A photographic method of measuring refractive indices

Abstract

As a sequel to experiments carried out since 1908 on the form of the curves of rotatory dispersion, it appeared desirable to undertake a similar study of refractive dispersion, in view of the fact that, whilst a few long series of precise observations had been made by physicists of the dispersion of quartz, sylvine, water, etc., the thousands of observations recorded in chemical literature were for four wave-lengths only and were of negligible value for the elucidation of this phenomenon. In order to fill this gap, two series of observations have already been made in conjunction with Colonel J. W. Gifford, the first on benzene and cyclohexane and the other on nicotine, i using in each case a hollow prism with three faces closed with quartz plates. Before undertaking a more extensive programme, however, it was thought desirable to discover a method by which the refractive indices could be followed into the ultra-violet region, and beyond the limits of transmission of light by the thick layers of liquid contained in the hollow prism. For this purpose an interference method was indicated, § and a method was finally developed whereby the interference fringes from a thin film of liquid in a quartz etalon can be focussed on the slit of a spectrograph, and the refractive index of the liquid deduced from comparative measurements of the number of fringes contained in a given width of the slit, as exhibited on successive lines of the image of the slit in the spectrograph. The optical system, indicated in fig.1, consists of a pair of half-platinised quartz plates, E 1 E 2 between two right-angled quartz prisms, P 1 P 2 . A quartz lens L 1 throws a parallel beam of light through the etalon, and a quartz quartz-fluorite triplet system L 2 L 3 is used to focus an image of the fringes in the film on the slit of a spectrograph. The system L 1 , P 1 , P 2 is rigidly mounted and in permanent adjustment, whilst L 2 L 3 is furnished with a rack and pinion movement for exact focussing. Two fine wires mounted on the slit of the spectro­graph provide reference lines on the photographic plate.