Colour ellipsoids

In triclinic crystals there are no restrictions at all on the position of the indicatrix with respect to the crystal axes. No axis of the ellipsoid need coincide with any one of the crystal axes. Consequently the position of the ellipsoid may vary with the light w avelength for all crystal orientations incomplete extinction with abnormal interference colours at the position of minimum illumination may therefore be seen for any crystal orientation. 

Fig, 26,23 Threshold values. Brown and MacAdam ellipsoids in C.I.E. colour body... 

The MacAdam optical sensitivity chart shown in Fig. 26.21 does not take luminence into account and Silberstein (Phil. Mug., 1946, 37, 126) showed that when these ellipses were transferred to the three-dimensional version of the ehromatieity chart they would be ellipsoid as shown in Fig. 26.23. In order that a really satisfactory universally applieable speei-fieation for tolerances in colour matching can be established it is desirable that these ellipsoids should be spherical. This, unfortunately, is not possible and the nearest approach is the Simon and Goodwin system whieh takes the form of about 100 different charts in which, by alteration of the angle of the eo-ordinates of the ehromatieity chart the ellipses are COn-verted to eireles. The scale is such that each half-inch is equivalent to a threshold value of discrimination Fig. 26.24 (Shiot and Goodnih, Amer. Dyes Rep., 19.58, 105). 

Fig. 4. 32 Solving the stracture of the ligand 4 -azido-2,2 6, 2"-terpyridine. (a) One photographic frame from the single crystal X-ray data collection showing diffraction spots. ORTEP drawings of the molecular structure of 4 -azido-2,2 6, 2"-terpyridine from data collected at (b) 123 K and (c) 293 K with ellipsoids plotted at a 50% probability level. The H atoms were refined isotropically (see text). Colour code for atoms C, grey N, blue H, white. [Data M. Neuburger and N. Hostettler, University of Basel R. Al-Fallahpour et al. (1999) Synthesis, p. 1051.]...

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