250 COSMOS. 



determination of the distances oi the fixed stars from the 

 sun, by ascertaining their parallax; and the conjecture as ix> 

 the part in universal space towards which our planetary system 

 is moving are three problems in astronomy, which, through 

 the means of observation already successfully employed in 

 their partial solution, are closely connected with each other. 

 Every improvement in the instruments and methods which 

 have been used for the furtherance of any one of these difficult 

 and complicated problems, has been beneficial to the others. 

 I prefer commencing with the parallaxes and the determination 

 of the distances of certain fixed stars, to complete that which 

 especially relates to our present knowledge of isolated fixed 

 stars. 



As early as the beginning of the seventeenth century, 

 Galileo had suggested the idea of measuring the " certainly 

 very unequal distances of the fixed stars from the solar 

 system," and indeed with great ingenuity, was the first to 

 point out the means of discovering the parallax : not by 

 determining the stars' distance from the zenith or the pole, 

 " but by the careful comparison of one star with another 

 very near it." He gives, in very general terms, an account 

 of the micrometrical method, which William Herschel> 

 (1781,) Struve, and Bessel subsequently made use of. 

 " Perche io non credo," says Galileo, 18 in his third dialogue 

 (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 si trovasse col telescopic qualche piccio- 

 lissima stella vicinissima ad alcuna delle maggiori, e che 



16 Opere di Galileo Galilei, vol. xii. Milano, 1811, p. 206. 

 This remarkable passage, which expresses the possibility and 

 the project of a measurement, was pointed out by Arago ; 

 see his Annuaire pour 1842, p 382. 



