SELECTION, ETC., OF OBJECTS FOR PHOTOGRAPHING. 95 
index of this substance is nearly the same as that 
of the blood serum in which the organism is 
suspended. This substance is deeply stained by 
iodine, and the result is that in the photograph 
it appears as a well-marked aureole surrounding 
the Micrococci, and separating them from the 
dried blood serum which forms the background of 
the field. 
The method of mounting bacterial organisms 
in general, for the purpose of photographing them, 
is essentially that employed in the instance given. 
A drop of the fluid containing them is spread out 
upon a very thin and perfectly clean glass cover. 
This is allowed to dry, and the Bacteria are thus 
attached to the cover in a very thin and tolerably 
uniform layer. 
The am of the operator in preparing unicellular 
organisms or vegetable and animal tissues for photog- 
raphy, should ahvays be to secure a single layer of cells ; 
for when the cells are piled upon each other, 
those in the background are necessarily out of 
focus, and interfere with the beauty of the 
picture. 
The aniline colors are especially adapted for 
staining the Bacteria. Of these the methyl violet 
and aniline brown are the most generally useful. 
Other colors are useful for the differentiation 
of certain species, e. g., the bacillus of tuber- 
culosis described by Koch. <A good solution of 
methyl violet for the purpose is found in the 
violet ink to be purchased for a trifle from 
almost any stationer. This should be filtered 
