As a matter of fact, the actual refractive index is only stated in most tables of optical glasses for sodium light (D line), the dispersive properties of the glass being indicated by tabulating the differences between the refractive indices for the various lines, the table thus containing columns marked C-D, D-F, F-G′. These figures are usually described as the “dispersion” of the glass from C to D, D to F, etc. In addition to these figures it is usual to tabulate what is called the “mean dispersion” of the glass, which is simply the difference between the refractive indices for C and F lines; this interval is usually taken as representing that part of the spectrum which is of the greatest importance for visual purposes. A further constant which is of great importance in the calculations for achromatic lenses is obtained by dividing the mean dispersion into the refractive index for the D line minus one (usually written (C-F)/(n_{D}-1)=ν). This term, for which no satisfactory name has yet been suggested, characterises the ratio of the dispersive power of the glass to its total refracting power. It is usually denoted by the Greek letter ν. The following table (taken from the Catalogue of the Optical Convention, 1905) gives a list of optical glasses produced by Messrs. Chance, of Birmingham. This list, although it is not nearly so long as that issued by the French and German firms who manufacture optical glass, contains examples of the most important types of optical glass which are available at the present time. Those, however, who wish to use the data for the purpose of lens calculation are advised to consult the latest issues of the optical glass-makers’ catalogues, since the range of types available, and even the actual figures for some of the glasses, are liable to variation from time to time. TABLE OF OPTICAL PROPERTIES.