A cleavage fragment of smithsonite of known refractive indices is mounted permanently in a hole in a microscope slide. The cavity is filled with liquid of high but otherwise unknown index and covered with a cover glass. The slide is then mounted on a universal stage equipped with a water cell for temperature control. The crystal is rotated in such a way that its principal section remains parallel to the polariser. When the indices of the crystal and liquid agree the amount of rotation from the optic axis position is used as the ordinate of a graph--the abscissa gives the refractive index. Proximate curves serve to indicate dispersion.