88 The Electron Microscope 



slight disadvantage is however a very great advantage in the 

 taking of electron photographs, as no extreme accuracy is re- 

 quired in focusing. 



Whereas in bacteriology proper the electron microscope could 

 add relatively little to the vast body of knowledge accumulated 

 with the optical microscope, viruses and other submicroscopic 

 organisms presented it with a magnificent field for discovery. 

 One of its most important successes was the identification of 

 the influenza virus. "^^ But as the influenza virus is, even in elec- 

 tron micrographs, a rather unimpressive-looking particle, it ap- 

 pears preferable to select as an example a discovery which is 

 spectacular as well as important. 



Bacterial cultures are sometimes attacked by mysterious 

 epidemics. F. \V. Twort, in 1915, and F. d'Herelle, in 1918, 

 were the first to investigate them. D'Herelle gave the name 

 bacteriophage to the cause of the epidemics, and maintained that 

 the phage is an ultramicroscopic living organism, parasitic in 

 bacteria and reproducing itself. Direct proof was lacking until, 

 in 1940-42, Pfankuch and Kausche, Ruska, and Luria and 

 Anderson discovered and identified them with the electron 

 microscope.^^ Figure 29 and figure 30 are reproductions of two 

 brilliant photographs obtained by Luria and Anderson with the 

 R.C.A., Type B instrument. Figure 29 was taken in a culture 

 of Escherichia coli which was dried on to a thin membrane 

 5 minutes after it was injected with a drop of dilute suspension 

 of the Bacteriophage anti-coli PC. The drying process has caught 

 the phages which look remarkably like tadpoles vigorously 

 swimming toward the bacteria. A few have already managed to 

 bury their heads in them. The result of their activity is shown 

 in the second picture, which was taken after the coli bacillus 

 was exposed to the phage for 30 minutes. It shows destruction 

 so complete that the phages themselves cannot be detected. 

 There is ample evidence that, far from being destroyed, they have 

 multiplied their numbers by a factor of about a hundred while 

 feeding on the bacteria, which corresponds to a generation 

 lasting only 4 minutes. It is very striking that the phage which 

 seems to have so many of the attributes of animals contains 



