Remarks on a Micrometer , &c. 427 
the Wires; or by comparing its whole via¬ 
ble Superficies with one or more of the lit-? 
tie Squares formed by the Interfedion of 
the fame Wires. 
And the true Size of the magnified Image 
being thus known, the true Magnitude of 
the Objedt itfelf will be known alfo, if the 
magnifying Power of the Ohjedt Glafs is but 
given: and this will eaiily be found in the 
Following practical Manner : 
Let a minute Objedt of a known Size, be 
carefully viewed in the Microfcope: as for 
Example, Part of a fine diagonal Scale divid¬ 
ed into the Millefimals of an Inch; and,let it 
be examined how many of thofe Millefimals 
anfwer to the Diflance of two of the Wires' 
abovementioned, remembering to eftimate 
that Diftance from the Outfide of one of 
the Wires to the Infide of the other. For 
as 2 o are to that Number of Millefimals juft 
found, fo is the Length or Breadth of any 
Image meafured by the Micrometer to the 
Length or Breadth of the Objedt itfelf: or as 
400 are to the Square of the fame Number 
of Millefimals, fo is the magnified SuperfL 
cies of any Image, to the true fimilar Super¬ 
ficies of the Objedt. 
And again, as that Number of Millefimals 
of an Inch juft found is to 20, fo is Unity to 
a Number exprefling how many Times the 
Length or Breadth of any Image eftimated 
by the Micrometer, is greater than the true 
Length 
