Wooden slide rule type calculator for exposure. Flat wooden plate marked with 3 scales. Speed 0.5-300. Exposure 0.005-60seconds. Lens f/2.8-64. Slide marked with weather: Very dull, dull, mean, bright, very bright. With speed index mark. Card insert carries time of day and year scale for North latitude 7 degrees and 30 foot. Hurter and Driffield. c.1898.