Optical apparatus—known in Europe as early as the thirteenth century—basically consisting of a room, with a hole in one of its walls. The light rays coming from the outside enter the hole and, thanks to the rectilinear propagation of light, form an inverted image of the landscape on the opposite wall of the room; the image is visible to an observer inside the room. Not all camera obscuras were full-sized rooms: others were made in the form and size of a box, in which the side where the image forms is made of ground glass. The image produced in these models is not very bright, since the amount of light passing through the hole is necessarily small; however, if the aperture diameter is increased, the image loses definition. The camera obscura was significantly perfected by placing a lens ("objective") in the hole; this arrangement increased the aperture diameter and therefore the brightness, while improving the definition of the image on the ground-glass plane. The first photographic devices were essentially camera obscuras in which a photosensitive plate was inserted.