186 SPECIAL RESULTS IN THE TJRANOLOGICAL PORTION 



with great acuteness, proposed the means of finding the 

 parallax, not by the determination of the distance of a star 

 from the zenith or the pole, but " by the careful com- 

 parison of one star with another very near to it." Although 

 expressed in very general terms, this is the micrometric 

 method subsequently employed by William Herschel 

 (1781), Struve and Bessel. Perche io non credo/' 

 said Galileo ( 305 ) in his third discourse (Giornata terza), 

 "che tutte le stelle siano sparse in una sferica superficie 

 egualmente distanti da un centra ; ma stimo, che le loro 

 lontananze da noi siano talmente varie, che alcune ve ne 

 possano esser 2 e 3 volte piu remote di alcune altre; talche 

 quando sitrovasse col Telescopic qualche picciolissima steMa 

 vicinissima ad alcuna delle maggiori, e che pero quella fusse 

 altissima, potrebbe accadere, che qualche sensibile mutazione 

 succedesse tra di low" The promulgation of the Coper- 

 nican system carried with it the grounds for requiring the 

 numerical assignment by measurement of the change of 

 direction, which the half-yearly change of place of the Earth, 

 in her orbit round the Sun, must produce in the apparent 

 position of the fixed stars. But as the angular determina- 

 tions of Tycho Brahe, so happily employed by Kepler, 

 (although, as I have already said, they might be considered 

 certain to one minute of arc), still did not shew any such 

 change arising from parallax, the Copernicans long satis- 

 fied themselves by replying to such requirements, that the 

 diameter of the Earth's orbit, (41-J- German, 165 English 

 millions of geographical miles), was too small in proportion 

 to the exceedingly great distance of the fixed stars. 



The hope of being able to discover by observation the ex- 

 istence of parallax, must therefore have been seen to be de- 



