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The Camera Obscura
A Catechism of Photography (London: Cassell, Petter, and Galpin, 1859), p.2-3.
Q. What is a camera obscura ?
A. A box fitted with a lens, through which the images of exterior objects are received, and transmitted to a piece of ground glass, placed at the back of the camera.
Q. By whom was the camera invented ?
A. By Giovanni Baptiste Porta, a Neapolitan physician, about two centuries ago.
Q. What suggested the invention ?
A. The discovery that if light were admitted through a small hole into a darkened chamber, all the objects without, from which reflected rays could reach the hole, would be pictured on the opposite wall.
Q. Is a glass or lens necessary to produce this effect?
A. Although images may be received without the lens, they are coni'used and indistinct. A lens is essential for proper definition, and in proportion to the power of the lens is the image clearly defined.
Q. For what purpose was the camera obscura formeiiv employed ?
A. It was used in drawing, especially by landscape and panorama painters, as sketches could thus be obtained with facility and accuracy.
The accompanying figure represents one of these old-fashioned cameras. It consists of an oblong box, into which the rays of light, R, are admitted through the lens B, and form an object on the opposite side, O; but as the rays encounter a glass mirror, M, they change their direction, and the image is formed on the glass screen, N. Upon this a sheet of paper may be placed, and the outline of the image readily traced. A is simply a lap or screen to intercept the light, which would otherwise render the image on the glass invisible. The box consists of two parts, sliding in a groove, and is so arranged for the purpose of obtaining a clearly-defined image whatever may be the distance of the object.
Q. In what way is the camera employed in photography ?
A. The image being clearly defined on the ground glass of the camera, that glass is removed, and its place occupied by the prepared paper or plate, which receives and retains precisely the same image as that which was previously seen on the glass.