Camera Obscura

Camera Obscura, or dark room, invented by Porta, a small chamber within which a clear inverted picture of the external surroundings may be presented on a screen, by a process similar to that which holds in the case of the camera lucida (q.v.). Light from the outside is allowed to pass through a small aperture into the chamber, and there be totally reflected on to the screen from the inside surface of a glass prism or from a plane piece of glass mirror. The image is more clearly defined if a convex lens be placed at the aperture to concentrate the rays. Many such cameras as are used in public gardens, etc., combine the lens and mirror in a single glass prism with curved faces where the light enters and leaves it, and with a plane face where the light is totally reflected within the prism.