58 PREPARATION AND MOUNTING 
The method of finishing, however, which is mostly used 
at the present time, is to lay a coating of varnish upon the 
edge of the thin glass, and extend it some little way on the 
slide. When a black circle is required, nothing serves the 
purpose better than the gold-size and lampblack, or the 
asphalt and india-rubber varnish, neither of which is liable 
to chip; but when used for this, the latter should be rather 
thinner, as before advised. Some:of these varnishes are 
preferred of different colours, which may be made by using 
the different kinds of sealing-wax, as described in Chapter I1.; 
but they are always liable to the defects there mentioned. 
This circle cannot be made in any other way than by one of 
those contrivances called turntables. A very little practice 
will enable the young student to place his slide so that the 
circle may be uniform with the edge of the thin glass. 
The slide is now complete, except the addition of the 
name and any other particulars which may be desirable. 
For this purpose one of the methods described in Chapter IT. 
must be employed. 
Amongst the various classes of microscopic objects now 
receiving general attention, the Diatomaceze may be placed 
in a prominent position. They afford endless opportunities 
of research, and some very elaborate works have already 
been issued concerning them. Professor Smith’s may be 
mentioned as one containing, perhaps, the best illustrations. 
The young student may wish to know what a diatom is. 
The “Micrographic Dictionary” gives the following defi- 
nition :—“‘ A family of confervoid Alge, of very peculiar 
character, consisting of microscopic brittle organisms.” 
They are now looked upon by almost all of our scientific 
men as belonging to the vegetable kingdom, though some 
few still assign them to the animal. They are almost 
invariably so exceedingly small, that the unaided eye can 
perceive nothing on a prepared slide of these organisms but 
minute dust. Each separate portion, which is usually seen 
when mounted, is termed a “frustule,” or “testule:” this 
consists of two similar parts, composed of silica, between and 
