The refractive index of an optically heterogeneous specimen will generally vary in directions parallel and normal to the microscope axis, but a mean index can be defined such that the optical thickness of the specimen is equal to the product of the mean index and the specimen thickness. If such a specimen is examined under an interference microscope, the fringe displacement will be zero at those parts of the specimen for which the mean index is equal to the index of the immersion liquid. Thus, by gradually changing the index of the liquid relative to that of the specimen, one can determine the manner in which the mean index varies from region to region of the specimen. In the present work these changes in relative index have been effected by varying either the wavelength of the incident light or the temperature of the system.
The Baker two-beam interference microscope has been successfully used for such investigations. Three types of interference pattern have been tried: (i) a single fringe is made to fill the field of view and the intensity differences between the specimen and the background are studied; (ii) monochromatic fringes are formed in a quartz wedge and their linear displacements are examined; and (iii) a quartz plate is used, channelled spectra being produced by the spectrographic analysis of the interference colours. The first two types can be used with either the temperature or the wavelength variation method, but the third was specifically designed for use with the wavelength variation method. Although these experiments have mostly been carried out on birefringent fibres, the techniques are applicable to any kind of object. The attainable accuracy increases with specimen thickness; under favourable circumstances the index at any point of a specimen 10 µ thick should not be in error by more than †3x10-4
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