72 
PRACTICAL PHOTO-MICROGRAPHY. 
for oil apertures—both are fairly good, and neither is 
very expensive—but we would prefer, for general work, 
the dry achromatic mentioned above. 
Various “stops” are supplied with a condenser; 
some of these may be seen in fig. i^. The stops opaque 
in the centre are for providing either a “hollow” cone of 
light, or for “ dark background ” work ; the others are 
for oblique lighting, of which more will be written 
later in this book. Moreover, there are in existence, 
though practically out of use, various kinds of illumi¬ 
nating apparatus which may come under this head of 
Condensers ; for instance, there are “ spot lenses,’ 
‘‘ parabolic reflectors ” for use above the stage, and 
others. The appliance known as a “ Lieberkiihn ” is 
interesting, and in a few cases useful, and will be 
briefly noticed later. Regarding the 
Use of the Condenser. 
Fig. 20, from Mr. Nelson, shows the illumination 
when a condenser and an objective of equal aperture 
are used together, the condenser being in focus; here 
the object lies in the plane of the conjugate foci of 
objective and condenser ; and if we look at the back of 
