428 Remarks on a Micrometer, &c. 
Length or Breadth of the Objedl itfelf : and 
this laft Number I call the magnifying Power 
of the Objedi: Glafs. 
This Trial is to be made with Care once 
for all, for every different Qbjedl Glafs to be 
tafed with the Micrometer : and their dif¬ 
ferent refulting magnifying Powers, once re- 
gifiered in a Table, will be ready upon ail 
O.ccafions. For thus the Length or Breadth 
of any Image efeimated by the Micrometer, 
will when divided by the magnifying Power 
of the Objedi Glafs, exprefs the true Length 
or Breadth of the Objedi itfelf: or the Su¬ 
perficies of any Image eflimated by the ML 
crometer, in fquare Millefimals of an Inch, 
will in like Manner exprefs the true fimilar 
Superficies of the Objedi itfelf 5 if it is divided 
by the Square of the magnifying Power of 
the Object Glafs ufed. And for this Purpofe 
if may be of Ufe to fet down in another Co¬ 
lumn of the Table, the refpedtive Squares 
of the Numbers exprefling the magnify¬ 
ing Powers of the feveral different Objedi 
Glaffes. 
As fome Difficulty may be found, in ap¬ 
plying the diagonal Scale abovementioned to 
the larger Magnifiers : chiefly thro’ want of 
Light when the Objedt is to be brought very 
near to the Glafs ; that Defedf may be fop- 
plied by the Ufe of feme minute natural Ob- 
jpdl, whofe true Size has already been care¬ 
fully determined by one of the leifer Magni¬ 
fiers* 
