LONDON. Royal Society, June 2.—Prof. C. S. Sherrington, president, in the chair.—Bakerian lecture by Dr. T. M. Lowry and Dr. C. P. Austin: Optical rotatory dispersion. Although no case is known in which Biot's law of inverse squares, α=k/λ2, is accurately true, the rotatory dispersion in a very large number of organic compounds can be expressed by the simple dispersion formula, α=k/(λ2-λ02), which differs from Biot's formula only in the introduction of a “dispersion constant” λ02. This formula is a special case of the general formula α=∑kn/(λ2-λn2) introduced by Drude as an approximation based upon the electronic theory of radiation and absorption of light. Substances which require more than one term of this equation are said to show complex rotatory dispersion. Tartaric acid and its esters give dispersion curves which frequently show an inflexion, a maximum, and a change of sign; they are described as cases of anomalous rotatory dispersion. These can be represented by two terms of Drude's equation, while the rotatory dispersion in quartz was represented by a similar equation, in which the dispersion-constant of the negative term was negligible. In order to express recent measurements it is necessary to assume finite values for both dispersion-constants and to introduce a term to express the influence of the infra-red absorptions; this can be taken as a constant. The anomalous dispersion of tartaric acid was attributed by Arndtsen in 1858 to the presence of two modifications of the acid differing in the sign of their rotations and in the magnitude of their dispersions. This view has been confirmed (1) by the proof that the complex rotatory dispersion of the acid and its derivatives can be expressed as the sum of two simple dispersions, and (2) by the discovery of certain “fixed” derivatives of tartaric acid which exhibit simple rotatory dispersion. Attention is directed to some analogies between tartaric acid and nitrocamphor, which give two isomeric compounds in solution.