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 challenging to operate. Though Slide Cube Projectors are no longer manufactured, cubes, bulbs and a few replacement parts as well as used projectors and cubes remain available on the second hand market. See also Franke & Heidecke Rolleiscop Slide projectors