20 6 Dr. Herschel on the Quantity 
given, the angle of the direction of its motion with the visual 
ray will still remain unknown. 
As hitherto we have consulted only those proper motions 
which have a marked tendency to a parallactic centre, we 
ought now, when the question is to determine the velocity of 
the solar motion, to have in view the real motion 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 follows that a general 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 followed with all the rest. 
Proportional Distance of the Stars. 
We are now come to a point no less difficult than essential 
to be determined. Neither the 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 repre- 
sent the relative distances we require. 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 we 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 perfectly evident that all those of the first magni- 
tude must differ as much in distance as they certainly do in 
