Here described is a simple refractive index comparison instrument designed especially for use in mixing and testing the binary, quartz‐matching, immersion liquids which are now so widely used in quartz inspection and orientation instruments. The optical system comprises only a slit and a Z‐cut quartz prism immersed in the liquid, and a lens contacting the liquid surface. By observing the slit both through and to the sides of the prism it is possible to detect a &Dgr;N=0.001 mismatch between the liquid index and the desired quartz index. Further, without additional scales or other means, the degree of mismatch may be closely estimated, by comparison with the &Dgr;N=0.01 birefringence in the quartz prism itself.