ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
413 
But 
For the eye-piece 
O e + T 
m 
7 -> 
w = g+i, 
where D is the distance of distinct vision 
,\ M = mm' = 
(D +F) (T -/) 
r / 
The equivalent focal length of the eye-piece can be easily calculated, if 
the focal length of the eye-lens is known. Thus by the formula for the 
combination of two lenses 
j, - /, + f T f , f „ 
where f'f" are the focal length of eye-lens and field-lens respectively, 
and d is the distance between them. 
But in the case of a properly constructed negative eye-piece f = 3 f 
2 
and d = 2/, so that /' = - F. 
o 
To determine /' the magnification of the eye-lens can be measured 
by the use of the camera lucid a. A micrometer is placed at the 
diaphragm of the eye-piece, and the Microscope is inclined until the 
optical centre of the eye-lens is 250 mm. (distance of distinct vision) 
above the paper, on which the camera lucida projects the figures of the 
micrometer. The magnification m ' is thus directly determined and f 
given by the formula 
, B 
m ~ 7 + 
Fig. 50 explains tbe theory of the negative eye-piece, and shows how 
Fig. 50 
^he effect of the field-lens is to diminish the magnification by two-thirds. 
The rays rr converging from the objective to the point Q, are made 
1891. 2 f 
