238 Dr . Herschel on the Direction of the 
depend, admits of the greatest variety of directions. We are 
only acquainted with the plane in which the motion must be 
performed, and with the length of the arch in seconds by 
which that motion may be measured. We may add that the 
chords of the arches representing the three motions are the 
smallest velocities of these motions that can be admitted ; for 
in every other direction but at right angles to the line of sight, 
the actual space over which the star will move must be greater 
than the arch or chord by which its motion is represented. 
Now, since a motion of the sun will occasion parallactic 
motions of the stars, it follows that these again must indicate 
a solar motion ; but in order to ascertain whether parallactic 
motions exist, we ought to examine those stars which are most 
liable to be visibly affected by solar motion. "This requisite 
points out the brightest stars as the most proper for our pur- 
pose ; for any star may have a great real motion, but in order 
to have a great parallactic one, it must be in the neighbourhood 
of the sun. And as we can only judge of the distance of the 
stars by their splendour we ought to choose the brightest, on 
account of a probability that, being nearer than faint ones, they 
may be more within the reach of parallax, and thus better 
qualified to show its effects. 
We are also to look out for a criterion whereby parallactic 
may be distinguished from real motions ; and this we find in 
their directions. For if a solar motion exists, all parallactic 
motions will tend to a point in opposition to the direction of 
that motion ; whereas real motions will be dispersed indiscri- 
minately to all parts of space. 
With these distinctions in view, we may examine the proper 
motions of the principal stars ; for these, if the sun is not at 
