160 HISTORY OF ASTRONOMY. 



telescope discloses, there may be those whose light re- 

 quires hundreds, and perhaps thousands of years to travel 

 down to us. 



The difficulty of measuring, by direct meridional ob- 

 servations, a quantity so minute as the parallax of the 

 stars, has led astronomers to try a system of differential 

 observations, susceptible of far greater accuracy. Suppose 

 there are two stars at unequal distances from us, so situat- 

 ed as to appear nearly on the same line of vision. Their 

 apparent places must be alike affected by aberration, pre- 

 cession, nutation, refraction and instrumental errors; so 

 that although it is difficult to determine the true right 

 ascension and declination of either star within one second 

 of arc, we may measure the difference of position of one 

 star from the other with extreme precision, without the 

 necessity of taking account of the preceding corrections. 

 Now the difference of position of the two stars, if meas- 

 ured for every season of the year, gives us their difference 

 of parallax; and if one star is several times more distant 

 than the other, this difference of parallax will be sensibly 

 the entire parallax of the nearer star, This method was 

 first proposed by Galileo more than two centuries ago, 

 but it does not appear that he ever attempted to reduce it 

 to practice. Sir William Herschel attempted to reduce 

 this method to practice, and for this purpose he selected a 

 great number of double stars which in consequence of the 

 inequality of the component members, appeared to be well 

 adapted to this object. His labors led to the discovery 

 of a physical connection between the bodies composing 



