The memory window was a device invented by the Celestial Toymaker. It took the form of a video screen built into the chest of a life-sized toy robot.
Both Steven and Dodo looked into the memory window when they arrived in the Celestial Toyroom. Steven saw images from his adventures on Kembel in 4000 and Paris in 1572, while Dodo saw herself in school uniform on the day her mother died. The First Doctor told them the window was hypnotic, and that the Toymaker would have used it to dominate their minds. (TV: The Celestial Toymaker)