LONDON. Optical Society, June 9.—T. H. Harrison: The use of photo-electric cells for the photometry of electric lamps. A description is given of apparatus and experiments designed for giving the highest accuracy and precision in the photometry, using photo-electric cells, of electric lamps. Although the methods adopted are not novel, yet every care has been taken to obtain the highest sensitivity in the photo-electric current measuring apparatus and to maintain the lamps at a steady, accurately known voltage. It is claimed, therefore, that the results are useful in showing the maximum capabilities of photo-electric cells when used in the usual manner with a sensitive electrometer or electroscope. Sources of error and their elimination and the computation of the accuracy of the results are discussed.—R. Kingslake: An experimental study of the best minimum wave-length for visual achromatism. A special telescope is described, in which the chromatic aberration can be varied continuously without introducing any other undesirable aberrations. By the aid of this apparatus, many determinations have been made as to the best type of achromatism for visual observations, in daylight, in artificial light, and on astronomical objects. Several observers were employed to make settings, and their opinions as to the most desirable type of achromatism are tabulated and compared. In general it is found that a moderate amount of undercorrection is required to give a truly colour-free image, that less undercorrection is needed if the best definition is to be obtained, and that a slight overcorrection is desirable for astronomical work.—S. K. Datta: On Brewster's bands (Part ii.). The nature of the patterns obtained by the superposition of two systems of Haidinger's rings when the actual law of spacing of the rings is considered is discussed.