156 PREPARATION AND MOUNTING 
tions where the sections are required to show the position of 
the different vessels, &c.; but, as before stated, is very valu- 
able for all soft substances. As an instance of this, it may 
be mentioned, that it is frequently used in cutting sections 
of sponges; but as these are often very full of spicula, 
it is much better to press the sponge flat until dry, and then 
cut off thin shavings with a very sharp. knife; these shay- 
ings will expand when placed in water. After this they 
may be laid betwixt two flat surfaces and dried, when they 
may be mounted as other dry objects, or, when desirable, in 
balsam. 
Valentin’s knife is very much used in taking sections of 
skin, which are afterwards treated with potash solution, 
acids, &c., to bring out in the best way the different por- 
tions. Dr. Lister’s mode, however, of getting these‘is thus 
given in the Microscopic Journal :—“ But I afterwards 
found that much better sections could be obtained from 
dried specimens. A portion of shaved scalp being placed 
between two thin slips of deal, a piece of string is tied 
round them so as to exercise a slight degree of compression ; 
the preparation is now laid aside for twenty-four hours, 
when it is found to be dried to an almost horny condition. 
It then adheres firmly by its lower surface to one of the 
slips, and thus it can be held securely, while extremely thin 
and equable sections are cut with great facility in any plane 
that may be desired. These sections, when moistened with 
a drop of water and treated with acetic acid, are as well 
suited for the investigation of the muscular tissue as if they 
had not been dried.” 
There are many who almost confine their attention to 
polarized light and its beautiful effects. Such would not 
deem these efforts to aid the student in cutting sections 
complete, without some notice of those which are taken 
from various crystals, in order to display that curious and 
beautiful phenomenon, the rings with a cross, The arrange- 
ment of these is somewhat changed by the crystal which 
affords the section; but nitrate of potash gives two sets of 
