THE STELLAR UNIVERSE. 365 



sun S. When viewed from C, the star B will be above the star A, and when 

 viewed from D, it will be below it. During the intermediate six months the 

 relative change of position would gradually be effected, and the one star would 

 thus appear either to revolve annually round the other, or would oscillate semi- 

 annually from side to side of the other. The extent of its play compared with 

 the diameter C D of the earth's orbit would supply the data necessary to de- 

 termine the proportion which the distance of the stars would bear to that di- 

 ameter. 



The great problem of the stellar parallax seemed thus to be reduced to the 

 measurement of the small interval between the individuals of double stars ; and 

 it happened, fortunately, that the micrometers used in astronomical instruments 

 were capable of measuring these minute angles with much greater relative ac- 

 curacy than could be attained in the observations on greater angular distances. 

 To these advantages were added the absence of all possible errors arising 

 from refraction ; errors incidental to the graduation of instruments ; from un- 

 certainty of levels and plumb-lines ; from all estimations of aberration and pre- 

 cession ; in a word, from all effects which, equally affecting both the individual 

 stars observed, could not interfere with the results of the observations, what- 

 ever they might be. 



These considerations raised great hopes among astronomers that the means 

 were in their hands to resolve finally the great problem of the stellar parallax, 

 and Sir William Herschel accordingly engaged with all his characteristic 

 ardor and sagacity in an extensive series of observations on the numerous 

 double stars, to the original discovery of which science was already so deeply 

 indebted to his labors. ' He had not, however, proceeded far in his researches, 

 when phenomena unfolded themselves before him indicating a discovery of 

 a much higher order and interest than that of the parallax which he sought. 

 He found that the relative position of the individuals of many of the double 

 stars which he examined were subject to a change, but that the period of this 

 change had no relation to the period of the earth's motion. It is evident that 

 whatever appearances can proceed from the earth's annual motion, must be not 

 only periodic and regular, but must pass annually through the same series of 

 phases, always showing the same phase on each return of the same epoch of 

 the sidereal year. In the changes of position which Sir William Herschel ob- 

 served in the double stars, no such series of phases presented themselves. 

 Periods, it is true, were soon developed, but these periods were regulated by 

 intervals which neither agreed with each other nor with the earth's annual 

 motion. 



Some other explanation of the phenomena must therefore be sought for, and 

 the illustrious observer soon arrived at the conclusion that these apparent 

 changes of position were due to real motions in the stars themselves ; that 

 these stars in fact moved in proper orbits in the same manner as the planets 

 moved around the sun. The slowness of the succession of changes which 

 were observed rendered it necessary to watch their progress for a long period 

 of time before the motions of these bodies could be certainly or accurately 

 known ; and accordingly, although these researches were commenced in 1778, 

 it was not until the year 1803 that the observer had collected data sufficient 

 to justify any positive conclusion respecting their orbitual motion. In that and 

 the following year Sir William Herschel announced to the Royal Society, in 

 two memorable papers read before that body, that there exist sidereal systems 

 consisting of two stars revolving about each other in regular orbits, and consti- 

 tuting what he called binary stars, to distinguish them from double stars, gene- 

 rally so called, in which no such periodic change of position is discoverable. 

 Such stars may be only accidentally double, and, as we have already explained, 



