30 PREPARATION AND MOUNTING 
very small objects the grain which all such paper has 
when magnified detracts a little from the merit of this 
background; and lately I have used a small piece of thin 
glass covered on the back with black varnish, and placed the 
object upon the smooth untouched side; but a solution of 
the best Egyptian asphalt in benzole of moderate thick- 
ness may be painted on with this further advantage, 
that im mounting such opaque objects as foraminifera, &c., 
it will be sufficient to arrange them in the positions they 
are to occupy, when by slightly warming the slide they 
will adhere to the asphalt. 
Another method of making these cells is as follows :— 
Two punches, similar to those used for cutting gun-wads, 
are procured, of such sizes that with the smaller may be cut 
out the centre of the larger, leaving a ring whose side is not 
less than an one-eighth of an inch wide. These rings may 
be readily made, the only difficulty being to keep the sides 
parallel; but a little care will make this easy enough. For 
this purpose close-grained cardboard may be conveniently 
used. It must have a well-glazed surface, else the varnish 
or cement used in affixing the thin glass cover sinks into the 
substance, and the adherence is very imperfect. When this 
takes place it is easily remedied by brushing over the sur- 
face of the cardboard a strong solution of gum or isin- 
glass; and this application, perhaps, closes also the pores 
of the card, and so serves a double purpose. But, of 
course, the gum must be perfectly dried before the ring is 
used. 
For cardboard, gutta-percha has been substituted, but 
cannot be recommended, as it always become brittle after, 
a certain time, never adheres to the glass with the required 
firmness, and its shape is altered when worked with even a 
little heat. Leather is often used, and is very convenient ; 
it should be chosen, however, of a close texture, and free 
from oil, grease, and all those substances which are laid 
upon it by the dressers. 
Rings of cardboard, &c., have been rejected by persons of 
