A simple, straightforward method is described for determining experimentally the matrix of any, crystalline plate. The method involves three measurements of the state of polarization of light transmitted by the plate, and also a measurement of its transmission factor for natural light. These measurements determine the matrix uniquely, except for a phase factor whose practical significance is small. The conditions are stated for the application of the method to the more general type of optical system described in V. A statement of the content of future papers in this series is included.