THE DARK SLIDE.
The dark slide should be a double one, and its size will vary with the size of the ledge of joinery. The base board is 36 in. long, b,1 in. wide, and in. thick. Upon it is fixed a wooden framework with grooves on the top, on which the camera slides. The camera is IS in. long, in. deep, and 61 in. wide ; in the front is a circular opening covered by a short tube for connecting with the microscope ; at the back is an opening 4 in. x 3 in. with grooves up each side, in which the dark slide rests ; there is a piece of wood at the bottom and a hinge[l flap subsequently mentioned for producing two on the same plate. The microscope and the lamp are fixed on a separate board, which revolves on a pivot, and can be clamped with a thumbscrew in any position ; this is for the purpose of focussing preparatory to connecting up with the camera. The light in this case is the incandescent gas light or a paraffin lamp. When a low camera, either quarter, half, or whole plate. If the slide is larger than quarter plate it should be provided with carriers to take the smaller sizes of plates. The mahogany dark slides are better than the metal ones, as the slides usually draw out more easily, and there is therefore less vibration imparted to the apparatus ; if they are at all stiff a little blackhead should be rubbed on the grooves. It is
not often that a picture larger than quarter plate is required ; in fact, for most objects a smaller plate even than that could be used with advantage. A simple device for taking two pictures upon a quarter plate may be made as follows : In the back of the camera (Fig. 736) a piece of wood is fixed upon two hinges ; when raised this brings the bottom half of a quarter plate dark slide into the centre. An opaque slide for the back of the camera is then cut in sheet zinc, and a rectangle 3 in. x 2 in. is cut out of this, so that the opening thus made corresponds with the bottom half of the photographic plate. After the first exposure the slide is opened in the dark room, and the sensitive plate turned round so that in the second exposure it will also be in front of the opening in the zinc plate. With such a device two pictures