INVISIBLE MICROORGANISMS ' 1 19 



during filtration. The time of filtration should not exceed one hour. 

 The filtrate should remain free from all visible bacteria as shown by 

 microscopic examination and cultural tests. The filtrate should 

 possess the specific disease-producing quaUties of the unfiltered material. 

 Animals infected with the filtrate should yield material which, after 

 filtration, will in its turn possess the attributes of the original unfiltered 

 material. Recent suggestive developments have thrown some light on 

 the possible nature of filterable viruses. The reader is referred to the 

 work of Flexner and Noguchi since 1912, published in the Journal 

 of Experimental Medicine; he is also requested to read the article by 

 Lohnis and Smith already mentioned on page 97. 



