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. 1 



A measure of the quality of an objective 

 lies in its ability to make clear any fine 



and delicate details of structure. It is, FlG ' * nk Zeiss Variable 



Objective. 



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 light-collecting and light-transmitting power; this in 

 turn is governed by the numerical aperture and by the particular 

 wave-length of light 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 fills 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 light. 



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. 



