34 T he Magnifying Power of Glajjes . 
To go one Step farther: If a convex Glafs 
be fo fmall, that its Focus is no more than 
one twentieth of an Inch diftant; we fhall 
find that eight Inches, the common Dif- 
tance of Sight, contains an hundred and fix- 
ty of thefe twentieth Parts ; and, in Confe- 
quence, the Length and Breadth of an Ob¬ 
ject, when feen through fuch Lens, will each 
be magnified an hundred and fixty Times ; 
which multiplied by an hundred and fixty, 
to give the Square, will amount to twenty- 
five thoufand fix hundred; and fo many 
Times, it is plain, the Superficies of the Ob¬ 
ject mult appear larger than it does to the 
naked Eye at the Diftaftce of eight Inches. 
Therefore, in a fingle Microfcope, to 
learn the magnifying Power of any Glafs, 
no more is necefiary than to bring it to its 
true Focus; the ex ad Place whereof will 
be known, by an Objed’s appearing perfed- 
]y diftind and fliarp when placed there. 
Then, with a Pair of fmall Compafies, mea- 
fure, as nearly as you can, the Diftance from 
the Center of the Glafs to the Objed you 
was viewing, and afterwards applying the 
Compafies to any Ruler with a diagonal 
Scale of the Parts of an Inch marked on it, 
you will eafily find how many Parts of an 
Inch the faid Diftance is : When that is 
known, compute how many Times thofe 
Parts of an Inch are contained in eight 
Inches, the common Standard of Sight, and 
8 that 
