CELESTIAL MEASURINGS AND WEIGHINGS. 197 



it is found that one star appears to be subject to a regu- 

 larly recurring annual displacement, such as that which 

 the earths orbital motion might cause, and others in its 

 near neighbourhood show no signs of it, we can accept 

 the double conclusion that the one is and the others are 

 not at a measurable distance. 



(21.) As the ancients had no knowledge of the earth's 

 motion, so they could have had no conception of this 

 annual displacement of the stars or of their " parallax." 

 Tycho Brahe who rejected the Copernican system, might 

 perhaps have been led to do so from his not having been 

 able to perceive any such displacement of the pole star, 

 which, from the rudeness of his means of observation, he 

 could not possibly have done. Much more lately, in the 

 latter years of the iyth and beginning of the i8th cen- 

 turies, the attention of many eminent astronomers was 

 drawn, in consequence of the improvements then intro- 

 duced in the construction of astronomical instruments, 

 to a regularly recurring annual displacement of certain 

 stars observed by them to a very considerable amount, 

 which was at first supposed to be parallax, but which 

 proved to be what is now called " aberration," and to be 

 common to all the stars ; and when this was recognized 

 finally by Bradley as a result of the motion of light, the 

 idea of a measurable "parallax" was abandoned in de- 

 spair; to be revived again by Dr Brinkley in 1810 ; who 

 from his observations with a very fine circle in the Royal 

 Observatory of Dublin thought he had detected a paral- 

 lax of i" in the bright star Lyra (corresponding to an 

 annual displacement of 2"). This however proved to be 



