54 
Guy, on Crystals of Arsenious Acid. 
servations, and inferences by which I was gradually possessed 
of my existing knowledge of these interesting crystalline forms. 
Something I learnt from actual examination ; such, for in- 
stance, as the common appearances of the perfect octahedron, 
and the fact of the existence of plates of various forms, as well 
as of crystals other than the octahedron. Something more I 
learnt byinferences drawn from the close examination of models 
of the crystal and half-crystal, opaque and transparent. I 
understood at once the twin- crystal, or made. I inferred that 
the equilateral triangle mounted on a half-triangle as its base, 
the hexagon with three-shaded points, and the triangular prism 
were merely phases of the half-crystal; and I thought it 
likely that some of the detached equilateral triangles and 
some of the hexagons might be explained in the same manner. 
But I remained quite satisfied with the belief that a con- 
siderable number of the long narrow crystals were prisms. 
I was not quite satisfied of the existence of triangular plates 
or of hexagonal plates. 1 spoke doubtfully about cubes, and 
had not been able to make out the rhombic dodecahedron ; 
and I felt that my views concerning the large part played by 
the half-crystal, though highly probable, were still only pro- 
bable. But under the binocular microscope all my doubts 
were dissipated, all my errors corrected, some surmises con- 
firmed, and most of my inferences justified. That which had 
been a work partly of observation, and partly of reasoning, be- 
came a simple matter of sensation. If there is any one who 
doubts the value of this form of the microscope, or is disposed 
to treat it simply as a philosophical toy, I will ask him to ex- 
amine these crystals with the monocular microscope by trans- 
mitted light, and with the binocular microscope by reflected 
light; and I would especially commend to his attention the 
crystalline and globular sublimate (crystals of arsenious acid, 
and globules of metallic arsenic) shown in the capillary reduc- 
tion-tube. The fine relief and perfect roundness of the tube 
and its contents is, at one and the same time, a proof of the 
utility and of the faithfulness of the binocular microscope. 
With a view to give completeness to this paper, I will first 
briefly describe and illustrate by appropriate engravings, 
corresponding with the large diagrams and models shown at 
the meeting, the various attitudes and appearances of the 
entire octahedron and of the half- crystal, as deduced from 
the study of models of wood and glass,^ and then exhibit some 
* Since the paper was read, I have added studies of the rhombic dode- 
cahedron, similar to those of the octahedron which were shown in the 
diagrams exhibited at the meeting. This addition goes far towards ex- 
hausting the crystalline forms of sublimed arsenious acid. 
