In a paper communicated to the Royal Society, Mr E. T. Günther and the author gave an account of the results obtained from the examination of two samples of water taken from Lake Urmi, and amongst other determinations of a chemical and physical nature, were those of the refractive indices, which were performed with the aid of the Royal Society's large quartz prism and spectrometer, the latter reading by means of micrometers to 2″ of arc. On comparing the values obtained for the refractive indices of the two samples of water with those obtained for the relative densities, it was at once apparent that the former differentiated the two samples quite as distinctly as the latter.