HERSCHEL THE DISCOVERER. 29 



stars : of these he himself discovered 227. In 

 December 1784 he forwarded another catalogue, 

 containing 434 stars. He soon found that he 

 was unable to measure stellar parallax, and the 

 idea dawned on him that the double stars were 

 physically connected by the law of gravitation, 

 though he made no announcement to that effect 

 for many years. On July 1, 1802, Herschel in- 

 formed the Royal Society that " casual situations 

 will not account for the multiplied phenomena 

 of double stars. ... I shall soon communicate 

 a series of observations, proving that many of 

 them have already changed their situation in a 

 progressive course, denoting a periodical revolu- 

 tion round each other." In 1803 he showed that 

 many stars were revolving round their centres 

 of gravity, proving them, in his own words, to 

 be "intimately held together by the bond of 

 mutual attraction." In other words, Herschel 

 discovered that the law of gravitation prevailed 

 in the Stellar Universe, as well as in our Solar 

 System that the law which Newton ascertained 

 to prevail in the Solar System extended through- 

 out the depth of space. 



Herschel did not merely prove the revolution 

 of the binary stars; he assigned periods to 

 those which he had particularly studied. He 

 believed the period of Castor to be 342 years; 



