A modified spindle stage is described, with certain advantages over previous models. Conoscopic observations can be made using the short working distance of an ordinary polarizing microscope; the orientation of a mounted crystal can be adjusted by means of perpendicular slides and a ball-joint. Thus direct determinations of optic constants, including optic axial angle, are possible.

The construction, uses and operation of the stage are described and some results obtained during an investigation of the potassium-barium feldspar series are presented.

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