THE PATHOGENIC PROTOZOA 151 



The study of these viruses is far from being advanced ; one 

 only, that of cattle pleuropneumonia, has been seen (and even its 

 form is subject to discussion), obtained in pure cultures and treated 

 like an ordinary bacterium. In vaccinia, small-pox, sheep-pox, 

 hydrophobia, trachoma and molluscum contagiosum, microbes 

 have been described but they are still hypothetical ; the proof 

 is still to seek. 



The expression, " invisible microbes," originally employed to 

 describe these, has been abandoned as inexact, and they have 

 been called the " so-called invisible," and later the " filtrable 

 viruses." Invisible microbes are simply microbes which have 

 not yet been seen ; for example, the syphilitic virus was classed 

 among them till the day when Schaudinn discovered the 

 Spirochaete. The classic example of pleuropneumonia shows 

 that a microbe may traverse a porcelain filter without being 

 invisible. Little vibrios and even little protozoa (Micromonas 

 Mesnili, of Borrel) have been found in water; these pass 

 through a filter and can yet be quite easily seen. A filtrable 

 microbe is not necessarily visible. 



The name of ultra-microscopic microbes is the most correct, 

 because many of these micro-organisms, too small to be seen 

 under the microscope, can be studied with the ultra-micro- 

 scope. Everyone has heard of this improvement, consisting 

 in examining the object not as lighted from below and seen 

 by transmitted light, but lighted from the side so as to appear 

 as a bright point or line on a dark field. In the observation 

 of microbes which are perfectly visible the ultra-microscope is 

 not the instrument of choice for studying the structure ; a well- 

 stained preparation is still the best. The ultra-microscope 

 furnishes to medical bacteriology above all an economy of 

 time and trouble ; it makes the finding of the microbes a 

 more rapid process, for example, when there . are very few 

 trypanosomes in the blood or spirochsetes in the fluid from a 

 lesion suspected of being syphilitic; but these are quite 

 visible microbes. 



Are the ultra-microscopic viruses always microbes ? May 

 it not be a question, at least in certain cases (as Beijerinck has 



