^ournS/AlXTsea] far MicTOSCopic Examination^ 241 
The examination of rocks, especially of the more compact or ap- 
parently homogeneous ones, by transmitted light, is of much greater 
importance than when they are merely looked at superficially as 
opaque objects ; and in order to do so it becomes necessary, at least 
in the case of most rocks, to render them as translucent as possible, 
by reducing them to the form of their uniform plates or sections ; 
it is therefore proposed in this communication to give a short de- 
scription, showing how such sections can be prepared, in order to 
induce more of our fellow-microscopists to direct their attention to 
this interesting subject, and thereby to assist in advancing the know- 
ledge of the structure of minerals and rock masses. 
If a lapidary's bench is at disposal, the preparation of such 
sections is greatly facilitated ; but it is not at all imperative for the 
amateur microscopist to possess one, since with, it is true, some- 
what more labour and patience, the finest sections can be made by 
hand, not only equally well, but in the case of some of the softer 
rocks — metamorphic schists and various others, which, especially 
when making cross-sections, are liable to tear or break up under the 
rougher and less easily controllable movements of the machine — 
larger and more perfect sections can be executed by hand than on 
the lapidary's wheel. 
The first point to be attended to is to secure a proper fragment 
of the rock under consideration, which can be easily done in the 
case of massive homogeneous or crystalline rocks, by chipping olf 
a piece about from one quarter to three quarters of an inch across, 
as flat as possible, with the aid of a steel chisel and hammer. Should 
the specimen, however, be a very small, rare, or valuable one, or if 
it be required that the line of section should pass along some given 
direction not easily followed by chipping with a chisel, it is neces- 
sary to use a toothless iron saw worked with emery and water, or to 
resort to what is termed " slitting," an operation performed by any 
working lapidary (at a trifling expense per square inch of cut 
surface), by means of a thin steel disk, charged on its edge with 
diamond powder and set in rapid rotation in his lathe. 
One surface of the rough fragment or thin shce thus obtained is 
now ground down with emery and water, by holding it against the 
rapidly-rotating pewter plate of a lapidary's wheel, or, in^default of 
this, by the motion of the hand on a circular plate of cast zinc, 
iron, or stone (from 4 to 6 in. diameter, and J to 1 in. in thickness), 
using in succession various sizes of emery powder down to fine dust, 
until a perfectly flat surface is obtained ; after this, it is now again 
worked with water alone, upon a small, circular, Turkey hone, until 
the under-surface becomes beautifully smooth, free from scratches, 
and almost polished in appearance. 
Having thus obtained one finished surface, this is cemented with 
Canada balsam to a small piece of plate-glass about f in. square and 
