186 cosmos. 



time, therefore, before the beginning of his last and painful 

 illness, led Bessel, the greatest astronomer of our time, to the 

 conviction " that stars whose variable motion becomes appar- 

 ent by means of the most perfect instruments, are parts of 

 systems confined to very limited spaces in proportion to their 

 great distances from one another." This belief in the exist- 

 ence of double stars, one of which is devoid of light, was so 

 firmly fixed in Bessel's mind, as my long correspondence with 

 him testifies, that it excited the most universal attention, 

 partly on his account, and partly from the great interest 

 which independently attaches itself to every enlargement of 

 our . knowledge of the physical constitution of the sidereal 

 heavens. " The attracting body," this celebrated observer 

 remarked, " must be very near either to the fixed star which 

 reveals the observed change of position, or to the sun. As, 

 however, the presence of no attracting body of considerable . 

 mass at a very small distance from the sun has yet been per- 

 ceived in the motions of our own planetary system, we are 

 brought back to the supposition of its very small distance 

 from a star, as the only tenable explanation of that change 

 in the proper motion which, in the course of a century, be- 

 comes appreciable."* In a letter (dated July, 1844) in an- 

 swer to one in which I had jocularly expressed my anxiety 

 regarding the spectral world of dark stars, he writes : "At 

 all events, I continue in the belief that Procyon and Sirius 

 are true double stars, consisting of a visible and an invisible 

 star. No reason exists for considering luminosity an essen- 

 tial property of these bodies. The fact that numberless stars 

 are visible is evidently no proof against the existence of an 

 equally incalculable number of invisible ones. The physical 

 difficulty of a change in the proper motion is satisfactorily 

 set aside by the hypothesis of dark stars. No blame attaches 

 to the simple supposition that the change of velocity only 

 takes place in consequence of the action of a force, and that 

 forces act in obedience to the Newtonian laws." 



A year after Bessel's death, Fuss, at Struve's suggestion, 

 renewed the investigation of the anomalies of Procyon and 

 Sirius, partly with new observations with Ertel's meridian- 

 telescope at Pulkowa, and partly with reductions of, and com- 

 parisons with, earlier observations. The result, in the opin- 

 ion of Struve and Fuss,f proved adverse to Bessel's assertion 



* Schum., Astr. Nachr., Nos. 514-516. 



t Struve, Etudes d'Astr. Stellairc, Texte, p. 47, Notes, p. 26, and 51- 

 57 ; Sir John Herschel, Outl., $ 859 and 860. 



