II. MINERALS, ETC. 895 



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. 



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 polarised light. Drawn by contributor. 



Rev. Nicholas Brady, M.A. 



PYRAMIDAL AND RHOMBOHEDRAL SYSTEMS. UNIAXAL. 

 1. Calcite, negative, plane polarised, analyser and polariser, parallel. 

 45 apart. 



3. ?5 j? ^ )) 90 



4. elliptically polarised 45 

 Axis of wave film inclined about 22^ to plane of polariser. 



5. Quartz rotary polarisation. Analyser and polariser. 90 apart. 



6. Two similar plates of quartz superposed, the lower showing left-handed 



rotary polarisation, the upper right-handed analyser and polariser, 90 

 apart. This phenomenon is known as Airy's spirals. 



7. Quartz, circularly polarised, analyser and polariser, 90 apart, and axis of 



wave film 45 from plane of polariser. 



PRISMATIC OBLIQUE AND ANORTHIC SYSTEMS. BIAXAL. 



8. Nitre, plane polarised, analyser and polariser, parallel line joining optic 



axes = to polariser. 



9. Nitre plane polarised analyser and polariser, 45 apart, line joining optic 



axes 45 from polariser. 



10. Nitre plane polarised analyser and polariser, 90 apart, line joining optic 



axes 45 from polariser. 



11. Nitre plane polarised analyser and polariser, 90 apart, line joining optic 



axes parallel to polariser. 



12. Aragonite optic axes, widely separated, only one is seen, analyser and 



polariser, 90 apart, line joining optic axes parallel to polariser. 



3487. Large Wire Model, containing one example of each 

 species of simple form in the cubic system of crystallography, with 

 its corresponding hemihedral forms inscribed within a sphere of 

 the chief zone circles of the system. Rev. Nicholas Brady, M.A. 



The coloured wires indicating the junction of the faces are painted as follows : 

 Cube, vermilion ; octahedron and tetrahedron, French blue ; dodecahedron, 



