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17. Refractometers for Solids and Liquids.

These instruments have been designed by Dr. G. F. Herbert Smith, M.A., D.Sc., of the British Museum, for the identification of substances by a measurement of their refractive indices, on the principle of total-reflexion. In the yellow light of a sodium flame the boundary between the light and dark fields is sharply defined. Two types :—

(1) Refractometer for Solids. (Fig. 10).—The solid, e.g., a faceted gem stone, should have at least one flat surface, and is placed on the glass. The range extends from 1'300 to 1'795, the second decimal place being read direct from the scale, and the third by estimation. No calculation is required. A drop of methylene iodide removes the air-film under the gem. By rotating a doubly refractive stone, so that one flat surface remains in contact with the glass of the refractometer the amount of double refraction may be measured. The light enters from below. Gem-stones may be identified with unfailing certainty and there is no necessity to remove them from their setting.

Fig. 10.

(2) Refractometers for Liquids.—Liquids require a refractometer of higher refinement than solids. In these instruments the scale reads directly to the third place of decimals, and the fourth is found by estimation. The range is selected to suit various classes of liquids. The temperature is controlled by a water jacket to which a small thermometer is attached. The thickness of the liquid-film is controlled by a screw to secure the best definition of the shadow-edge. With white light the shadow-edges are coloured, but their definition may be much improved by the introduction of the red glass disc.

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