268 
Mr. Herschel on the 
stars, and for stars nearer than 5" the amount is of course 
proportionally greater. Thus for two stars, at only 1" dis- 
tance from each other, of which the one is affected by 
parallax to the amount of 1", and the other not all, the an- 
nual variation in position will amount to upwards of 53 0 . 
When the distance between two stars amounts to no more 
than 3", and in many cases even when they are still nearer, 
Mr. South’s and my own experience in the use of the position 
micrometer, as well as Mr. Struve’s numerous and excellent 
measures recorded in the Dorpat Observations, lead me to 
believe that a single degree in the angle of position is a 
quantity distinctly appreciable in the mean of several sets of 
measures carefully taken on different nights of observation, 
even when a considerable inequality in the stars, or other 
unfavourable circumstances exist ; and were observations con- 
tinued for a series of years at the proper times, and made 
with the care so delicate a subject of research would peculiarly 
call for, there can be no doubt that a greater degree of pre- 
cision might be obtained ; and it certainly seems not too much 
to assume, that half that quantity, or 30' of variation in the 
angle of position, if regularly periodical, must ultimately be 
detected. This conclusion appears warranted by the inter- 
esting re-examination by Mr. South of stars pointed out in 
our joint paper on Double Stars (Ph. Tr. 1824. iii.) as having 
probably a relative angular motion, or as otherwise remark- 
able, lately communicated by him to the Royal Society, (See 
Part i. of the present vol.) in which we find instances of 
coincidence between calculated and computed motions falling 
within the limit assumed at the outset — too frequent, and too 
remarkable to be the effect of mere accident. 
Now, the tangent of 30' to a radius of 3 " corresponds to a 
