This paper presents the results of investigation of the optical system of the ore microscope with the purpose of evaluating various sources of error in measurements of rotation properties. The following sources of error are considered: the polarizer, the reflecting plate, the objective, the polished surface itself, and the analyzer. Errors from these sources are found to be due largely to incorrect adjustment of the microscope. Strain in objectives, however, can be a source of serious errors, and the use of strained objectives is undesirable. The reflecting plate introduces errors in angles of rotation and apparent rotation. Measured values must be corrected to true values by reference to a curve constructed for the particular microscope used.