Slide cube projector
A slide cube projector is a slide projector and system created by Bell & Howell, marketed through the 1980s. The system derives its name from the plastic slide storage cubes. The cubes are about 5.5 cm in each dimension (a bit larger than a slide), and hold 36–44 slides depending on the mount thickness. Unlike the trays used in straight-tray or round-tray slide projectors, the slides in a cube are not separated from each other; they are stacked one on top of the other. The slide advance mechanism depends on a slide dropping from the loaded  cube into a slide-size hole in a circular plate. The plate is thinner than a slide and so should grab only a single slide and move it forward, first to a preview section, then to the projection position, then to a holding position from which it can be returned to allow for a single slide's worth of backup, and then finally to a position in which it drops into a stack that can be pushed back up into the cube after projection is completed. Slide cube projectors were popular at one time because the cubes were cheaper than  trays and they allow for higher storage density – e.g. a book or drawer of 16 forty-slide cubes (640 slides) fits in the space of a single round tray that holds at most 140 slides. The stack storage and preview feature also make for easier editing of slide shows; slides can be added to or removed from the show without having to shift all of the remaining slides up or down in their slots one at a time. Some tray-based projectors (e.g. some Kodak round-tray and Hähnel straight-tray slide projectors) can use stack loaders to view a  stack of slides, but they don't lend themselves to this type of operation and there is no associated storage system. The slide cube had drawbacks, including the inability to reverse to more than one slide, the fragility of the cubes, and the advancing plate's tendency to jam — interrupting slide shows and making the projectors unsuitable for the less mechanically apt. Slide cube projectors are no longer manufactured, but cubes, bulbs and a few replacement parts as well as used projectors and cubes remain available.