150 DISCOVERY OH. 



The fact that stars undergo a measurable annual 

 displacement upon the vault of heaven is an indirect 

 proof of the revolution of the earth around the sun, but 

 a direct proof of this movement is available in the 

 " aberration of light." The observations which led 

 James Bradley (1693-1762) to the discovery of this 

 important .astronomical effect were made in the coal 

 cellar of a house at Wanstead in the early part of the 

 eighteenth century. Those were the days when very 

 long telescopes were used as a means of avoiding the 

 coloured fringes which appear around the images pro- 

 duced by an object-glass consisting of a simple lens. 

 The lens of Bradley's telescope was on the roof, and the 

 eye-piece end was placed in the coal cellar, so as to be 

 as far away as possible ; holes being cut in the intervening 

 floor to allow the instrument to pass through. 



Bradley used this instrument to measure the position 

 of a star with the view of discovering the apparent 

 effect, known as " parallax," which must be produced 

 by the change in the earth's point of view an effect 

 sought for by Tycho Brahe and other astronomers, but 

 not then established beyond question, though its exist- 

 ence was a fundamental consequence of the Copernican 

 theory. Bradley did not find what he expected, but 

 his observations showed that all stars seemed to shift 

 slightly from their true places in periodic fashion and 

 by the same amount. The paths followed were similar 

 to what would be produced if the earth's orbit around 

 the sun were projected in miniature upon the back- 

 ground of space. This wandering or aberration could 

 not be confused with parallax due to the change in the 

 point of view, for the shift was the same for all stars, 

 whereas apparent displacement due to parallax would 

 differ for stars at different distances. 



