262 Mr. herschel on the proper Motion 
angular difance in antecedentia (reckoned upon the ecliptic 
from the far towards which the fyfem moves) is lefs than 
180 degrees, will decreafe in longitude. And that, on the 
contrary, every flar, whofe difance from the fame far 
(reckoned upon the ecliptic but in confequentid) is lefs than 180 
degrees, will increafe in longitude, in both cafes without alte- 
ration of latitude. 
From thefe principles it would be eafy to draw general 
theorems for every poflible direction of the motion of the folar 
fyfem, by which we might find what alteration of longitude 
or latitude would take place in any given far ; but it will be 
time enough for thole invef igations when we fhall have more 
immediate occalion for them. What we are now chiefly to 
endeavour at is, the fipeedy method of obtaining fufficient fads 
to proceed upon. 
The immenfe regions of the fixed f ars may be confidered as 
an infinitely expanded globe, having the folar fyfem for its 
center. With this idea it will occur to us, that no method 
can be fo proper for finding out the direction of the motion of 
the fun as to divide our obfervations on the lyfl ematical paral- 
lax of the fixed fars into three principal zones. Thefe, for 
the convenience of fixed inf ruments, may be aflumed fo as to 
let them pafs around the equator and the equinodial and fol- 
f itial colures, every one being at redangles to the other two, 
according to the three dimenfions of folids. And fince no ob- 
fervations can be fo conveniently made to afcertain fmall rela- 
tive proper motions among the fixed fars as thole on double 
fars, I have continued my refearches in that line with great 
application, and can now furnifh out thefe three zones, with a 
very complete fet of double fars for fucli obfervations. We 
have the greatef reafon to hope for fuccefs in this attempt ; for, 
if 
