74 
PRACTICAL PPIOTO-MICROGRAPHY. 
If the condenser is racked down, as shown by the 
dotted lines, the rays cross before they reach the object; 
if it is racked up, they cross above the object, and in 
either case the field is more evenly lighted, but damage 
is done. If, when the condenser is in focus, it is found 
that the held is not evenly lighted all over, the proper 
plan is to use a longer focus condenser, or to bring the 
radiant nearer if no paralleliser is in use, or to use a 
paralleliser. 
The iris diaphragm, be it noted, though much more 
convenient to use in a condenser, is not so accurate a 
device as the ordinary plate with apertures properly cut. 
The iris aperture is not strictly circular, the “ leaves ” 
do not answer perfectly to the actuation, especially at 
the small apertures, and it is difficult to set the dia¬ 
phragm accurately to any desired aperture. 
