The object of this work was to design and build an instrument for mapping optical heterogeneities in fused silica blocks, with a sensitivity at least an order of magnitude better than any system currently available (prompted by a requirement of the Stanford Gravity Probe B project). The sources of inhomogeneities and the environment for containing these blocks to achieve measurements at this level are discussed with the implications for the instrument design. After a review of homogeneity testing methods, it was decided to develop an instrument system for use with several measurement methods. The most sensitive of these methods was selected to be fully developed in order to realise the measurement aim, the others being facilitated in the design. The design and development of a novel mechanically scanned heterodyne interferometric mapping system is described-following an error budget driven methodology. The critical 1mK thermally stable environment required for the liquid immersion measurement method employed, was exceeded by the design and development of a large computer controlled thermal enclosure to house the whole instrument system-sub-millidegree stabiliy achieved over periods of hours. Also described is the computer integration and software for execution and co-ordination of all measurements. The developed system meets the initial aim, being capable of an optical heterogeneity measurement sensitivity of 5 x 10-8 in refractive index in a 50mm thick sample. This was calibrated indirectly in a comparison with a Talystep surface profiling instrument. Further development work suggested could enhance this sensitivity by another factor of two.