METHODS 133 



methylene blue and stain very feebly with Loeffler's blue ap- 

 plied with heat. According to da Rocha-Lima diluted carbol 

 fuchsin and carbol thionin, as employed for bacteria, stain 

 rickettsia feebly if at all, while concentrated solutions of carbol 

 fuchsin and carbol gentian violet will stain them but not more 

 heavily than bacteria can be stained with the dilute solutions. 



With Giemsa's stain, used as described above, the small 

 forms of Rickettsia prowazeki stain clear rose red, much less 

 intensively than bacteria which (in the same preparation) 

 stain deep purple or bluish red. On the whole the staining of 

 rickettsia, in color and in intensity, is like that of spirochaetes. 

 Certain larger thread-like or filamentous forms as well as the 

 substance intermediate between the poles of small forms stain 

 a light clear blue. (Figs. 8, 9, 11, etc., plate II.) 



Rickettsia may also be demonstrated by Burri's method. 

 Nigrosin in saturated aqueous solution is more satisfactory 

 than India ink; while the use of an aqueous solution of cyano- 

 chin blue, which Weigl called to our attention, gives the com- 

 bined advantage of a dark background and a faint tinting of 

 the intermediate substance. While these modifications of 

 Burri's method have contributed no new information in our 

 study of Rickettsia prowazeki, they demonstrate that the dif- 

 ferential staining, polar bodies, and intermediate substance, 

 correspond with physical differences (Fig. 10, plate II). 



Dark field illumination with preparations from heavily 

 infected lice shows Rickettsia prowazeki as paired and single 

 ovoid bodies without motility but exhibiting molecular motion 

 to a degree shown by other particles of equal size. The dark 

 field is reliable for the identification of Rickettsia prowazeki 

 only when they are present in numbers. We employed dark 

 field illumination on nineteen occasions for the examination of 

 suspensions of louse organs, and recognized rickettsia in eleven 

 instances in material which showed the organisms in stained 

 preparations; but we failed to identify them on two occasions 

 when stained preparations revealed them. 



The refractivity of Rickettsia prowazeki seems to us to be 

 less than that of bacteria and more like that of spirochaetes. 



