In 2007 Yin, Zhang and Tian [1] [Yin et al., 2007] derived the expressions of the refractive indices of biaxial crystals evaluated from the refractive indices ellipsoid equation. In the past we have researched about the simultaneous measurement of birefringence and optical activity in different crystals [2] [Hernández-Rodríguez et al., 2000], [3] [Hernández-Rodríguez and Gómez-Garrido, 2000], [4] [Herreros-Cedrés et al., 2003], [5] [Herreros-Cedrés et al., 2005] and [6] [Herreros-Cedrés et al., 2007], and recently, when we used their methods for the study of nonlinear crystals such as KTiOAsO4 (KTA) and KTiOPO4 (KTP), we found some errors in some expressions in their paper which were used by other authors [7] [Gao et al., 2003].