THE WORLD OF MICROSCOPIC LIFE 55 



The study of bacteria involves the use of an 

 especially refined technique. It is a study full of 

 pitfalls, and in which many errors have been made 

 in the past. One great difficulty arises from the 

 fact that it is often very difficult to isolate par- 

 ticular kinds and to keep them free from admix- 

 ture with foreign species. Even with the most 

 careful treatment our cans of fruit and vegetables 

 sometimes spoil, and the investigator occasionally 

 finds his material contaminated after all possible 

 precautions had been taken to prevent the entrance 

 of outside germs. Perhaps the difficulty in keep- 

 ing material free from outside bacteria can best be 

 realized by watching the dust particles revealed by 

 a beam of light entering a room. For each par- 

 ticle that we can see there are many others too 

 small to be seen, and even the smallest visible par- 

 ticle would appear under a powerful microscope to 

 be quite a large object many thousand times the 

 size of a bacterium. The floating matter of the 

 air, as has been shown by the extensive studies of 

 the great English physicist, John Tyndall, is widely 

 distributed even in the air at great heights. It is 

 responsible for the blueness of the sky and the red- 

 ness of the sunset. A beam of light is visible only 

 on account of the dust particles in its course. And 



