LONDON. Optical Society., Feb. 12.—T. Smith: Modern optical glass as exemplified by the list of the Parsons Optical Glass Co., dated September 1926. The optical positions (μv) of the glass types catalogued by the Parsons Optical Glass Company and the relations between the dispersions for several segments of the visible spectrum are exhibited graphically. A knowledge of the refractive indices of any glass for three wave-lengths is sufficient to specify the index for the whole of the visible spectrum, and a knowledge of two indices is almost sufficient. In particular, the partial dispersive ratios are almost functions (nearly linear functions) of v only. This implies that there are no glasses suitable for making apochromatic telescope objectives of large relative aperture. The graphs show that the general standard of accuracy of the measured indices is high. A new graphical method of interpolating refractive indices for glass is obtained.—J. Guild: On the fixed points of a colorimetric system. The paper discusses the significance of the constants which enter into the specification of colour on the trichromatic system, and suggests certain fundamental considerations of a metrological character which ought to govern the choice of such constants in a standard reference system. Various proposals which have been put forward from time to time are discussed in the light of these considerations, and the basis of the system which has been adopted at the National Physical Laboratory is explained.