GvY, on Crystals of Arsenious Acid. 



oo 



of 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 crys- 



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). 



