LONDON. Optical Society, December 10.—Col. H. S. L. Winterbotham: General principles of photographic surveying. A brief outline is given of the develop ment of photogrammetry from terrestrial and aerial positions, and an indication of some of the points to be considered in the design of instruments required by the photogrammetric surveyor in Britain.—Lieut.-Col. W. N. MacLeod: Perspective conditions of photo grammetry. The perspective conditions which must be satisfied in aerial survey work are discussed and the methods of reconstructing these conditions from the photographs obtained are described. Attention is directed to the desirability of designing new apparatus with which to carry out the necessary reconstruction with the requisite accuracy.—Lieut. M. Hotine: Some precision problems in air survey. The various factors upon which accuracy in air survey measurements depends are described and suggestions are made as to some of the conditions which should be fulfilled in apparatus employed for such measurements.