An Interferometric Method of Studying Local Variations in the Refractive Indices of a Solid

Many small transparent solids possess local variations in their refractive indices that cannot be detected by the standard immersion methods. This difficulty has been overcome by placing the immersed solid between the two parallel, metal-coated plates of an interferometer illuminated at normal incidence by a parallel beam of white light The immersion liquid is so chosen that its dispersion curve intersects that of the solid at some wavelength within the visible spectrum An image of the immersed specimen is then projected on to the slit of a spectrometer, whereupon one can observe coloured fringes of equal chromatic order (channelled spectra) These fringes generally suffer a displacement on crossing from the liquid to the liquid-solid region of the interferometer; this displacement does, however, vanish at that wavelength at which the refractive index of the liquid equals that of the solid The method is equally applicable to both isotropic and birefringent specimens, the index measurements under favourable conditions being accurate to within ± 0.0001 The paper is illustrated by several examples, particular reference being made to the possibility of studying the skin effect in rayon fibres