28 PREPARATION AND MOUNTING 
a dial like that of a watch, whereupon are figures at fixed 
distances. When the lever is separated from the metal 
which is stationary, the other end being connected with the 
“finger,” of the dial, that “ finger” is moved in proportion 
to the distance of the separation. The thin glass is, there- 
fore, thrust betwixt the end of the lever and fixed metal, and 
each piece is measured by the figures on the dial in stated 
and accurate degrees. This kind of apparatus, however, is 
expensive, and when not at our command, thin glass may 
be placed edgewise in the stage forceps, and measured very 
accurately with the micrometer, or by the calliper eye- 
piece described by Dr. Matthews in No. 8, for October, 1869, 
of the Journal of the Quekett Microscopical Club. 
Cleanliness with thin glass is, perhaps, more necessary 
than with the sides, especially when covering objects which 
are to be used with a high power; but it is far more difficult 
to attain, on account of the liability to breakage. The 
usual method of cleaning these covers is as follows :—T'wo 
discs of wood, about two inches in diameter, are procured, 
one side of each being perfectly flat and covered with clean 
wash-leather. ‘To the other side of these a small knob is 
firmly fixed as a handle, or where practicable, the whole 
may be made out of a solid piece. In cleaning thin glass, 
it should be placed betwixt the covered sides of the discs, 
and may then be safely rubbed with a sufficient pressure, 
and so cleaned on both sides by the leather. If, however, 
the glass be greasy, as is sometimes the case, it must be 
first washed with a strong solution of potash, infusion of 
nutgalls, or any of the commonly used grease-removing 
liquids; and with some impurities water, with the addition 
of a few drops of strong acid, will be found very useful, but 
this last is not often required. 
This method of cleaning thin glass should always be used 
by beginners; but after some experience the hand becomes 
so sensitive that the above apparatus is often dispensed 
with, and the glasses, however thin, may be safely cleaned 
betwixt the fingers and thumb with a cambric handkerchief, 
