BLACK BACKGROUNDS. 
Ill 
The best method for obtaining this result is to use 
one of the opaque centre stops (fig. No. 18), and the 
line to follow is to use the smallest of these that will 
give the object well lighted and clearly defined with the 
background quite dark. It will be found difficult to 
work this system with the high medium powers, but we 
are told that even the 3 mm. apochromatic glass has 
been so used. If with any objective it is found that 
the dark ground cannot be obtained by the procedure 
here given, the plan to follow will be to fix into the 
back of the objective a stop, cutting off part of the 
effective area of the back combination, or an iris may 
be fitted to several objectives. Thus we failed with the 
12 mm. apochromatic to get the black ground, but on 
procuring an iris stop for the back of the objective, as 
suggested, we overcame the trouble. 
The method of procedure is as follows : With one of 
the lower powers, as one inch, or a higher power with 
the stop, focus objective and condenser, using the 
paralleliser as usual ; then insert into the condenser 
the smallest “spot” stop. By racking the condenser, 
try to get the black ground; if this does not succeed, 
put in the next larger stop, and so on till the object is 
sharply and brightly lighted, and the ground quite 
black. The exposure will, of course, require to be long, 
and development will be puzzling till experience has 
been gained. It is a mistake to develop to great 
density; and, of course, the background must not be 
allowed to show development to any marked extent. 
Slight “ greying” under development will “ fix out.” 
