In evidence since the 11th century, the camera obscura is an optical device consisting of a small hole in the wall of a darkened room or container, allowing light to pass through which causes a projection of an inverted and mirrored image of the outside view. In Camera obscura, Eliasson's first work with this device, a mirror is used to reflect an image downward from above onto a vertical surface in the same way as a periscope functions. Here, as in the other camera-obscura works, a lens is used to focus the image on the surface.