PROPER MOTTON OF THE STARS. 265 



Duration of proper motion, that is to say, of the changes which 

 take place in the positions of self-luminous stars, throws some 

 light on two mutually dependent problems ; namely, the motion 

 of the solar system, 89 and the position of the centre of gravity in 

 the heaven of the fixed stars. That which can only be reduced 

 in so very incomplete a manner to numerical relations, must for 

 that very reason be ill calculated to throw any clear light on such 

 causal connexion. Of the two problems just mentioned, the 

 first- alone (especially since Argelander's admirable investiga- 

 tion) admits of being solved with a certain degree of satis- 

 factory precision ; the latter has been considered with much 

 acuteness by Madler, but according to the confession of this 

 astronomer himself, 30 his attempted solution is, in consequence 

 of the many mutually compensating forces which enter into it, 

 devoid " of anything like evidence amounting to a complete 

 and scientifically certain proof." 



After carefully allowing for all that is due to the precession 

 of the equinoxes, the nutation of the earth's axis, the aber- 

 ration of light, and the change of parallax caused by the earth's 

 revolution round the sun, the remaining annual motion of 

 the fixed stars comprises at once that which is the con- 

 sequence of the translation in space of the whole solar sys- 

 tem, and that also which is the result of the actual proper 

 motion of the,fixed stars. In Bradley's masterly labours on 

 nutation, contained in his great treatise of the year 1748, we 

 meet with the first hint of a translation of the solar system, 

 and in a certain sense also with suggestions for the most 

 desirable methods of observing it. 31 " For if our own solar 

 system be conceived to change its place with respect to abso- 



K Cosmos, vol. i. p. 136. 



* Madler, Astronomic, s. 414. 



31 Arago, in his Annuaire pour 1842, p. 383, was the 

 first to call attention to this remarkable passage of Brad- 

 ley's. See, in the same Anmiaire, the section on the trans- 

 lation of the entire solar system, pp. 389-399. 



