LONDON. Optical Society, May 9.—Mr. S. D. Chalmers in the chair.—T. Y. Baker and Major L. N. G. Filon: Spherical aberration. The authors had considered the subject from the point of view of an optical design for a system of co-axial thin lenses (separated by air) in which the focal lengths and separations of lenses are determined from general consideration of the functions that the instrument has to perform, and from the necessity for correcting for colour. A design carried out in this manner leaves available for the correction of spherical aberration the forms of the various lenses. For a thin lens of definite focal length made of a definite variety of glass the difference of curvature of the two faces of the lens is fixed, but the mean of these two curvatures is arbitrary. When aberrations of the second order have to be included, the semi-cubical parabola is no longer a sufficiently close approximation to the caustic, which, in general, develops two new cusps off the axis. The general appearance of such a caustic was examined, as well as the possibility of deriving the two parameters from trigonometrically calculated rays. The authors urged that the full import of the higher-order aberrations could best be understood by an actual construction of the caustic in the several media, from which the trained optical calculator would be able to tell from the shapes of the successive curves how the aberrations of different orders would affect the final image formation, and also to form an idea as to which lenses were having most serious effect, and how changes in the forms of the lenses would enable him to diminish the spherical aberration of the final image.