114 PRACTICAL PHOTO-MICROGRAPHY. 
under the object to be photographed. The chief 
objection to this appliance is that we require a “ Lie- 
berkiihn ” for each focal length of objective used; 
otherwise we shall not get proper illumination. The 
exposure here is not so protracted as with the two last 
illuminators. 
The “ Lieberkiihn ” fits rather loosely on to the 
Fig. 30. 
Fig. 31. 
objective, and, after the object is in focus, the “ Lieber- 
kiihn ” is moved up and down till the brightest and 
most even illumination is found. 
We give no attention to “ spot lenses,” “ parabo¬ 
loids,” and the like, believing them to be wholly inferior 
to the instruments we have noticed. 
Very useful photographs of microscopic objects have 
been produced by polarised light. A polariser and 
analyser of the usual kind are employed, choice being 
made of prisms as large as may be obtained, and in 
most cases very prolonged exposure will be required. 
We strongly recommend practice with polarised light, 
as its revelations are of great importance, and we have 
