OF MICROSCOPIC OBJECTS. 29 
having first slightly damped the ends of the fingers employed 
to obtain firm hold. When the dirt is very obstinate, 
breathing upon the glass greatly facilitates its removal, and 
the sense of touch becomes so delicate that the breakage is 
inconsiderable; but this method cannot be recommended to 
novices, as nothing but time spent in delicate manipulation 
can give the sensitiveness required. 
It has been before mentioned that ordinary glass sides are 
sometimes worthless, especially for fine objects, from having 
a rough surface, which presents a dusty appearance under 
the microscope. This imperfection exists in some thin glass 
also, and is irremediable; so that it is useless to attempt to 
cleanse it; nevertheless, care should be taken not to mistake 
dirt for this roughness, lest good glass be laid aside for a 
fault which does not really belong to it. 
When any object which it is desired to mount is of con- 
siderable thickness, or will not bear pressure, it is evident 
tkat a wall must be raised around it to support the thin 
glass—this is usually termed a “cell.” There are various 
descriptions of these, according to the class of objects they 
are required to protect; and here may be given a description 
of those which are most generally used in mounting “ dry” 
objects, leaving those required for the preservation of liquids 
until we come to the consideration of that mode of mounting. 
Many have used the following slides. Two pieces of hard 
wood of the usual size (3 in. by 1 in.), not exceeding one- 
sixteenth of an inch in thickness, are taken, and a hole is 
then drilled in the middle of one of these of the size required. 
The two pieces are then united by glue or other cement, 
and left under pressure until thoroughly dry, when the cell 
is fit for use. Others substitute cardboard for the lower 
piece of wood, which is less tedious, and is strong enough 
for every purpose. This class of “cell” is, of course, fitted 
for opaque objects only where no light is required from 
below ; and as almost all such are better seen when on a dark 
background, it is usual to fix a small piece of black paper 
at the bottom of the cell upon which to place them. For 
