210 



KNOWLEDGE 



[November 1, 1892. 



observation. The extreme sensitiveness of modern plates 

 causes a little li,g;lit to go a long way in producing legible 

 impressions; and tbese impressions firmly hold their ground 

 against aerial vibrations as regards position, suffering only 

 some slight prejudice in point of definition. 



Since 1887, accordingly, Dr. Vogel, aided very effectually 

 by Dr. Soheiner, has been engaged in perfecting the 

 " spectrographic " method, and with so prosperous an issue 

 as to ensure its wide adoption. The accuracy attained, in 

 fact, is, under favourable circumstances, of so high an 

 order as to give hope of an eventual reactive effect ia 

 correcting the values at present assigned to the dimensions 

 of the solar system ; for the orbital travelhug; of the 

 earth, now in a positive, now in a negative direction, 

 necessarily records itself, in varying but determinable 

 proportions, on each plate exposed to the prismatic rays 

 of the stars. The measured displacements have then to 

 be cleared of its effects, their residue giving the constant 

 quantity of stellar movement towards or from the sun. 

 The terrestrial speed is of necessity, at the actual stagvi of 

 the enquiry, taken as known, the star's movement being 

 treated as unknown. But with some further advance in 

 exactitude, this relation may in the future be inverted, and 

 the earth's rate of circulation determined anew by sub- 

 tracting from the gross amounts of measured velocity the 

 known movement of some star. Thus the great astronomical 

 unit of the sun's distance will perhaps before long come to 

 be more satisfactorily ascertained, through seemingly in- 

 significant modifications of stellar spectra, than it has yet 

 been by means of transits of Venus, oppositions of Mars, or 

 laborious experiments on the swift travelling of light. 



Spectroscopic determinations of motion might almost be 

 said to be unconditioned by either time or space. They 

 are independent of distance, provided only the concen- 

 trated rays they deal with retain sufficient intensity for the 

 purposes of analysis ; and they give results absolutely, 

 and at once. " Proper," or thwartwise motions have to 

 be gathered in slowly, decade by decade, even century by 

 century ; but end-on motions put no strain of waiting on 

 human impatience. Five years have hence sufficed for 

 the execution of the immediate part of the task under- 

 taken at Potsdam. Those five years, moreover, were 

 mainly spent in the development of untried methods, in 

 the invention and construction of novel apparatus. The 

 work was of the pioneering sort, and could not proceed 

 with the same rapidity as if the path of its progress had 

 been long ago smoothed and straightened. Its results, 

 now published in detail," claim a peculiar and unique 

 interest. They are the first of their kind, and they imply 

 the promise of much more than they actually give. Their 

 substantive importance, nevertheless, is great. The fifty- 

 one stars measured are all those suitably situated, and 

 bright enough for the grasp of a 12-inch refractor. With 

 a larger instrument, in course of construction, Dr. Vogel 

 hopes to deal similarly with a longer list of fainter stars, 

 and thus to collect sufficient materials for solving the 

 problem of the sun's motion in space — above all, one may 

 add, for fixing the rate of that motion, so far entirely 

 unknown. 



The first notable point about the motions in line of sight 

 just now published is their moderate speed. The average 

 velocity of the collection comes out no more than 10'4 

 English miles per second. And this, it must be remem- 

 bered, includes the effect of the solar translation, which 

 may accordmgly be surmised to progress at a slower pace 

 than had of late been more or less conjecturally assigned 



* " Publicationen cles Astropliysikalisclien ObsevTatoi-iuras zu 

 Potsdam." Von H. C. Vogel. Baiid VII., Tliell I. Potsdam, 1892. 



to it. The highest velocity determined at Potsdam 

 belongs to Aldebaran, which recedes from us by thirty 

 miles a second, y Leonis coming next with twenty-four 

 miles of approach. There is a total absence of startling 

 records ; nicety of observation has been strongly operative 

 in slackening pace. Dr. Vogel's stars travel at moderate, 

 decorous, and explicable rates. No such celestial projectiles 

 occur among them as y. Cassiopeia? or Groombridge 1830, 

 no correlatives even of ij ToucaniP or 40 Eridani. More- 

 over, the average rate of advance along great circles of the 

 sphere of fifty-one stars with ascertained proper motions, 

 and at more or less reliably known distances, appears to be 

 about thirty-four miles a second, or more than thrice the 

 average raclial speed of an equal number of — it may be 

 well to remark — much brighter stars. The disparity is 

 striking, but it may possibly be reconciled by further 

 research. 



The " goal of the sun's way " may now be placed, with 

 some confidence, in the neighbourhood of the brilliant 

 Vega — let us say, taking the mean of M. Oscar Stumpe's 

 recent determinations, in r. a. 28.5°, Dec. + 38- . In that 

 quarter of the heavens, then, movements of approach must 

 considerably outweigh movements of recession, while, in the 

 opposite quarter near the sun's "anti-apex," the relation 

 is doubtless inverted. lUit here the stars are, for the most 

 part, invisible in northern latitudes, so that negative motion 

 — motion, that is to say, serving to curtail distance — 

 ought, on the whole, to predominate in the Potsdam list. 

 Of the stars included in it, accordingly, thirty-one are 

 approaching, only twenty receding from our system ; while 

 the proportion of stars possessing a negative to those 

 showing a positive velocity above the average of 10-4 miles 

 a second in either sense, is eleven to seven. The 

 inequality would, however, presumably be removed if the 

 observed stars were fairly divided between the northern 

 and the southern hemispheres. 



The adopted velocities in the Potsdam table are the 

 means of independent measures by l)rs. Vogel and 

 Scheiner. These, in most cases, agree well ; in some, 

 they are nearly identical. Vet discordances are not wholly 

 absent. Little reliance, for instance, can be placed on the 

 ostensible result for y Cassiopeife, which, according to 

 Vogel, recedes from the earth at the rate of 2-5 miles, yet 

 was found by Soheiner to have an approaching speed of 

 6-0 miles a second. Both observers, on the other hand, 

 agreed as to the negative direction of the movement of 

 Sirius ; but the velocity assigned by Vogel was of 8-4, by 

 Scheiner of 12-5 miles a second. This discrepancy is 

 indeed minute compared with those which afl'ected the 

 earlier visual estimates of this star's motion. They wore, 

 too, a delusive aspect of periodicity difficult to be accounted 

 for by the mere vagaries of instrumental error. First, the 

 recession attributed to the star gradually diminished its 

 rate between 18(38 and 1882, from twenty-five to two miles 

 per second ; then the direction of motion seemed to become 

 reversed, and approaching velocities, increasing with the 

 same semblance of regularity, were registei'ed. The change, 

 although ten times greater than could be accounted for 

 by the known revolutions of Sirius round its dim 

 companion, appeared to proceed with such steadiness and 

 consistency that few questioned its genuineness. Yet, it 

 is now fully admitted to have been altogether iUusory. 

 The Sirian system, there can be little doubt, is transported 

 towards the solar system at about the same rate of ten 

 miles a second with which it moves across our visual line. 

 But the sun is travelling away from the star, which is 

 situated in the immediate vicinity of the solar anti-apex. 

 Its radial motion hence represents a gain upon the sun. 

 Besides the measurable ten miles a second, Sirius possesses, 



