128 
Dr. Wollaston on the methods of 
passing through two opposite edges of the prism, in order to 
make two other wedges which are to be cut in different direc- 
tions from the two portions, and to have each the same angle 
Let one of the halves thus obtained be slit in a plane which 
meets the surface of bisection in one of the edges of the 
original prism, and consequently, in a line parallel to the 
Let the remaining half be cut by another plane not vertical, 
but inclined to the vertical plane at an angle of 20°, and meet- 
ing it in a line parallel to the base, or at right angles to the 
axis. This may be called a vertical wedge. 
We have thus three wedges cut in different directions [at 
right angles to each other, and, accordingly, having their 
axes of crystallization differently placed in each. 
In the first, or horizontal wedge, the axis is at right angles 
to the first surface. In the second, or lateral wedge, the axis 
is parallel in the first surface, and parallel to its acute edge. 
In the third, or vertical wedge, the axis is also in the first 
surface, but it is at right angles to the acute edge. 
An object seen through the first wedge in the direction of 
the axis, does not appear double; but, since rays transmitted 
through the second or third, pass at right angles to the axis, 
both of these wedges give two images of any object seen 
through them. 
There are obviously three modes in which these wedges 
may be combined in pairs, by placing two of them together 
with their acute edges in opposite directions. 
The first pair may be represented by L H ; the 
second by V H ; the third by V L. In the two first cases 
of 20 degrees. 
axis. The wedge thus formed may be called a lateral wedge. 
