. XXXViL DOUBLE STABS. 365 



globe would of course be proportionally greater. I speak 

 now not of periodical but secular changes. But the ar- 

 gument is complicated with the consideration of the 

 possibly imperfect transparency of the celestial spaces, 

 and with the cause of that imperfect transparency which 

 may be due to material non-luminous particles diffused 

 irregularly in patches analogous to nebulae, but of greater 

 extent to cosmical clouds in short of whose existence 

 we have, I think, some indication in the singular and 

 apparently capricious phenomena of temporary stars, 

 and perhaps in the recent extraordinary sudden increase 

 and hardly less sudden diminution of rj Argus." Mr. 

 Goodricke has conjectured that the periodical changes 

 in the stars may be occasioned by the revolution of some 

 opaque body coming between us and the star, and ob- 

 structing part of its light. Sir John Herschel is struck 

 with the high degree of activity evinced by these changes 

 in regions where, " but for such evidences, we might 

 conclude all to be lifeless." He observes that our own 

 sun requires nine times the period of Algol to perform 

 a revolution on its own axis ; while on the other hand, 

 the periodic time of an opaque revolving body sufficiently 

 large to produce a similar temporary obscuration of the 

 sun, seen from a fixed star, would be less than fourteen 

 hours. 



Many thousands of stars that seem to be only brilliant 

 points, when carefully examined are found to be in 

 reality systems of two or more suns, sometimes revolving 

 about a common center. These binary and multiple 

 stars are extremely remote, requiring the most power- 

 ful telescopes to show them separately. The first cat- 

 alogue of double stars, in which their places and relative 

 positions are determined, was accomplished by the tal- 

 ents and industry of Sir William Herschel, to whom 

 Astronomy is indebted for so many brilliant discoveries, 

 and with whom the idea of their combination in binary 

 and multiple systems originated an idea completely 

 established by his own observations, and recently con- 

 firmed by those of his son and other astronomers. The 

 motions of revolution of many of these stars round a 

 common center have been ascertained, and their periods 

 determined with considerable accuracy. Some have, 



