THE CONDENSER. 
73 
the objective we shall find it wholly illuminated. But 
if we cut down the aperture of the condenser by any 
means we shall get the state shown in fig. 21, where 
part of the aperture of condenser and objective is lost. 
In these figures C is the condenser, L the objective, 
O the object, and BL the back combination of the 
objective, as seen down the tube without the ocular. 
Sometimes it is found that when both objective and 
condenser are in the proper focus, the area of even 
illumination is not large enough to cover the held, and 
to overcome this some workers deliberately put the 
condenser out of focus, generally by racking it down. 
This certainly spreads the light out over the held, but 
Fig. 22. 
for two reasons the practice must be condemned. 
First, a certain amount of the aperture of the condenser 
is lost, which may or may not matter ; but, second, the 
state of affairs shown in hg. 22 is brought about, and 
this is clearly wrong, for the object no longer lies in the 
focus of the condenser. 
O is the object, C the condenser. Continuous lines 
represent the condenser in focus; dotted lines the con¬ 
denser racked downwards ; interrupted lines the con¬ 
denser racked upwards. 
