vtiwed.by the Microfiope . 47 
to him by Dr. Jtjrin. Vide Philofophical 
P’ranfactions, Numb. 377. 
Mr. Martin, in his Optics*, gives ano¬ 
ther Way, fufficiently eafy and ready on, all 
Occafions. On a circular Piece of Glafs, 
let a Number of parallel Lines be drawn 
carefully, with the fine Point of a Diamond, 
at the Diftance of one fortieth Part of an 
Inch from each other. If this be placed in 
the Focus of the Eye-glafs of a Microfcope,' 
the Image of the Objedt will be feen upon 
thefe Lines, and the Parts thereof may be 
compared with the Intervals, whereby its 
true Magnitude, or Dimenfions, may be very 
nearly known : for the Intervals of thefe 
Lines, tho’ fcarce difcernible to the naked 
Eye, appear very large through the Micro¬ 
fcope. A Contrivance of this Kind may alfo 
be invented for fuch Microfcopes as a Glafs 
cannot be applied to in the above Manner, 
by placing it under or behind the Objedt, 
which will anfwer the fame Purpofe. 
Here it will be eafy to find what Pro¬ 
portion an Objedt, or any Part thereof, bears 
to an Interval between two Lines, and then, 
determine it in Parts of an Inch : for if the 
Width of an Objedt appears juft one Inter¬ 
val, we fhall know it to be juft one fortieth 
Part of an Inch; if half an Interval, the 
eightieth; if a Quarter of an Interval, the 
one hundred and fixtieth ; if one fifth, only 
the two hundredth Part of an Inch. 
Vol. I. E 
Dr, 
