GENERAL ZOOLOGY 



Fig. 2.2. Lis;ht 

 Optical Co.) 



microscope with illuminator. (Courtesy Bausch and Lomb 



the distinctness of images seen with the compound microscope in these stained 

 cells (Fig. 2.2). 



It was found, however, that in some cases the fixation and staining produced 

 undesirable modification of cell structures as compared with living cells. On 

 the other hand, even when living cells were isolated so that adequate light 

 was transmitted for microscopic examination, it was extremely difficult to 

 distinguish the exact limits of fine structures because of the similarity of their 

 optical densities. It was like trying to see glass beads in a vial of cedar 

 wood oil. The introduction of new diaphragms and lenses into the optical 

 system of the microscope made it possible to gain sharper contrast between 

 cellular parts hitherto virtually indistinguishable. Light microscopes equipped 

 with such diaphragms and lenses are known as phase-contrast microscopes 

 and were first sold in this country about 1944. 



Even under the best conditions the smallest object that can be seen clearly 

 with the light microscope is 0.25 micron (ja) or 2500 angstrom units (A.) 

 in diameter. A micron is 0.001 mm.; an angstrom unit is 0.0001 fx . An 

 entirely different kind of microscope, known as the electron microscope, makes 

 it possible to identify objects as small as 10 A. in diameter (Figs. 2.3 and 

 2.4); this instrument was first available commercially in this country in 1941. 

 In the electron microscope, the smallest of which is many times larger than 



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