GcY, on Crystals of Arsenious Acid. 55 
A the leading forms as seen under the monocular microscope 
by transmitted light, and under the binocular microscope by- 
reflected light. I also append, at the desire of the editors 
of the Journal, a short account of the best mode of obtain- 
ing the crystals of arsenious acid for microscopic examination. 
1. The entire crystal. 
a. The crystal adhering by one of its edges, and 
displaying two sides (fig. 1). 
b. The crystal adhering by one of its faces, and 
displaying three sides (fig. 2) . 
c. The crystal adhering by one of its faces, and 
so seen as to display four sides (fig. 3). 
d. The crystal adhering by a solid angle, so as 
to show four equal faces (fig. 4) . In this 
position the crystals appear by transmitted 
light as black squares. 
e. The crystal adhering by one of its faces, and 
showing the lights and shadows of the 
transparent model (fig. 5). 
2. The half- crystal. 
The regular octahedron may be divided into 
two symmetrical bodies — 
1. By a plane parallel to two faces of the crj^s- 
tal (fig. 6). 
The sections thus formed are bounded by a 
hexagon and by an equilateral triangle, 
and they have the appearance shown in 
fig. 7. 
2. By a plane passing through four edges of 
the crystal, each section being a four-side 
pyramid on a square base (fig. 8). 
3. By a plane cutting the equilateral triangular 
faces of the crystal into two equal right- 
angled triangles, each section presenting 
a rhombic face (fig. 9). 
