54 PREPARATION AND MOUNTING 
means of a thick varnish ard camel-hair pencil, to form a 
ring of the desired depth; but should the varnish not be of 
sufficient substance to give such “walls” at once, the first 
application may be allowed to dry, and a second made upon 
it. A number of these may be prepared at the same time, 
and laid by for use. When liquids are used (see Chapter V.), 
' Dr. Carpenter recommends gold-size as a_good varnish 
for the purpose, and this may be used in dry mountings 
also. I have used the asphaltum and india-rubber (men- 
tioned in Chapter IT.), and found it to be everything I could 
wish. The cells, however, must be thoroughly dry, and when 
they will bear the heat they should be baked for an hour at 
least in a tolerably cool oven, by which treatment the latter 
becomes an excellent medium. All dry objects which will 
not bear pressure must be firmly fastened to the slide, 
otherwise the necessary movements often injure them, 
by destroying the fine hairs, &c. For-this purpose thin 
varnishes are often used, and will serve well enough for 
large objects, but many smaller ones are lost by adopting 
this plan, as for a time, which may be deemed long enough 
to harden the varnish, they exhibit no defect, but in a while 
a “wall” of the plastic gum gathers around them, which 
refracts the light, and thus leads the student to false 
conclusions. In all finer work, where it is necessary to 
use any method of fixing them to the slide, a solution of 
common gum, with the addition of a few drops of glycerine 
(Chapter IT.), will be found to serve the purpose perfectly. 
It must, however, be carefully filtered through blotting- 
paper, otherwise the minute particles in the solution interfere 
with the object, giving the slide a dusty appearance when 
under the microscope. 
When mounting an object in any of these cells, the glass 
must be thoroughly cleaned, which may be done with a 
cambric handkerchief, after the washing mentioned in Chap- 
ter II. If the object be large, the point of a fine camel-hair 
pencil should be dipped into the gum solution, and a minute 
quantity of the liquid deposited in the cell where the object 
