4 Of Microscopes in General. 
Center, in an Angle proportionable to the 
Convexity of the Glafs, and meet in a Point, 
at a greater or lefs Diflance from the Glafs, 
as it is lefs or more convex. This Point, 
where the Rays meet, is called the Focus ; 
and this Focus is nearer or farther off, ac¬ 
cording to the Convexity of the Glafs: for 
as a little Convexity throws it to a confi- 
derable Diflance, when the Convexity is 
much, the Focus is very near. Its magni¬ 
fying Power is alfo in the fame Proportion 
to the Convexity: for as a flat Glafs mag¬ 
nifies fcarce at all, the lefs a Glafs departs 
fromFlatnefs, the lefs ofcourfe it magnifies; 
and the more it approaches towards a glo¬ 
bular Figure, the nearer its Focus is, and 
the more its magnifying Power. 
People's different Length of Sight depends 
on the fame Principle, and arifes from a 
more or lefs Convexity of the Cornea and 
Cryjlalline Humour of the Eye : the rounder 
thefe are, the nearer will the Focus or Point 
of ?neeting Rays be, and the nearer an Ob- 
jedl mufl be brought to fee it well. The 
Cafe of fhort-fighted People is only an Over- 
roundnefs of the Eye, which makes a very 
near Focus; and that of old People is a 
Sinking or Flattening of the Eye, whereby 
the Focus is thrown to a great Diflance: 
fo that the former may properly be called 
Eyes of too jfhort, and the latter. Eyes of too 
long a Focus. Hence too the Remedy for 
the 
