sdvi The Microfcope explained. 
A method of meafuring ©bjeds that are viewed 
in compound microfcopes. 
T HE general practice being formed tinder the con- 
fideration, that a perfon of a common fight fees 
©bjeds diflind at about eight or ten inches diftance 
from his eye, let us fuppofe ten inches 3 if the fame per¬ 
fon looks upon the fame objed with a lens of one and 
i-quarter inch focus, he will fee this objed eight in¬ 
ches nearer to his eye ; and as the angle formed by the 
image on the retina, appears eight times greater, fo 
knowing the focus of any fmall lens, it will be eafy to 
find how much the appearance of the objed is magnified, 
by finding how many times its focus is contained in 
eight or ten inches, See. but a$ this method is doubtful 
in feyeral refpeds, we fhall only mention the difficulty 
of coming at the precife focus of a very fmall lens ; and 
alfo obferve, that thofe who are fhort-fighted, cannot 
comply with a bafe of eight or ten inches for their cal-» 
culations 3 and thence proceed to the following eafier 
method. 
All convex lenfes of any foCus, have the property of 
doubling the apparent diameter of an objed, and confe-\ 
quently of quadrupling the fa rface, * provided the ob¬ 
jed is at the focus of the glafs on one fide, and the eye 
in the oppofite focus on the other fide. 
Take a double convex lens of eight or fix inches fo¬ 
cus, fig. 21. and fix it at A, perpendicular to a rule 
FG, divided into inches and parts upon a Aiding piece 
at B j flick a piece of white paper as at D, two or three 
tenths of an inch broad, and three inches long, on 
which draw three black lines which fhall divide the 
breadth into four equal parts, and obferve that the mid¬ 
dle 
* Journal CEconomique pour le Mois d’Aout, 1753- 
