SELECTION, ETC., OF OBJECTS FOR PHOTOGRAPHING. 93 
The photographic method is especially adapted 
to the study of the Bacteria; and these minute 
organisms possess a peculiar interest at the present 
day, because of the important rdle which they 
have recently — within fifteen or twenty years — 
been shown to play in the economy of nature, 
as the cause of the putrefaction and fermentation 
of organic substances, and of certain infectious 
diseases, and because there is yet much to be 
learned about them. Bacteria of various forms 
and dimensions are found in vast numbers in 
animal or vegetable infusions exposed to the air; 
but im this case different species are commonly 
mingled together, and the Micrococci, especially, 
are rarely found unmixed with other forms. There 
are various methods of obtaining a “pure culture,” 
but the limits of this work oblige the writer to 
confine his remarks to that adopted in the experi- 
ment which furnished the material for the photo- 
micrograph under consideration. 
It is well known that a variety of bacterial 
organisms are constantly found in the mixed 
salivary secretions present in the healthy human 
mouth. A little saliva scraped from the surface 
of the tongue, dried upon a thin glass cover, and 
stained with a solution of aniline violet, will invari- 
ably be found to contain several different species.’ 
Among these is a micrococcus which does not 
present precisely the same appearance as that 
1 See paper by the present writer published in “Studies from the 
Biological Laboratory,’? Johns Hopkins University, Vol. Il. No. 2, 
p. 157. 
