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object existed beyond Uranus would have been regarded 

 as an interesting statement which remained to be 

 proved. 



Another case in which the existence of an unknown 

 mass was suspected before the body itself was seen is 

 that of the Dog Star, Sirius. This star the brightest 

 in the heavens is moving through space, as, indeed, 

 all stars are, but its change of position is not uniformly in 

 one direction. The difference is very slight, and requires 

 good instrumental means to detect it, but it could not be 

 disregarded when once it had been established, and an 

 explanation had to be found for it. This was given by 

 Bessel in 1844, who suggested that a dark body near 

 Sirius was exerting an influence upon its movements. 

 Referring to the matter in a letter to Sir John Herschel, 

 he wrote, " Light is no real property of mass. The 

 existence of numberless visible stars can have nothing 

 against the existence of numberless invisible stars," and 

 upon this idea he based the opinion that Sirius was a 

 double star consisting of an invisible body as well as the 

 visible one, the two forming a couple united by the bonds 

 of gravitation. At that time it seemed scarcely credible 

 that there were stars which could not be seen as well as 

 those revealed by the telescope, but Bessel's belief has 

 since received ample justification. Twenty years after 

 he had given expression to it a faint star was discovered 

 near Sirius, and it proved to be the body which causes 

 the bright star to swerve from a straight path. 



Stars with bright companions were discovered by Sir 

 William Herschel towards the end of the eighteenth 

 century, and their majestic march around one another 

 was recorded. Many of these twin suns are nearly equal 

 in brightness, but in most cases the two stars are badly 

 matched in regard to visual appearance, a brilliant star 



