thirty-two-inch photographic refractor, 

 lately ordered by Dr. Vogel, the inquiry can be extended 

 to several hundred fainter stars ; and materials will then 

 at last be at hand for a really authentic determination of 

 the sun's course through space, and of the speed with which 

 it is pursued. No treatment of stellar proper— th3,t is, 

 telescopic — motions, avails to elicit, apart from doubtful 

 assumptions, the latter element ; but the general upshot 

 of a large mass of recent work on the subject has been to 

 shift the solar apex from the point where Hersehel placed 

 it in 1783, towards Lyra and the plane of the Milky Way. 

 But the limits of uncertainty are, if anything, wider than 

 they were left by Argelander's discussion in 1837. 



The first evidences of motion in nebulfe were obtained 

 by Prof. Keeler in 1890 from spectroscopic observations 

 made visually with the great Lick equatorial. The 

 light-grasp of the instrument, and the stillness of the 

 air on Mount Hamilton, contributed, with his exceptional 

 skill, to make his results trustworthy. They applied to 

 ten planetaries, found to possess radial movements of at 



least the average stellar velocity. The thwartwisf motion 

 of nebulae has yet to be detected. Their appaivnt fixity 

 is doubtless due, in part to their remoteness, in part to 

 inaccuracies in the places assigned to them. 



The discovery of " spectroscopic binaries" is one of the 

 most striking fruits of the line-of-sight method. Miss 

 Maury's examination of the Harvard photographs showed, 

 in 1889, that the lines in the spectrum of ? Ursag Majorig 

 separate and close up again once in fifty-two days, 

 obviously as a consequence of the orbital revolution in 

 twice that time of a pair of suns. Soon afterwards, ,3 

 Aurigfe was simUarly decomposed, the spectral duplications 

 taking place, however, on alternate days ; and two 

 unequal couples, with still shorter periods, have since been 

 discovered at Harvard. Systems equally rapid are formed 

 by the union of bright with dark components. They are 

 recognized by the occurrence of periodical opposite dis- 

 placements in the single spectrum of the obscurely 

 attended star. Spica Virginis is of this kind. Its peculiar 

 character was detected photographically at Potsdam. A 

 number of variables belong to the same type, notably Algol, 

 and other eclipsing stars. Less intelligibly, J Cephei and 

 its congeners, in which the law of light-change appears to 

 exclude eclipses, range themselves unmistakably iu the 

 category. Some of these extraordinary couples circulate 

 in a few hours, most likely with enormous velocities. 



The combination of lucid with obscure, or partially 

 obscure, masses appears to be a regular part of the cosmical 

 scheme. The presence of dark stars in ordinary stellar 

 systems is not unfrequently betrayed by their gravitational 

 effects upon the movements of their bright fellows. 



Sir Geobge AiKr. 



Examples are met with in ^ Canon and 70 Ophiuchi. 

 The dusky attendants, too, of Sirius and Procyon were 

 predicted by Bessel as necessary to explain the undulating 

 proper motions of the Dog Stars long before they were 

 actually seen ; the former, by Clark in 18G2 — the latter, 

 probably, by Schaeberle in 1890. The " astronomy of the 

 invisible," however, achieved its highest triumph in the 



