The holograms also are capable of knocking people back making it (and the accompanying sword) the only known holograms to be able to interact with solid matter and become solid. Merlin had one in his vault in Avalon, to test the "worthiness" of any who would enter. He also had one guarding his library in Camelot. It is unknown if the rest of the Ancients used these but it is unlikely, although the solid hologram technology was most likely used in other Ancient bases for different purposes. (SG1: "Avalon, Part 2", "Camelot")