XII. ELECTRON MICROSCOPY 409 



the amount of information. the electron microscope can provide in 

 any given problem. 



Under ideal conditions, including perfect operation of the instru- 

 ment and with the most suitable specimens, present day electron 

 microscopes can be expected to yield a resolving power approaching 

 20 A. (2 myu). In a few special cases instruments have been built 

 that, under these ideal conditions, will provide resolving powers 

 approaching the theoretical limit of the lenses in use at the present 

 time — approximately 10 A. Nevertheless, the resolution actually 

 demonstrated in an electron micrograph of a biological specimen may 

 be considerably worse than this; it could be as much as 5 m/x for 

 viruses and even as much as 50 m^t for internal structure of some of 

 the larger bacteria. This is due to the fact that the observation of 

 the structure of the specimen in the electron micrograph is dependent 

 upon that structure producing differences in intensity in the image. 

 Such differences in intensity arise only when there are differences in 

 the electronic scattering power of adjacent portions of the structure 

 in the specimen. Unfortunately, all organic materials are composed 

 of light elements — mostly carbon, oxygen, and nitrogen — have ap- 

 proximately the same density and, hence, show very uniform scatter- 

 ing power for the electron beam. In a biological specimen it is pos- 

 sible to have a complex structure of different chemicals that never- 

 theless appear homogeneous in the electron microscope image. Thus, 

 very often the electron microscope investigations of biological mate- 

 rials involve the development of techniques for introducing contrast 

 in the image. For instance, as might be expected, heavy metals or 

 compounds containing heavy metals will scatter electrons consider- 

 ably more than organic materials and, hence, there exists the possi- 

 bility of using such chemicals as a means of selectively staining the 

 structures to be examined. Considerable preliminary work has been 

 done on the use of such stains (e.g., phosphotungstic acid or osmic acid 

 vapor), though the method has by no means been fully exploited as 

 yet. 



There is also an instrumental method for increasing contrast, 

 which is analogous to using a diaphragm in the objective of the light 

 microscope. It consists of inserting a small opening in the objective 

 lens of the electron microscope so that the effect of the differences 

 in the scattering powers of various portions of the specimen are en- 

 hanced. At the present time the theoretical expectations of this 

 method have been onlj' partially achieved in practice, mainly due 



