Aroi'ST 24, 188U.J 



SCIENCE. 



■235 



existing between the oo-onliuates of eacU zone with 

 tliose of it3 neighbor. 



It is probable, however, that the experience of Gill 

 will be repeated on a larger scale. In 1S7S he solicited 

 the co-operation of astronomers in the determination 

 of the co-ordinates of twenty-eight stars, which he 

 desired to employ in the reduction of his heliometer 

 observations of the planet Mars for the purpose of 

 obtaining the solar parallax. The results obtained 

 at twelve observatories of the first class are published 

 in volv xxxix. p. 99, of the monthly notices of the 

 Royal astronomical society. Notwithstanding the 

 fact that the final values obtained at each observa- 

 tory depend upon several observations, the averjige 

 difference between the least and the greatest results, 

 obtained by different observers for each star, isU.2J\ 

 ill right ascension and 2.3" in declination. In four 

 eases the difference in right ascension exceeds ;J.O , 

 and in four cases the difference in declination ex- 

 ceeds 3.0." 



Kven after the results are reduced to a homogeneous 

 system, the following outstanding deviations from a 

 mean system are found : — 



The observations of a second list of twelve stars, 

 one-lialf of the number lieing comparatively bright, 

 and the remaining half faint, showed no marked im- 

 provement, either with respect to the magnitude of 

 errors which could be classed as accidental, or in 

 regard to the systematic deviations from a mean 

 system. 



This discussion revealed one source of discordance 

 which will doubtless affect the zone observations; 

 viz., the difference between right ascensions deter- 

 mined by the eye-and-ear method, and those deter- 

 mined with the aid of the chronograph. 



Tlie programme of the gesellschaft makes no pro- 

 vision for the eliniiiiaticm of errors which depend 

 upon the magnitude of the stars observed; but 

 special observations have been undertaken at several 

 observatories for the purpose of defining the relation 

 between the resull.4 for stars of different magnitudes. 

 At Harvard-college observatory, the direct effect of a 

 reduction of the magnitude has been ascertained by 

 reducing the aperture of the telescope by means of 

 <liaphragnis. lieside this, the observations have been 

 arranged in such a nuinner that an erior depend- 

 ing ui)on the magnitude can be derived from an in- 

 vestigation of the observations upon two successive 

 nights. 



At Leiden, at Albany, and perhaps at other 

 observatories, the effect of magnitude has been deter- 

 mined by observations through wire gauze. But 



notwithstanding all the precautions wlilch have been 

 taken in the observations, and which may be taken 

 in llie reductions, it will undoubtedly be found that 

 the final results obtained will involve errors which 

 cannot be entirely eliminated. 



In the experience of the speaker, two other sources 

 of error have been detected. It has been found, that 

 there is a well-defined equation between the observa- 

 tions, whicli is a function of the amount, and the 

 character of the illumin.ition of the field of the tel- 

 escope. It has also been found that ob,servations 

 made under very unfavorable atmospheric conditions 

 differ systematically from those made under favorable 

 conditions. When the seeing was noted as very bad, 

 it is found that the observed riglit ascensions are 

 about .OS'too great, and that the observed declina- 

 tions are about 0.8" too great. 



There are doubtless other sources of error which 

 the discussion of the observations will bring to liglit. 

 The effect of the discovery of these and other errors 

 will probably be to hasten tlie repetition of the zone 

 observations under a more perfect scheme, framed in 

 such a manner as to cover all the deficiencies which 

 experience has revealed, or may yet reveal. One 

 would not probably go far astray in naming the year 

 liKX) as the mean epoch of the new survey. If the 

 observations are again repeated in lli.lO, sufficient 

 data will then have been accumulated for at least an 

 approximate determination of the laws oi siderial 

 motion. 



What is the present state of our knowledge upon 

 this subject? It can be safely said that it is very 

 limited. First of all, it cannot be affirmed that there 

 is a sidereal system in the sense in wliicli we speak 

 of the solar system. In the case of the solar system, 

 we have a central sun about which the planets and 

 their satellites revolve in obedience to laws which 

 are satisfied by the hypothesis of universal gravita- 

 tion. Do tlie same laws pervade the inter-stellar 

 spaces ? Is the law of gravitation indeed universal ? 

 What physical connection exists between the solar 

 system and the unnumbered and innumerable stars 

 which form the gala.xy of the heavens ? Do these 

 stars form a system which has its own laws of rela- 

 tive rest and moti<ui ? or is the solar system a part 

 of the stupendous whole ? Dues the solar system re- 

 ceive its laws from the sidereal system? or has Kepler 

 indeed pierced the depths of the universe in the discov- 

 ery of the laws wliich gave him immortality ? Are we 

 to take the alternative stated by Hall, — either that our 

 sidereal system is not an entirely isolated object, or 

 its bodies must be vastly more numerous or more 

 massive than even our most liberal interpretation of 

 observations would seem to warrant ? Are we to 

 conclude, for example, that stars like IWO Groom- 

 bridge and a Centauri, "after having travelled from 

 an infinitely greivt distance on one side of the heavens, 

 are now passing through our system for the first and 

 only lime, and that after leaving our system they 

 will retreat again into the depths of space to a dis- 

 tance which, fur any thing we can tell, may be pr.ic- 

 tically regarded as infinite " ? Can we assert with 

 Xewcomb, that in all probability the stars do not 



