a color or black-and-white positive photographic image on a transparent base (glass or film), viewed against a light source or projected on a screen. Reversed or special (diapositive) photographic films or plates are used to prepare diapositives (slides). Diapositives are used in educational and other visual demonstrations. A set of diapositives dealing with a common topic is sometimes called a slide sequence. Sometimes the designation “diapositive” is applied to sketches, pictures, and drawings produced on photographic plates or film (after the fixative process).