APPLICATION OF ‘FOCAL SCREENING’ TO MEASUREMENT OF INDICES OF REFRACTION BY THE IMMERSION METHOD

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Abstract
A parallel ray-bundle of white light, crossing the solid/liquid boundary at a small angle, is spread into a spectrum when the index of the immersion liquid approaches that of the solid. This is due to the greater dispersion of liquids and forms the basis of the three variants of the "focal screening" method, using appropriate masks in the focal plane of the objective: (1) In apertural screening the ray-bundle is parallel to the microscope axis, and an iris diaphragm in the focal plane of the objective is restricted until only the undeviated wavelengths pass. Thus at the grain edges only the color for which the index of liquid and solid match is seen. (2) In unilateral screening the ray-bundle is inclined and the diaphragm is restricted to block certain of the rays and reveal different colors on opposite edges of the grain. (3) In central screening the ray-bundle is again parallel to the microscope axis, and a dot mask at the center of the focal plane holds back the undeviated rays. The edge color is a recognizable combination of those of the deviated rays. Indices are said to be measurable to .001 with the first arrangement, more accurately with the second, and "more accurately than by the Becke method" with the third. Advantages include the benefits of chromatic variation without requiring a variable monochromator, visibility of grains at the match point, and less disturbance by inclusions. The effectiveneSs of this method is increased by quantitative data on the dispersion of the liquids

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