XI. WHEN TO USE SPECIAL MICROSCOPES 347 



cal aperture of the objective used is de.siral)le. Much, greater mag- 

 nification fails to reveal new detail and is referred to as empty magni- 

 fication. Since the brightness of the image varies directly as the 

 square of the numerical aperture but inversely as the square of the 

 magnification, i.e., as (N.A.)VM^ too great magnification may give 

 so little light as to make the image scar(!ely visible. The same rule is 

 useful for photomicrography, although the necessary magnification 

 will depend on the resolution of the emulsion and the viewing distance 

 at which the print will be examined (7). Some excess or empty 

 magnification may be helpful in measurement. 



Monochromatic light passing through the edge of a simple magni- 

 fying lens is focused closer to the lens than that passing through the 

 central part of the lens. This variation from a single focal point is 

 called spherical aberration. Chromatic aberration arises when long 

 wavelength red light comes to a focus farther from the lens than 

 shorter wavelength blue light. To correct these and other aberra- 

 tions (coma, astigmatism, curvature of field, and distortion) positive 

 and negative lenses of various kinds of glass or crystalline materials 

 are combined. Since available optical materials do not permit per- 

 fect simultaneous correction of all aberrations, each manufacturer 

 must list many types of lenses. Each type is designed to be the best 

 possible compromise for some specific application. To be capable, 

 a microscopist must know which lenses are best for a given problem. 



WTien a microscope is focused on a reasonably transparent speci- 

 men, the object is not all seen at once, but only section by section be- 

 cause of the limited depth of field of the objective. The structure of 

 the specimen in depth is comprehended by slowly focusing up and 

 down throughout its depth. Lenses of lower numerical aperture 

 have greater depth of field. Lenses of great numerical aperture have 

 very little depth of field. The latter are useful for optical sectioning 

 (17). 



Few specimens occur naturally in suitable form for examination 

 with the compound microscope. They must be thin enough to 

 transmit light or reflect enough light to be seen, and of suitable shape 

 for manipulation. Many methods have been developed for the prep- 

 aration of the specimen, e.g., sectioning, staining, isolating, and 

 orienting the specimen. Some specimens require high skill and special 

 equipment. Many books give detailed instructions and should be 

 consulted (6,9) . Unless the specimen has been properly prepared, its 

 examination with a microscope is likely to be useless. 



