26 PREPARATION AND MOUNTING 
may be employed, which thoroughly and speedily cleanses 
glass from all traces of grease or balsam. 
We have before said that any object to be viewed in the 
microscope must have its support; but if this object is to 
be preserved, care must be taken that it is defended from 
dust and other impurities. For this purpose it is necessary 
to use some transparent cover, the most usual at one time 
being a plate of mica, on account of its thinness; this sub- 
stance is now, however, never used, thin glass being substi- 
tuted, which answers admirably. Sometimes it is required 
to “ take wp” as little space as possible, owing to the short- 
ness of focus of the object-glasses. It can be procured of 
any thickness, from one-fiftieth to one-two-hundred-and- 
fiftieth of an inch. On account of its want of strength, and 
probable defect of due annealing, it is difficult to cut, as it 
is very liable to “fly” from the point of the diamond. To 
overcome this tendency as much as possible, it must be laid 
upon a thicker piece, previously made wet with water, which 
causes the thin glass to adhere more firmly, and conse- 
quently to bear the pressure required in cutting the covers. 
The process of cutting being so difficult, especially with the 
thinner kinds, little or nothing is gained by cutting those 
which can be got from the dealers, as the loss and breakage 
is necessarily greater in the hands of an amateur. It is 
convenient, however, to have on hand a few larger pieces, 
from which unusual sizes may be cut when required. 
If the pieces required are rectangular, no other apparatus 
will be required save a diamond and a flat rule; but if 
circles are wanted, a machine for that purpose should be 
used (of which no description is necessary here). There are, 
however, other contrivances which answer tolerably well. 
One method is, to cut out froma thick piece of cardboard a 
circle rather larger than the size wanted. Dr. Carpenter 
recommends metal rings with a piece of wire soldered on 
either side; and this, perhaps, is the best, as cardboard is 
apt to become rough at the edge when much used. A friend 
of mine uses thin brass plates with circles of various sizes 
