The aberrations and variation in transmission of a beam of light converging to a focus and passing through plates of calcite have been investigated, in order to estimate possible photometric errors due to these effects when a crossed calcite plate is used to calibrate photographic plates. The aberrations are calculated in terms of ray deviations whose magnitudes are derived, and the transmission coefficient is determined by solution of electromagnetic boundary conditions. The resultant formulae are evaluated numerically, and set limits on the photometric accuracy of images formed by a Schmidt telescope used with a calcite plate calibrator.