I 10 
Mr. herschel on the 
Let the triangle ab/3, fig. n. reprefent the fituation of the 
ftars, ab = d being their diftance feen at O, their dis- 
tance feen at E. That the change bj3 which is produced by 
the earth’s motion will be truly expreffed by P, may be 
proved as before, by fuppofing the ftar a to have been placed 
at a. Now let the angle of pofition baa. be taken by a micro- 
meter*, or by any other method that may be thought Suffi- 
ciently exaCS; then, by Solving the triangle aba, we Shall have 
the longitudinal and latitudinal differences aa and ba of the 
two flats. Put aa — x, ba~y, and it will be x + b(2~ aq. 
whence D = v X -f- 
m 
-M 
Mm 
P -t-jyy ; 
and 
2 X M 2 
Til 
■ jfMm 
in 
M 
= P. 
If neither of the Stars fhould be in the ecliptic, nor have the 
Same longitude or latitude, the laft theorem will ftill Serve to 
•calculate the total parallax whoSe maximum will lie in E. 
There will, moreover, ariSe another parallax, whoSe maximum 
will be in the conjunction and oppofition, which will be di- 
vided, and lie on different fides of the large Star ; but as we 
know the whole parallax to be exceedingly Small, it will not be 
neceffary to inveftigate every particular caSe of this kind ; for, 
by reafon of the divifion of the parallax, which renders obser- 
vations taken at any other time, except where it is greatest, 
very unfavourable, the forms would be of little uSe. 
Tofinifh this theory, I Shall only add a general observation 
on the time and place where the maxima of parallax will 
happen. 
* The pofition of a line palling through the two liars, with the parallel of i 
declination of the largeft of them, may be had by the micrometer I invented for 
this purpofe in the year 1779, of whieh a defcription has been given in a former 
paper; whence, by fpherical trigonometry, we eafily deduce their pofition b ax fig. 
11, with regard to the ecliptic. 
When 
