Actinograph exposure calculator of cylindrical type. With a revolving scale to set date and time and the speed index of the plate is set on the speed scale. Slide is moved until the aperture can be read off for five light conditions: very dull, dull, medium, bright and very bright. Each of the H. & D. actinogrpahs was sold for one particular latitude. Hurter & Drifield, c.1888.