The Microfcope explained. xlvii 
die of this paper correfponds to the center of the lens * 
an eye-piece E muft alfo Hide upon the rule. Being 
thus prepared, go into the darkeft and moft remote part 
of a room from the window, and diredt the glafs A to¬ 
wards any remarkable objedt without doors, and move 
the Hiding piece B, until the focus is determined upon 
by the rule, by a diftindt reprefentation of the objedt j 
count the inches and parts between the edge of the Hide? 
B, and center of the lens at A, and Hide the fight E 
on the other fide of the lens to the fame diftance, this 
will give the double focus. 
Place one eye clofe to the fight at e, and with the 
other eye look at the objedt D j you will then find the 
image feen through the glafs A, and exprelfed by the 
dotted lines, to be double the breadth of the fame image 
feen at the fame time by natural vifion ; this holds good 
whatfoever the focus of the lens may be with which you 
make the experiments : the fame proportionable diftance' 
being attended to between the eye and the lens, and 
from thence to the objedt, the breadth of the objedt will 
always be double to that feen by the naked eye. 
In fig. 22. Let the angle A F B, be equal to that 
made by the naked eye without a glafs, from the rays of 
light, palling by the extremities of the objedt till they 
unite in the eye at the point F. 
The angle D F E, is equal to that which firft came 
parallel from the fides of the objedt A B, to IK, from 
whence they were refradted through the lens to the point 
F its focus. 
CO, is equal to the focal diftance of the lens on the 
fide next the objedt; cF equal thereto on the fide next 
the eye F O, the diftance between the eye and the ob¬ 
jedt. 
It is evident the naked eye would fee the objedt from 
the point C, of double the fize it would appear if viewed 
from the point F; as the diftance F O, Is twice that of 
CO, 
