SELECTION, ETC., OF OBJECTS FOR PHOTOGRAPHING. 111 
It is always desirable to obtain a field in which 
the objects do not overlie or cross each other; and 
with long plant hairs, like cotton, this is not an 
easy matter unless they are carefully arranged 
one by one. A good plan both for long plant 
hairs and animal hairs is to place several side by 
side on a dry glass slide, fixing the ends to the 
edges of the slide with sealing-wax. When they 
are adjusted in position the central portion is wet 
with alcohol, then with water, and finally with 
glycerine, if it is to be used. A thin glass cover 
is then applied. 
ANIMAL Harrs. 
A series of photo-micrographs of animal and 
vegetable hairs would be extremely interesting 
and instructive. Reagents will often be required 
to show the structure of animal hairs, which is 
not so simple as that of those from the vegetable 
kingdom. The thickness of these hairs makes it 
desirable that photo-micrographs should be made 
with low-power objectives, as these have the 
greatest penetrating power. At the same time 
good definition is required to show the outlines 
of the imbricated cells in wool, for example. 
Wool, ready dyed, of any shade required, is to 
be had by picking out a little end of colored 
worsted. 
Hints in regard to mounting have already been 
given in speaking of vegetable hairs, and the 
limits of this volume permit of little more than 
hints with reference to this part of the technique, 
