PHOTOGRAPHY is the recording angel of the arts and sciences, but it is to be hoped that angels of other classifications do not have to endure the insistent requests for materials possessing incompatible properties which are demanded of the photographic chemist. Perhaps the most difficult problems are presented by the physicists and astronomers, who require grainless emulsions of extremely high speed, sensitive to all the visible spectrum, or any portion of it-with extensions into the ultra-violet and infra-red regions!