408 JAMES HILLIER 



in the electron microscope image vary greatly with focus, and in so 

 doing change the apparent size of small particles appreciably (6). 



Other Information in the Image. At present most research 

 with the electron microscope depends on its use as an aid to our 

 vision. In other words, an experiment is performed and the result 

 carefully but qualitatively observed with the help of the electron 

 microscope. An obvious example of such an experiment is the fol- 

 lowing of the growth of bacteria. The method used may be a time 

 sampling of a large number of preparations in which the organisms 

 are made to grow directly on the supporting membrane (7). In the 

 electron microscope the obvious development of the colony can then 

 be followed, also the time interval of the observations can be narrowed 

 down so that the morphological changes that occur during the life 

 cycle of the individual cells can also be observed. Once the investi- 

 gator has become familiar w'ith these changes he can then extend his 

 experiment further; for instance, by treating the cells of a certain age 

 with different chemicals in an effort to observe (jualitatively the effect 

 on the structure in an indirect effort to determine the chemical changes 

 actually going on in the life process. 



It is obvious from this rather brief example that the information 

 obtained will be intelligible if the experiments are properly designed 

 but will be in a somewhat intangible form that defies simple char- 

 acterization. The realization that the electron microscope is merely 

 a device to permit the brain to obtain visual information from the 

 microscopic world is probably the biggest step in the education of an 

 electron microscopist. Once this realization has come to the in- 

 vestigator, he will no longer have diflficulty dismissing the complex- 

 ities of the electron microscope from his own problem. 



D. LIMITATIONS OF ELECTRON MICROSCOP\ 



1. Instrumental Limitations 



The electron microscope has two fundamental limitations: 

 resolving power and image contrast. The first of these, i.e., the 

 limiting resolving power of the electron microscope, does not need to 

 be considered in detail in this chapter since it has only a slight bearing 

 on the results obtainable with the electron microscope in biological 

 problems. The latter, on the other hand, is quite important since 

 it is one of the most important instrumental factors determining 



