Camera Obscura, an instrument invented in the middle ages, according to some by Roger Bacon, and to others by Battista Porta, in which the image of illuminated objects formed by a convex lens is received upon a screen in a darkened chamber, or in a box. It was for centuries used as an amusement, or as a guide in drawing outlines, but by the invention of Daguerre was suddenly invested with new value as the main instrument in photography. The figure represents a camera obscura intended for use in drawing. It consists of a rectangular box made of two parts, one of which slides into the other so that it may be lengthened or shortened, according to the distance of the object. The rays of light from the object pass through a convex lens a, and are reflected from a mirror m, placed at an angle of 45°, upon a horizontal plate of ground glass n, where they form an uninverted image, which may easily be traced with a pencil.