THE EXPOSURE OF PLATES.
The apparatus required for this is the following : A camera with a dark slide, a lens, a camera stand, and a focussing cloth.
The photographic camera is simply a portable form of the camera obscura. It is a box, at one end of which is fixed the lens, whilst at the other there is a piece of ground glass on which falls the inverted image which the lens forms of any brightly-lighted object which is opposite to it. The dark slide is a light-tight case to hold one or more, generally two, sensitive plates, and so constructed that when the ground glass is removed from the camera it (the dark slide) may slide into a groove, and so that after tins, on drawing out a thin slab of wood, the sensitive plate finds itself in the precise position in which the ground glass before was,—the image formed by the lens now of course falling on it in place of on the ground glass.
The lens will be fully described hereafter in a chapter on photographic optics. It may be called the chief of photographic tools. The first experiments may be made with any of the several forms of lenses. Every camera is fitted with an adjustment for altering the distance between the lens and the ground glass or sensitive plate so as to perform the operation known as focussing.
The tripod stand is the three-legged support for the camera, which must be familiar in appearance to all. In most modern tripods the legs slide one half into the other, and the whole packs into a very small space.
The focussing cloth is a piece of square opaque cloth to cover the head of the photographer and shut out the glare of light so as to enable him with ease to examine the image on the ground glass of his camera. The size may be from one yard to a yard and a half square according to the size of the camera. The best material is velvet. Black waterproof cloth is also good, but any cloth which is opaque and black will do very well.
We now commence our first experiment. The smallest size of plates which are sold should be used, as the first results may not be of great use, and the smaller the plates wasted the better. The smallest size of plate com
monly used is five inches by four inches. If the dark slide be constructed for larger plates it may be fitted with a carrier for the size mentioned. Dark slides for dry plates are now always made to hold two plates each, and are called "double dark slides." The dark slide is taken into the dark room, and is placed open on the table opposite the red light. All other light that may be in the room is now extin guished.
The box in which the dry plates are purchased is opened. A plate is taken out. On careful examination it will be seen that this plate is different on the two sides. One side shows the ordinary surface of a sheet of glass, the other is covered with emulsion, and looks something like a ground-glass surface. Care must be taken not to touch with the fingers this last-mentioned side. The plates should be handled by the edges. It is placed in the dark slide with the film side downwards, so that when the shutter of the slide is withdrawn this side will be exposed. The other plate is placed similarly, and the piece of blackened tin which is used with every dark slide goes between the two plates.
It must be explained that there are two ways in which double dark slides are constructed. In the one kind the slide opens on hinges into two divisions like a book, a plate is placed in each division, and the slide is then closed. In the other kind both plates are put into the slide from one side. In this latter case the first plate is placed in film side downwards, the blackened tin-plate is placed on it, and, last of all, the second plate is placed in the slide film side upwards. There is also a third kind, such as in our student and Oxford set cameras. In these the blackened tin-plate is fixed ; consequently the emulsion plates must be placed one on either side, with the film upwards. Whatever the construction of the slide, when it is closed the plates must be in it back to back, and with a piece of blackened tin or paper between them.