LONDON. Optical Society, October 12.—Prof. F. J. Cheshire, vice-president, in the chair.—L. C. Martin: A physical study of coma. A specially designed microscope objective and mounting, calculated to exhibit coma in the absence of spherical aberration and astigmatism, are described. Photographs of a star image, taken when the amount of coma is equivalent to that for which the light distribution has been calculated, verify the numerical work. The photometric examination of the photographic image is carried out by a special method.—F. W. Preston: Comparison of the structure of sand-blasted and ground glass surfaces. Glass surfaces smoothed or “greyed”by loose abrasives in the usual way are compared with those produced by sand blasting. The surfaces are practically indistinguishable either by the naked eye or the microscope, and the development of the structure by etching shows that the structure is virtually identical. Thus it appears that mere pounding of a glass plate can, and does, produce a surface which is structurally indistinguishable from a smoothed surface of a technical order.