MAGNIFICATION*. 
This expression, the ratio of tangent of the angle of view to the actual size 
of the object, is a measure of the magnification V as determined by the lens 
system alone. Since X' is usually very small compared with ', the magni- 
fying power of the instrument is measured approximately by the reciprocal 
, 
In case the eye is considered part of the lens system the magnification is 
defined by the ratio of the apparent size of the object at the distance of 
distinct vision / (250 mm. or 10 inches) to that of the image at the same 
distance, or 
of the focal length, . 
y/i II I y 
The distance of distinct vision is considered to be the nearest distance to 
which the normal eye can accommodate itself and distinguish details with- 
out appreciable fatigue ; for the normal eye which is at rest when focussed 
at infinity, this distance / is 250 mm. The magnification is therefore 
N = ( iH - J or approximately N = 
FIG. 33- 
An approximate measure for the magnification is accordingly the ratio of 
the distance of distinct vision to the focal length. In the case of the micro- 
scope the focal length/' is determined by the focal length /'i of the objective, 
by that of the ocular /'s, and by the distance D, the optical interval between 
upper focal point of the objective and the lower focal point of the ocu- 
f f 
lar as expressed in equation (11) f ~ The magnification for the dis- 
tance of distinct vision / is accordingly 
f f r 4' 
J J V 
(23) 
D 
The first part of this formula, , can be considered as the magnification due 
( x l 
to the objective of the power -7- for the distance D and the second part, -7-, 
/i /* 
as that due to the ocular acting as a magnifier. 
