The OS 51 series is a fascinating one. It consists of a mixture of paper maps and plans as well as almost 1,000 glass lantern slides, including images which were to be used on the cover of maps or booklets produced by the Ordnance Survey, as well as images of how the maps were created.
Glass lantern slides (also known as magic lantern slides) consist of a photographic image sandwiched between two square pieces of glass. The sandwich is then bound around the outside with a gummed tape, usually black. They were invented in the late 1800s and would have been used in a lantern slide projector to project the images for entertainment or educational purposes.