94 Mr, herschel on the 
know thefe centers to be but a few feconds diftant, it is plain 
how great an advantage we gain by fuch an enlarged fcale. 
Thefe experiments have but very lately pointed out to me a 
method of making a new micrometer, upon a conftrudtioja 
entirely different from any that are now in ufe, which I have 
been fuccefsful enough to put in pradtice, and by which I have 
already begun to determine the diftance of the centers of fome 
of the moft remarkable double bars to a very great degree of 
accuracy *. 
The powers that may be ufed upon various double ftars are 
different, according to their relative magnitudes : e Bootis, for 
inftance, will not bear the fame power as a. Geminorum, nor 
would it be difficult to affign a reafbn for it ; but as I here fhaii 
merely confine mylelf to facts, it will be fufficient in general 
to mention, that two ffars, which are equal, or nearly fo, 
will bear a very high power : with a. Geminorum I have gone 
as far as 3168 ; but with the former only to 2010. The diffi- 
culty of ufing high powers is exceedingly great ; for the field 
of view takes in lefs than the diameter of the hair or wire in 
the finder, and the effedt of the earth’s diurnal motion is fo 
great, that it requires a great deal of pradtice to find the 
object, and manage the inffrument. It appears to me very 
probable, that the diurnal motion of the earth will be the 
greateff obftacle to our progrefs in magnifying, except we can 
introduce a proper mechanifm to carry our telelcopes in a con- 
trary motion. 
Notwithftanding opticians have proved that tw T o eye-glafles 
will give a more correct image than one, I have always (from 
experience) perlifted in refuting the affiftance of a fecond glafs, 
which is lure to introduce errors greater than thofe we would cor- 
rect. Let us refign the double eye-glafs to thole who view objects 
* For a defeription of this micrometer fee a fubfequent paper. 
merely 
