A method is described using a divided-circle spectrometer with exit slit and hollow, 60° prism, in which the lens element is immersed, using a liquid of approximately the same index. The refraction of light through the cylindric edge of the lens is viewed through the exit slit. Instead of adjusting the liquid exactly to the index of the lens, an approximate match is made, and the residual difference of indices is determined by measuring the deviations at the two places on the edge, at which the angle of incidence is 45°; this is evidenced by the coincidence, as seen through the exit slit, of the bright line on the edge of the lens with marks previously placed there. The mean of the two measured deviations at minimum determines the refractive index of the liquid, and their difference gives the correction to be applied to the latter for the index of the lens. Two instrument set-ups are described, one using one spectrometer and low pressure mercury lamp with filters, the other using a spectrometer, monochromator, and high pressure mercury lamp.
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