1^0 A/r. herschel’s Defcription of 
jetted upoii the micrometer ; then, by the handle P , which com- 
mands the pofition of the arm, I raile or depiefs it fo as to bring 
the two lucid points to a fimilaf Situation with the two ftars ; and, 
by the handle D, I approach or remove the moveable lucid point 
to the fame diftance of the two ftars, fo that the two lucid 
points may be exactly covered by, or coincide with the ftars* 
A little practice in this bufmefs foon makes it ealy, efpecially 
to one who has already been ufed to look with both eyes open. 
What remains to be done is very fimple. With a proper 
rule, divided into inches and fortieth parts, I take the diftance 
of the lucid points, which may be done to the greateft nicety-, 
becaufe, as I obferved before, the little holes are made with the 
point of a very fine needle. The meafure thus obtained is the 
tangent of the magnified angle under which the ftars are feen 
to a radius of ten feet ; therefore, the angle being found and 
divided by the power of the telefcope gives the real angular 
diftance of the centers of a double ftar. 
For inftance, September 25, 1781, I meafured * Herculis 
with this mftfument. Having caufed the two lucid points to 
coincide exaftly with the ftars center upon center, I found the 
radius or diftance of the central lamp from the eye 10 feet 
4,15 inches; the tangent or diftance of the two lucid points 
50,6 fortieth parts of an inch; this gives the magnified angle 
and dividing by the power which I ufed, we obtain 
4" 34'" for the diftance of the centers of the two ftars. The 
fcale of the micrometer at this very convenient diftance, with 
the power of 460 (which my telefcope bears fo well upon the 
fixed ftars that for near a twelVe-fnonth paft 1 have hardly ufed 
any other) is above a quarter of an inch to a fecond ; and by 
putting; on my power of 022, which in very fine evenings is 
1 extremely 
