EXAMPLES OF PROCEDURE. 
93 
or a well-stained preparation of bacillus anthracis- 
These may be tried with a ^in. or £in. using an ocular 
and at a magnification of 350 to 450 diameters. In 
the first place, the objective will probably have a cor¬ 
rection-collar, which must be so used as to get the best 
definition, and probably the entire condenser will be 
used. As before, if the condenser is used too wide 
open, we may, unless we are using the finest objectives, 
“ flood” the objective, and, as before, it is well to look 
down the tube and so arrange the condenser-iris or stop 
that the back of the objective is about half to two-thirds 
illuminated. The condenser can, as before, be focussed 
by using the iris or disc in the paralleliser and a low- 
power objective; without this it will be impossible to 
make sure of the condenser being in focus ; but some 
prefer to omit the paralleliser for these powers. 
The projection oculars of Zeiss have certain numbers 
on their shoulder, and the eye-glass is fitted to a small 
tube which slides in the main tube by a spiral slot. 
This is to allow of the diaphragm of the ocular being 
focussed on the plane of the sensitive plate. The 
image of this diaphragm should be projected on to the 
ground-glass of the camera, and made quite sharp, by 
moving the eye-glass in its spiral slot. The position of 
the index of the ocular being once found should be 
noted, as it will serve for all cases where the same tube- 
length and camera-stretch are used. 
To find the amount of magyiification , in diameters, of 
any combination of objective, ocular and camera-stretch. 
—If objectives and oculars were accurately designated 
