DISEASES PRODUCED BY ULTRA-MICROSCOPIC ORGANISMS 477 



particles, even when smaller than can usually be seen by the un- 

 aided eye, became visible when thus illuminated. This principle is 

 applied to microscopic examination by sending the rays from an 

 arc light or similar source, so that they are concentrated in a 

 powerful beam, which is passed through the hanging drop or similar 

 preparation from side to side. Exceedingly minute particles may 

 thus be made visible. The use of this instrument has been found 

 to be less helpful in the fields of biological research than had been 

 hoped. Very few facts concerning the organisms that cause 

 disease, and particularly the ultra-microscopic organisms, have 

 been discovered by its aid. 



An attempt has sometimes been made to compare the relative 

 size of ultramicroscopic organisms by the use of porcelain filters 

 of different degrees of density. It has been found that the virus 

 of some diseases will pass through coarse filters, but not through 

 the finer ones. It does not seem to be entirely a matter of the 

 relative size of pores and organisms that may pass through the 

 filter. It is probably a phenomenon analogous to an adsorption 

 quite as much as mechanical filtration that removes the organ- 

 isms. 



Bacterial or Protozoan Relationships of Ultramicroscopic 

 Organisms. There is no practicable method telling certainly 

 whether or not an ultramicroscopic virus should be grouped with 

 the protozoa or with the bacteria. There are methods which may 

 sometimes be used that will give inferences, however. It has been 

 found possible in some cases to secure growth in culture-media, 

 such as is used for bacteria. Such organisms are probably bac- 

 teria. In others, the type of disease produced may resemble so 

 closely some other infection produced by a known organism that a 

 probable classification into protozoan or bacterial might be made. 

 The type of immunity developed may likewise be of importance. 

 In most instances these differences are not pronounced enough to 

 allow of certainty in the classification. 



The more important diseases which have been described as 

 due to ultramicroscopic organisms are contagious pleuropneumonia 

 of cattle, rinderpest or cattle plague, foot-and-mouth disease, hog 

 cholera, horse sickness, dog distemper, fowl plague, equine anemia, 

 fowl pox, yellow fever, and epidemic infantile paralysis. 



