Summary: A new method is described to replace the standard procedure for completing the adjustment of an optical symmetry plane, with conditions for the accurate use of the biquartz. Extinction measurements are plotted to show the wide range of sensitivity (for the adjustment of A₁ and A₂) at various directions in the symmetry planes of a plagioclase. It is proposed that A₁ and A₂should be adjusted at different settings of A₄ so as to obtain maximum sensitivity for each of those axes.