International
Tables for
Crystallography
Volume D
Physical properties of crystals
Edited by A. Authier

© International Union of Crystallography 2006
International Tables for Crystallography (2006). Vol. D. ch. 1.6, p. 164

Figure 1.6.4.15 

A. M. Glazera* and K. G. Coxb‡

a Department of Physics, University of Oxford, Parks Roads, Oxford OX1 3PU, England, and bDepartment of Earth Sciences, University of Oxford, Parks Roads, Oxford OX1 3PR, England
Correspondence e-mail:  glazer@physics.ox.ac.uk

[Figure 1.6.4.15]
Figure 1.6.4.15

(a) Acute bisectrix figure of a biaxial crystal with [2V \simeq 40]°. The optic axial plane is oriented NW–SE. First-order colours are confined to the two circular `eyes' or melatopes around the optic axes, with sensitive tint itself making a figure-of-eight shape (`dumbbell'). At the edge of the figure in the NE and SW directions the coloured fringes reach third-order pink. (b) The same figure with the sensitive-tint plate inserted (slow direction NE–SW). The isogyres have taken on the sensitive-tint colour. Between the optic axes (e.g. the centre of the figure), the original sensitive-tint dumbbell has become black, and to the NW and SE of the centre all the fringes have dropped by one order. Conversely, in the extreme NW and SE directions all fringes have gained an order. The little patches of yellow lying just inside the optic axes, and the accompanying blue patches just outside, indicate by analogy with the uniaxial figure that the crystal is optically negative.