CHAPTER X. 
BLACK BACKGROUNDS. REFLECTORS. 
THE LIEBERKUHN. THE POLARISER. 
HE illumination known as “black background” is 
* by no means so much appreciated or used as in 
our opinion it deserves to be. Not only is this illumin¬ 
ation a fine test for the qualities of objectives, as pointed 
out by Mr. Nelson, but there are not a few subjects 
besides diatoms which are shown in a highly beautiful 
and valuable manner by this treatment. We find its 
advantage when dealing with physiological and other 
objects stained by one of the “ metallic ” methods—- 
for instance, “ endothelial ” tissues, such as mesentery 
stained with silver nitrate ; nerves, and Pacini’s bodies 
with gold; and muscular and nervous tissues mounted 
au naturel; sections of unsoftened bone, and various 
other objects. Many low forms of animal life and 
microscopical plants may also be photographed on a 
black ground. It will, however, be found difficult to 
treat in this way tissues stained in the ordinary styles, 
especially if they are multiple-stained; but in such a 
case we cannot expect to see more with a black back¬ 
ground than by the usual method. 
