MICROSCOPY 



97 



Since the numerical aperture is limited by the refractive index of the 

 least refractive medium in the light path, high-resolution microscopy uses 

 oil-immersion lenses. A drop of a transparent "oil" with a refractive index 

 of about 1.5 is placed between the cover glass and the objective lens 

 and usually also between the condenser and the slide (see Fig. 8-10). 

 The material to be examined is mounted between the slide and the cover 

 glass in another medium of high refractive index. The numerical aper- 

 ture is thus increased, reducing the limit of resolution. 



Fig. 8-11. A rotary microtome. (Courtesy American 

 Optical Company.) 



Optical Company.) 



An objective lens is moved upward or downward until the object is in 

 focus on the retina. A slight deviation upward or downward produces 

 an inferior image, but the eye might not notice the imperfections. Fur- 

 ther deviation, however, produces an image which is obviously out of 

 focus. The thickness of the object which seems to be in focus at any one 

 time is known as the depth of field. The greater the resolving power, the 

 thinner this depth will be. If great detail is to be seen, high numerical 

 aperture is required, but visibility of details is achieved with a sacrifice 

 of depth of field. Usually an object is examined first with a low power 

 objective (greater depth) and later with the high power objective. The 

 narrow depth of field in high power objectives is not entirely a disadvan- 

 tage because it permits "optical sections" of cells. If the center plane of 

 a cell is in focus, both the top and bottom will be invisible and thus 

 will not confuse the image of that center plane. The net result is the 



