SECTION VI 



INFECTIOUS DISEASES IN WHICH THE SPECIFIC 

 CAUSE IS NOT CERTAINLY KNOWN 



CHAPTER XLV 



DISEASES PRODUCED BY ULTRA-MICROSCOPIC ORGANISMS 



WITHIN the last two decades there have been described a 

 number of diseases in which the causal organism is said to be 

 ultramicroscopic. By this is meant that with the best powers of 

 the microscope no definite organism can be distinguished. Such 

 an organism is frequently called a filterable virus, because filtrates 

 passed through fine-pored porcelain filters retain their pathogenicity 

 for susceptible animals. 



It is a principle of optics that no object can be clearly differen- 

 tiated that is smaller than one-half the wave-length of the light 

 in which it is examined. This means that there is an apparently 

 insuperable physical obstacle to the observation of some of these 

 forms, as our best lenses approach moderately near to this limit 

 in their magnification. Our reasons for believing that there are 

 organisms this small will be discussed under the heading of various 

 diseases. 



Recently there has come into use an instrument known as the 

 ultramicroscope, which renders visible objects more minute than 

 had heretofore been observed. This instrument enables one to 

 observe objects by making use of the principle worked out by 

 Tyndall in determining the absence of floating dust-particles in the 

 air. He noted that when a ray of a very bright light was admitted 

 to a darkened room or box, this ray could be distinctly seen as 

 long as there were any floating particles, but became invisible 

 when these dust-particles had completely subsided. The motes or 



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