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Care in the optimum translational adjustment of a prism in order to permit the symmetrical use of all apertures is always necessary on account of aberrations inherent in lens systems. Such prism adjustments are advisable also because of slight curvatures of most prism surfaces. When prisms are at all times correctly located with respect to the axes of collimator, telescope, and spectrometer, the departures of the surfaces from planeness may then be appreciably greater than has hitherto been recognized as allowable. Moreover, it is shown that the latitude in collimation becomes sufficiently large to permit making all necessary refocusings with the telescope, even when using objectives with the usual type of color correction. Tolerances, corresponding to an error of ± 1 X 10~8 in index of refraction, are evaluated for curvature of prism surfaces, translational adjustment of the prism, eccentricity of prism-table axis, and collimator refocusing.

Permissible curvature of prism surfaces and inaccuracy of collimation in precise minimum-deviation refractometry