1 60 Mr . H£rschel*s Catalogue 
one Tingle degree of difference in their magnitudes. See fig. 6. 
for a reprefentation of thofe ftars with my power of 460. 
I do not mean to depreciate Mr. mayer’s method, the excel- 
lence of which is well known ; and with Tome ftars of my 
third, all thofe of the fourth, fifth, and fixth claffes, as well 
as with thofe ftill farther diftant, to which he has applied 
it with admirable Ikill, and “ magno labore, mnltifque 
“ no&urnis vigillis” (as he very juftly exprefles himfelf) a 
better can hardly be wifhed for ; but with ftars of the fecond 
clafs which generally differ no more than one, two or three- 
tenths of a fecond of time in A , and can never differ more 
than four tenths, the infufficiency of meafuring by time is 
obvious. In regard to the declination, it is all'o no lefs evident, 
that it is much more accurate to take an angle, which may be 
had true to 2 or 3 0 at moft, than to meafure its tangent, which 
in ftars of the fecoiid clafs is generally no more than 2, 3, or 
4" of a degree, and can never exceed five. I do not To much 
as mention the ftars of the firft clafs : they muft certainly, as 
to fenfe, pafs the meridian at the fame inftant of time. Their 
diftance has even eluded the attacks of my fmalleft (ilk-thread 
micrometer armed with an excellent power of 460 ; but I fhall 
Toon apply my laft new inftrument to them *, not without hopes 
of fuccefs. Now, though I have hitherto not been able to 
exprefs the diftance of the ftars of the firft clafs, otherwife 
than by the proportion it bears to their apparent diameters, I 
think it a very great point gained, that one of my inftruments at 
leaft ( viz. the crofs-hair micrometer) has laid hold of them : 
for their angle of pofition, I think, is within a very fmall quan- 
tity as well determined as it is in thofe of the fecond clafs. 
This fimple hut moft ufefui inftrument can, by actual meafure, 
* For a ejefeription of which fee p. 163. 
difeover 
