BINARY STAR a CENTAURI. 463 



But the most interesting part of the orbit is still to come, viz., the periaster in 

 1851, and that this be well observed is indeed to be earnestly hoped, for the peiiod 

 will be an eminently crucial one ; it proved so in the case of 7 Virginis, impera- 

 tively requiring an excessive alteration in all the elements except one, as pre- 

 viously calculated ; and in the case of « Centauri, the characteristic features of 

 the orbit are of a much more violently marked nature, besides being represented 

 altogether on a larger scale. 



The extreme importance of obtaining an abundance of observations at that 

 epoch may be further indicated by the mere statement, that it cannot yet be con- 

 sidered as fully proven, that the law of gravity extends absolutely unaltered to the 

 most distant parts of the sky, and the only mode of proof open to us is by ob- 

 serving the double stars. It is true that most of the orbits yet computed on the 

 theory of gravity have turned out very near the truth, but still not quite so near, 

 it must also be confessed, as could have been desired ; and in the luciferous case 

 of 7 Virginis, every orbit that has been computed for it yet, has persisted in giving 

 a minimum distance of not less than 0-5", while observation at the time of the 

 periastral passage made it certainly much smaller. 



I do not, ot course, by any manner of means, wish to express any doubt on 

 these grounds as to the sufficiency of gravity to explain all the observed pheno- 

 mena ; a great part of the onus, or the whole of it, may rest on the excessive diffi- 

 culty of the species of observation, and their inappropriateness for calculation in 

 all ordinary manners, caused by the exti-eme roughness of even the very best 

 procurable data ; resembling, indeed, those of the comet of 1556, whose return, 

 calculated on such wretched notices of its former perihelion passage, we have 

 been looking out for in vain so long. 



But whatever weight we may attach to the insuf3Bciency of our observations 

 and methods of calculation,* it is always proper to draw a distinct line of demar- 

 cation between those things which are proved and those which are merely inferred, 

 and not seek to enjoy a triumph before the victory has been decidedly achieved. 



* In a letter just received from Captain Jacob, he says that he thinks he has fallen on the right 

 orbit of 7 Virginis at last, having obtained one that expresses all the known observations very well, 

 and gives a minimum distance of 0'23". 



VOL. XVI. PART IV. 5 Z 



