and a remarkable Structure in Analcirne. 189 



while the crystal is made to revolve round that axis ; but when 

 the axis is inclined 45 to that plane, or when the diagonals of 

 any of the cubical faces are in the plane, we observe a black cross 

 in the directions AB, CD, Fig. 1., separating four luminous sec- 

 tors covered with the tints of polarised light. 



If ACBD, Fig. 3., representing a piece cut off the summit of 

 the icositetrahedron by a plane perpendicular to one of the axes, 

 is exposed to polarised light, it will exhibit, of course, the black 

 cross AB, CD, and the four sectors of polarised light. If the 

 crystal is now turned round CD as an axis, so that the part A 

 is brought near the eye, and B retires from it, the black cross 

 opens at the centre, and assumes the form of two curves A a C, 

 B d D ; but when B is brought near the eye, and A retires, these 

 curves have the position B b C, D c A. 



When a slice is cut from the summit with three planes, or that 

 which corresponds to one of the diagonals of the cube, as shewn in 

 Fig. 4., the three planes of no polarisation a d, bd, cd, are dis- 

 tinctly seen. If the line c d is placed in the plane of primitive po- 

 larisation, the sector Sadb, opposite to it, becomes dark, and the 

 same is true of the other lines ad, bd. If, instead of a slice of 

 the crystal, we use a complete icositetrahedron, and look along 

 the diagonal of the cube, we observe six sectors, as in Fig. 5. 

 The reason of this is, that the three planes at the opposite end 

 of the diagonal have an inverse position, the three edges of the 

 one, corresponding with the diagonals of the three trapezia in 

 the other, as shewn in Fig. 6. If the analysing plate has its 

 plane exactly perpendicular to that of primitive polarisation, all 

 the six sectors are equally whitish in minute crystals ; but by 

 turning the plate to the left, three of the alternate sectors be- 

 come dark, and by turning it to the right, the other three be- 

 come dark. The polarisation is a minimum along this and all 

 the other diagonals. When the plane T a d c of the slice shewn 

 in Fig. 4. is inclined, so that d is brought nearer the eye, and 



