MICROSCOPY 



95 



distinguish or resolve small objects is much more important. Further 

 magnification does not necessarily improve the ability to see details; only 

 an increase in resolving power can do this. 



The limit of resolution of any optical system is defined as the ability 

 to separate adjacent objects. Under a microscope two objects or lines can 

 be seen as separate and distinct. As two lines are moved closer together, 

 they eventually seem to merge into a single line. The distance between 

 the lines just before they can no longer be distinguished from each other 

 is the limit of resolution. This limit, also called resolving power, for the 

 human eye is about 0.1 mm. Since this is about the maximum size of 

 ordinary cells, the microscope becomes very necessary in biology. 



Fig. 8-9. In the figures, a is the angle between the axial 

 ray and the first diffracted ray. On the left, axial lighting 

 is used and the lens would capture the blue, but not the 

 red diffracted light. Oblique illumination (right) in- 

 creases the effective angle through which the lens gathers 

 light. 



The resolving power of a lens system depends upon the angle through 

 which it can gather light. Figure 8-8 also shows a pair of objective lens 

 systems. One has a long working distance and gathers light through a 

 small angle. The other gathers light over a wider angle and would be ex- 

 pected to resolve smaller objects. The angle has been called the angular 

 aperture (A. A.), and its size can vary from a few degrees to a theoretical 

 maximum of 180°. The refractive indexes of the lens, of the slide and 

 cover glass, and of the substance between the cover glass and the lens all 

 influence the resolving power, however, so that angular aperture alone is 

 not an adequate expression of the ability to resolve. Another expression, 

 the numerical aperture (N. A.), more nearly indicates the resolving 

 ability. 



N.A. ^wXsin^A.A. 



