Determining the sign of optical rotation in linearly birefringent crystal sections

The problem of determining the sign of optical rotation in a linearly birefringent crystal section is examined. This problem has not been fully addressed in recent publications describing techniques for these measurements. It is shown in this paper that there is considerable scope for error in the determination of the sign and particular care must be taken in certain situations. It is important to know this sign as theories that seek to correlate optical rotation with crystal structure cannot be tested without this information.