The widespread use of methods based oil total-reflection for finding the refractive indices of crystals either directly, by the Abbe-Czapski crystal reflectometer or by the Kohlrausch method, or indirectly by means of liquid mixture of known refractive intex which require to be verified and standardized from time to time, renders desirable a rapid graphical method of ascertaining with some accuracy the index of the substance under test, direct from the instrumental data without calculation.

An attempt in this direction was made by the author some years ago. The equation to be solved is n' = n sin θ, where n' is the index to be found, n that of the glass hemisphere, prism, or highly refracting liquid employed in the total-reflectometer, and θ the critical angle of total-reflection observed.