754 SEC. 17. MINERALOGY, CRYSTALLOGRAPHY, ETC. 



3480. Photograph of the Hitter sgriin Meteorite. 



Royal Mining Academy, Freiberg, Saxony. 



3481. Wall Map of the Natural History of the Mineral 

 Kingdom, by Dr. G. Seelhorst. P. C. G-eissler, Number g. 



The wall map consists of coloured plates, which are mounted on linen, with a 

 portfolio in which to keep the map when not hanging on a wall. The text is 

 printed on both sides of the map. The map is constructed to fold up in the 

 portfolio, as well as to hang on a wall, and consists of one sheet. 



3482. Diagrams (four), or " epures," illustrative of Appli- 

 cations of Descriptive Geometry to Crystallography, viz. : 

 two illustrating derivation of holohedral and hemihedral tesseral 

 forms ; and two showing construction of crystal projections from 

 angular measurements and from crystallographic formulae. 



Prof. Jos. P. O'Reilly, Dublin. 



3483. Model, mounted with movable arcs and core, for 

 demonstration of the fundamental forms in each crystalline system, 

 and of the geometric method of derivation of the different holohedral 

 and hemihedral forms from the fundamental one in each system. 



Prof. Jos. P. O'Reilly, Dublin. 



The model possesses a movable core adapting itself to the six systems and 

 rise of elastic cords to represent the intersections and edges of forms. 



3484. Seven Wire Models representing the symmetry of 

 five of the crystallographic systems (made by Mr. Sparrow of the 

 British Museum for Prof. Maskelyne). Prof. Maskelyne, F.R.S. 



These models represent the great circles in which the sphere of projection 

 is intersected by the planes of symmetry characteristic of the different crystal- 

 line systems ; each plane of symmetry being subdivided by a network of wires 

 intersecting at distances proportionate to the parametral ratios of some crystal 

 belonging to the particular system. They are 



I. The cubic system. 



II. The hexagonal system represented 



1. By quartz. 



2. By calcite. 



3. By tourmaline. 



III. The tetragonal system represented by apophyllite. 



IV. The prismatic system represented by barytes. 



V. The oblique system. 



3485. Diagrams illustrative of the Cubic System of Crys- 

 tallography, painted to match the models. 



Rev. Nicholas Brady, M.A. 



These give the general position on the sphere of projection of all the chief 

 varieties of the species in the system. Examples of the different species, 

 both holohedral and hemihedral, in orthographic perspective, in projection 

 on the plane of the paper, and giving the position of their poles on the sphere 

 of projection, with similar drawings for twin crystals, drawn by contributor. 



3486. Series of Twelve "Water-colour Drawings, illus- 

 trative of the Optical Phenomena seen in sections of Minerals 

 cut perpendicular to the optic axis or axes of the crystal under 

 the influence of polarized light. Drawn by contributor. 



Rev. Nicholas Brady, M.A. 



