DARK BACKGROUND ILLUMINATION 95 



Bacteria may be most beautifully studied by means of the dark 

 field method of illumination in which they appear luminious 

 against a black background. An arc light and an especial sub- 

 stage condenser are necessary. By mixing a bacterial emulsion in 

 fluid, such as blood in .saliva, with a mixture of India ink and 

 water and drying it on a slide, in examination the bacteria are not 

 stained but ai e sharply defined against the black ink in a beautiful 

 way. The various spirochaetas and trypanosomes may be studied 

 in these two ways very satisfactorily. 



