It is well known that in the Schroeder Van der Kolk method of comparing the refractive index of a crystal grain with that of the material in which it is mounted, the effect can be reversed by moving the microscope sub-stage condenser, and that the position of this condenser must therefore be carefully controlled if ambiguities in the interpretation of the observations are to be avoided. This fact is mentioned in the majority of text-books and is referred to in a general manner by Wright and by Winchell. No clear explanation of the reversal is offered, however, and students are consequently inclined to forget this important limitation in the method or, alternatively, to forget in which sense the movement of the condenser influences the effect: unless a control experiment is invariably performed errors may therefore result.