The difference between the index of refraction for green mercury light and for 7 other mercury emission wavelengths from 15 300 A to 3651 A has been measured using a Twyman-Green interferometer as a refractometer. A photoelectric method was used for the infrared determination. A method depending upon photography of interference fringes which were caused to appear crossing the lines in the mercury spectrum was used for the visible and ultraviolet measurements. The mean observed deviation of the measurements of (μ − 1) from the predictions of the Edlén formula was 1 part in 105.
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