ADDITIONS TO THE NEWTONIAN THEORY. 403 



which, as we have said, sound science prescribes, a Table to be habitu- 

 ally used for all observations. But great difficulties at this time em- 

 barrassed 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, 

 Bicker, 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 refraction. But there were still difficulties. According 

 to La Hire, though the phenomena of twilight give an altitude of 

 34,000 toises to the atmosphere, 1 those of refraction make it only 

 2000. John Cassini undertook to support and improve the calcula- 

 tions 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 refractive power 

 of air. 2 Newton calculated a Table of Refractions, which was pub- 

 lished under Halley's name in the Philosphical Transactions for 1721. 

 without any indication of the method by which it was constructed. 

 But M. Biot has recently shown, 3 by means of the published corres- 

 pondence of Flamsteed, that Newton had solved the problem in a 

 manner nearly corresponding to the most improved methods of modern 

 analysis. 



Dominic Cassini and Picard proved, 4 Le Monnier in 1738 confirmed 

 more fully, the fact that the variations of the Thermometer affect the 

 Refraction. Mayer, taking into account both these changes, and the 

 changes indicated by the Barometer, formed a theory, which Lacaille, 

 with immense labor, applied to the construction of a Table of Refrac- 

 tions from observation. But Bradley's Table (published in 1763 by 

 Maskelyne) was more commonly adopted in England ; and his formula, 

 originally obtained empirically, has been shown by Young to result 

 from the most probable suppositions we can make respecting the atmo- 

 sphere. Bessel's Refraction Tables are now considered the best of 

 those which have appeared. 



Sect. 2. Discovery of the Velocity of Light. Homer. 



THE astronomical history of Refraction is not marked by any great 

 discoveries, and was, for the most part, a work of labor only. The 

 progress of the other portions of our knowledge respecting light is 



i Bailly, ii. 612. 2 Ibid. ii. 607. 



8 Biot, Acad. Sc. Compte Renda, Sept, 5, 1333. 4 Bailly, iii. 92. 



