OBJECTIVES AND OCULARS 
7 
low powers, the chemist will find an objective of this sort an 
exceedingly great convenience. Fig. i shows a variable objec¬ 
tive as manufactured by Zeiss. Its range 
of magnification lies between 29 and 43 
diameters and its free working distance 
between the limits 53 millimeters and 13 
millimeters. To obtain a similar range with 
non-variable objectives requires four or five. 
Variable objectives do satisfactory work 
and are relatively inexpensive.^ 
A measure of the quality of an objective 
lies in its ability to make clear any fine 
and delicate details of structure. It is, 
therefore, customary to speak of the resolv¬ 
ing power of objectives and express this attribute in terms of 
the number of fine lines per unit length the different objectives 
will render distinctly visible, or, in other words, the resolving 
power of an objective can be defined as the minimum distance 
apart two lines or spots may be and yet appear as two distinct 
individuals. The resolving power of an objective is dependent 
upon its fight-collecting and light-transmitting power; this in 
turn is governed by the numerical aperture and by the particular 
wave-length of fight entering the lens system. 
In general it may be stated that in properly corrected objec¬ 
tives the resolving power is directly proportional to the numer¬ 
ical aperture. This is based upon the assumption that the 
illuminating cone of light completely Jills the aperture of the 
objective. In the case of ordinary objectives we find that, 
theoretically, the limit of resolution will be attained when the 
magnification of an objective reaches about 900 when using 
white fight. 
The chemist is not alone interested in the brightness of the 
image and in the resolving power of an objective, but he is vitally 
concerned with another property, namely, the ability of the 
> An excellent variable objective of great penetrating power is made by the 
Spencer Lens Co. of Buffalo, N. Y. The magnification of these objectives ranges 
between 5 and 20 diameters. 
