Of Microscopes in General. 5 
the laft is a convex Glafs, to fupply the 
Want of Convexity in the Eye itfelf, and 
bring the Rays to a Ihorter Focus ; whereas 
a concave Glafs is needful for the firft, to 
fcatter the Rays, and prevent their coming 
to a Point too foon. 
Nothing is morecommon than to obferve 
old People holding Objects they would exa¬ 
mine at a great Diftance from them, for 
the Reafon above-mentioned; and every 
body knows, Ihort-fighted People cannot 
diftinguilh any thing without bringing it 
very near their Eyes. Both Extremes are 
inconvenient; but thofe whofe Eyes are flat 
by Age Ihould remember with Satisfaction, 
that they have enjoyed the Pleafure of them 
for many Years ; and the Short-lighted may 
comfort themfelves, that they can diftin- 
guilh much fmaller Objects than long- 
lighted People: for the Qbjedt is magnified 
in Proportion to the Roundnefs of the Eye 
and the Nearnefs of the Focus, and confe- 
quently appears four times as big to an Eye 
whofe Focus is but four Inches off, as it 
does to one whofe focal Diftance is at eight 
Inches, They have alfo this farther Advan¬ 
tage, that Age improves their Eyes, by the 
fame Means it impairs other People’s, that 
is, by making them more flat. 
The nearer any Objedt can be brought to 
the Eye, the larger will be the Angle under 
which it appears, and the more it will be 
B 3 mag-. 
