THE STELLAR UNIVERSE. 



before the telescope had received the vast accession of power 

 which was given to it by the labour and genius of Sir William 

 Herschel. That astronomer observed and catalogued 500 double 

 stars ; and subsequent observers, among whom his son, Sir John 

 Herschel, holds the foremost place, have augmented the number 

 to 6000. 



49. The close apparent juxta-position of two stars in the firma- 

 ment is a phenomenon which might be easily explained, and which 

 could create no surprise. Such an appearance would be produced 

 by the accidental circumstance of the lines of direction of the two 

 stars as seen from the earth, forming a very small angle, in which 

 case, although the two stars might in reality be as far removed 

 from each other as any stars in the heavens, they would never- 

 theless appear close together. The fig. 5 will render this easily 

 understood. Let a and b be the two stars seen from c. The star 



Fig. 5. 



a will be seen relatively to b, as if it were at d, and the two 

 objects will seem to be in close juxta-position ; and if the angle 

 under the lines c a and c b be less than the sum of the apparent 

 semi-diameters of the stars, they would actually appear to 

 touch. 



50. If such objects were few in number, this mode of explaining 

 them might be admitted ; and such may, in fact, be the cause of 

 the phenomenon in some instances. The chances against such 

 proximity of the lines of direction are however so great as to be 

 utterly incompatible with the vast number of double stars that 

 have been discovered, even were there not, as there is, other con- 

 clusive proof that this proximity and companionship is neither 

 accidental nor merely apparent, but that the connection is real, 

 and that the objects are united by a physical bond analogous to 

 that which attaches the planets to the sun. 



But apart from the proofs of real proximity which exist respect- 

 ing many of the double stars, and which will presently be ex- 

 plained, it has been shown that the probability against mere 

 optical juxta-position such as that described above is almost 

 infinite. Professor Struve has shown that, taking the number of 

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