100 PHOTO-MICROGRAPHS. 
Occasionally a living infusorium may be quiet 
long enough to have its photograph taken; but 
usually the Infusoria are in rapid motion, and it 
will be necessary to arrest this motion by means of 
some chemical agent fatal to their vitality. A weak 
solution of iodine does this very effectually, and at 
the same time stains the protoplasm a brownish 
color. A ciliated infusorium killed by adding a 
drop of this solution (iodine 1 gr., potassic iodide 
2 gers. water 100 grs.) to a drop of the fluid in 
which it is swimming, remains for a time as if sud- 
denly frozen, with its cilia rigid, and projecting, 
like rays, from the surface of the body. This is 
the favorable time for photographing the creature, 
as, later, it is liable to undergo changes which 
destroy the internal structure. 
Another method is to place a drop of fluid con- 
taining the Infusoria in the centre of a clean glass 
slide, and to invert this over the mouth of a bottle 
containing a one-per-cent solution of osmic acid. 
A very brief time is sufficient to destroy the life 
of the Infusoria, which may then be selected 
under a low power and transferred to a drop of 
clean water. They must be mounted in the thin- 
nest possible stratum of fluid, otherwise they are 
likely to change position while the exposure is 
being made. 
As a general rule, transparent objects, like Amoebse 
and the Infusoria generally, should be mounted in an 
aqueous medium for photography, as this gives better 
photographic contrast than does a medium having 
a higher index of refraction, such as glycerine. 
