86 The Electron Microscope 



\^ 



(a) B. Subtilis 



• 



(b) Typhoid 



Fig. 28. Typhoid germ and Bacillus Subtilis 

 electron and light micrographs 



density. This for the time being is still a difficult and rather 

 uncertain process, and in the case of small objects their theory 

 requires correction for the reasons described in chapter 6. The 

 biologist will obtain in most cases sufficient information from 

 a few micrographs which show the bacilli in different positions 

 or directly from stereoscopic electron micrographs. By these 

 means it is possible to overcome the disadvantage of electron 

 micrographs which like X-ray diagrams have very great depth 

 of focus and make it difficult to locate details in depth. This 



The bacteriophages, or bacterial viruses, of which three strains have 

 been identified under the electron microscope, are the smallest known liv- 

 ing organisms. For some time it was doubtful whether they could be 

 classed as such. The strain here shown, Bacteriophage anti-coli PC, is 

 the largest of the three. The "head" has a diameter of about 800 A (0.08 

 microns), the "tail" has a length of about 1,300 A. One particle of phage 

 is sufficient to originate the lysis of a bacterial cell, and during the lysis 

 an average of about a hundred new phage particles are generated. This 

 photograph shows the phage particles swimming toward the bacteria. 

 They were immobilized by drying five minutes after a suspension of the 

 bacteria was injected with a drop of diluted phage solution. 



(By courtesy of Dr. V. K. Zworykin, R.C.A.) 



