SOLAR MOTION. 233 



of its motion with the visual ray will still remain un- investigation 

 known. of the proper 



As hitherto we have consulted only those proper mo- g^^^ 

 tions which have a marked tendency to a parallactic cen- 

 tre, we ought now, when the question is to determine the 

 velocity of the solar motion, to have in view the real mo- 

 tion of every star whose apparent motion we know; for 

 as it would not be proper to assign a motion to the sun, 

 either much greater or much less than any real motion 

 which may be found to exist in some star or other, it fol- 

 lows that a fieneral review of proper motions ought to be 

 made before we can impartially fix on the solar velocity ; 

 but as trials with a number of stars would be attended 

 with considerable inconvenience, I shall use only our 

 former six in laying down the method that will be fol- 

 lowed with all the rest. 



Proportio7ial Distance of the Stars. 



We are now come to a point no less difficult than es- 

 sential to be determined. Neither th« parallactic nor real 

 motion of a star can be ascertained till its relative distance 

 is fixed upon. In attempting to do this it will not be 

 satisfactory to divide the stars into a few magnitudes, and 

 suppose these to represent the relative distances we re- 

 quire. There are not perhaps among all the stars of the 

 heavens any two that are exactly at the same distance 

 from us ; much less can Ave admit that the stars which we 

 call of the first magnitude are equally distant from the 

 sun. And indeed, if the brightness of the stars is admitted 

 as a criterion by which we are to arrange them, it is per- 

 fectly evident that all those of the first magnitude must 

 differ as much in distance as they certainly do in lustre; ^ 



yet imperfect as this may be, it is at present the only 

 rule we have to go by. 



The relative brightness of our six stars, may be express- 

 ed as folloMS : Sirius - - . Arcturus - Capella 5 Lyra * . 

 Aldebaran , Procyon. 



The notations here used are those which have been ex- 

 plained in my first Catalogue of the relative Brightness of 

 the Stars* ; but to denominate the magnitudes of these six 



• Phil. Trans, for 1796, page 189. 



