xMu The Mi cr of cope explained, 
C O, we muft prove the angle A C B, to be equal ta 
that of I F K. 
The optical axis F O, being perpendicular to the glafs 
and furface of the objedf, the parallel rays A I, BK, 
flowing from the points A and B in their progreffion 
towards the eye* are alfo perpendicular thereto, until 
they arrive at the lens, where they project the diameter 
of the objedt at I K : hence F C=C O and I K—A B, 
the two triangles A C B, IF K, will be fimilar, and 
thence the two angles at the points C and F, equal j 
but the vifual rays F I, F K, continued to the furface of 
the objedt D E, will form the angle D F E, equal to 
the fir ft angle ABC; there/ore, as CO: AB : : F O: 
DE, whence it is plain, that the diameter of the objedf, 
appears double the fize when feen through the lens* . 
than when it is viewed with the naked eye, and confe- 
quently quadruple in furface. 
No notice has been taken of. the double bending of 
the rays in their paftage through the lens, it being a na¬ 
tural effedt, has no weight in the demonftration, as ap¬ 
pears from the line S S, which is in the diredtion of the 
pafiage of the rays through the lens, whence it may be 
fuppofed the rays bend only on the line G H. 
On the removal of the eye from F to M, half the 
focal diftance, the apparent diameter of the object will 
only be magnified one third. 
On the contrary, if the eye be removed from F to 
N, double the focal diftance ; the diameter of the objedt 
will appear tripple': whence it is plain, that if the point 
of fight be taken at any other diftance, but the focal 
point- of the lens, the apparent magnitude will be either 
greater or lefs. 
The fimpleft method to find the magnifying power of 
^ny compound microfcope, is to have a little ebony rule, 
three or four tenths of an inch broad, about one tenth 
thick, and feven inches long; at each inch is fixed a 
piece 
