360 Dr. Herschel's Account of the Changes that have happened 
systematical parallax, as we may call that apparent change of 
place of stars which is owing to the motion of the solar system. 
Let the real motion of x, aided by the effect of the same pa- 
rallax, be the cause of the changes in the angle of position 
which my observations have given. We may admit the largest 
of the two stars of our double star to be of the second magni- 
tude ; and, as we are not to place x too near a, we may suppose 
its distance from O to be to that of a from the same as 3 to 2. 
In this case, O will move from the parallel of a, in an angle of 
6 o° 37' north following, with an apparent annual velocity of 
,4536. The motion of « in right ascension, may be intirely 
ascribed to solar parallax ; but its change of declination, cannot 
be accounted for in the same manner. Let us therefore admit 
that the solar velocity, in the direction we have calculated, will 
produce an apparent retrograde motion in a, which, in 231- 
years will amount to 2",o85 in right ascension. But the same 
parallax will also occasion a change in declination, towards the 
south preceding, of 3", 701 ; and, as this will not agree with 
the observed motion of a, we must account for it by a proper 
motion of this star directly towards the north. The real annual 
velocity required for this purpose, must be 1,3925. 
The apparent motion of x, by parallax, at the distance we 
have placed this star, will be 2", 832 towards the south pre- 
ceding; and, by assigning to it an annual proper motion of the 
velocity 1,3354, in the direction of 73 0 10' north preceding its own 
parallel, the effect of the solar parallax and this proper motion 
together, will have caused the small star, in appearance, to 
revolve round «, so as to have produced all the changes in the 
angle of position which my observations have given ; and, at 
the same time, a, will have been seen to move from its former 
