54 Giv, 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 rliombic dode- 

 cahedron, similar to those of tlie 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. 



