ADDITIONS TO THE NEWTONIAN THEORY. 261 



and Newton, respecting the motions which belong 

 to the earth. 



The true laws of Atmospheric Refraction were 

 slowly discovered. Tycho attributed the apparent 

 displacement of the heavenly bodies to the low and 

 and gross part of the atmosphere only, and hence 

 made it cease at a point half-way to the zenith ; 

 but Kepler rightly extended it to the zenith itself. 

 D. Cassini endeavoured to discover the law of this 

 correction by observation, and gave his results in 

 the form which, as we have said, sound science 

 prescribes, a table to be habitually used for all ob- 

 servations. But great difficulties at this time embar- 

 rassed this investigation, for the parallaxes of the 

 sun and of the planets were unknown, and very 

 diverse values had been assigned them by different 

 astronomers. To remove some of these difficulties, 

 Richer, in 1762, went to observe at the equator; 

 and on his return, Cassini was able to confirm and 

 amend his former estimations of parallax and re- 

 fraction. But there were still difficulties. Accord- 

 ing to La Hire, though the phenomena of twilight 

 give an altitude of 34,000 toises to the atmosphere ', 

 those of refraction make it only 2000. John Cas- 

 sini undertook to support and improve the calcu- 

 lations of his father Dominic, and took the true 

 supposition, that the light follows a curvilinear 

 path through the air. The Royal Society of London 

 had already ascertained experimentally the refrac- 

 tive power of air 2 . Newton calculated a Table of 

 1 Bailly, ii. 612. * Ib. ii. (507- 



