The less expensive projects show one slide at a time in the order “ slide-across” holder or the chute type into which the slides drop. With automatic projectors slides are stacked in straight magazines, 36 at a time in 2in-square mounts, or in circular magazines containing up to 120 slides. These have push-button or other devices which allow the slides to be advanced, and a remote control focusing device. It is advisable to mark the slides clearly so that they are inserted the right way up and in the correct order. When not in use slides can be contained in view packs which allow convenient in section and handling without damage. Slides are more effective when in colour, but should not contain too much compressed information. The text and diagrams should be large and clearly visible from the back of the classroom. Clarity simplicity and visual impact should always be aimed at.