Convergent light figure

Fig. 52. Convergent light figures, a. Uniaxial crystal with optic axis parallel (left) and slightly inclined (right) to line of vision. 6. Biaxial crystal with acute bisectrix parallel (left) and inclined (right) to fine of vision.

Biaxial crystals under similar optical conditions produce convergent light figures like that shown in Fig. 52 6, when the acute bisectrix of the optic axes lies along the line of vision and the vibration directions... 

Fig. 55. Effect of quartz wedge on convergent light figures, a. Uniaxial positive. b. Uniaxial negative, c. Biaxial positive, d. Biaxial negative.

Fig. 57. Optical properties of twinned crystals, a. Gypsum, b. Calcium sulphate subhydrate. Note orientations of convergent light figures in the...

If a uniaxial convergent light figure is seen, the optical sign of the crystal may Tbe checked by the use of the quartz wedge in the manner described in the previous chapter. This is not necessary (except as confirmation) unless for any reason it is not possible to obtain actual measurements of both a> and e. 

The observation of convergent light figures may often provide confirmation of the orientation of the indicatrix. The plane of the optic axes is the ay plane, while the normal to this plane is the j8 vibration direction. For a positive crystal the acute bisectrix is the y vibration direction, while for a negative crystal it is the a vibration direction. 

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