This is a kind of double-bodied camera, one part of which * is provided with holders for the negatives, and has no lens; the other portion has a lens which can be moved so as to approach or recede from the negative, and has the usual ground-glass plate. The negative must be placed in its holder, screw on the lens, and adjust the lens to its proper distance from the negative. If the copy is wished to be larger than the negative, approach the lens to it, and farther from the ground glass; if it is required to be smaller, remove the negative farther from the lens. The light passing through the negative will show its image on the ground glass in the usual way. When the desired size is obtained, remove the ground glass, and replace by a frame having an ordinary wet sensitised plate. Use a diaphragm, with a small stop, and proceed as if for an ordinary negative. It is necessary sometimes, when considerable enlargements are required, to use a camera with a long body opening out like an accordion; the operations are similar when once the proper focal distances have been adjusted. The power of a lens is determined by its shape and diameter.

The larger it is in diameter, the more light it will admit; whilst the degree of curvature it has regulates its focal length, and determines the size of the image it will produce. It is the focal length of a lens, and not its diameter, which regulates the size of the image; and the distance it has to be from the ground glass screen determines the length of body required in the copying camera.

## Calculating Length Of Camera Required

Calculate the distance the ground glass must be from the back lens, thus: multiply the focal length of the lens used by the number of times of enlargement required, add the focal length to the product. The focus of a quarter-plate lens is generally 6 in.

Say the negative is to be enlarged 3 times, 6 x 3 = 18 + 6 in. focal length = 24 in., the distance required between the ground glass and the lens. The distance the negative is to be in front of the lens is always more than the focal length, but less than twice the focal length.