188 cosmos. 



may well be assumed, there exist, in the regions of space, 

 dark invisible bodies in which the process of light-producing 

 vibration does not take place, these dark bodies can not fall 

 within the sphere of our own planetary and cometary system, 

 or, at all events, their mass can only be very small, since 

 their existence is not revealed to us by any appreciable dis- 

 turbances. 



The inquiry into the quality and direction of the motion oj 

 the fixed stars (both of the true motion proper to them, and 

 also of their apparent motion, produced by the change in 

 the place of observation, as the earth moves in its orbit), the 

 determination of the distances of the fixed stars from the 

 sun by ascertaining their parallax, and the conjecture as to 

 the part in universal space toward which our planetary 

 system is movinsr, are three problems in astronomy which, 

 through the mean& of observation already successfully em- 

 ployed 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 par- 

 allaxes and the determination of the distances of certain fixed 

 stars, to complete that which especially relates to our pres 

 ent 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 sys- 

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

 point out the means of discovering the parallax ; not by de- 

 termining the star's 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 mi- 

 crometrical method which William Herschel (1781), Struve, 

 and Bessel subsequently made use of. " Perche io non credo," 

 says Galileo, 1 * in his third dialogue (Giornata terza), " die 

 tutte le stelle siano sparse in una sferica superficie egual- 

 mente distanti da un centro ; ma stimo, che le loro lonta- 

 nanze da noi siano talmente varie, che alcune ve ne possano 

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

 gi trovasse col telescopio qualche picciolissima Stella vid- 



* Opere di Galileo Galilei, vol. xii... Milano, 1811, p. 206. This re- 

 markable passage, which expresses the possibility and the project of 

 a measurement, was pointed out by Arago ; ^ee his Annuaire pour 1842 

 p. 382. 



