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82 MOVING PICTURES chap. to the exposure hole is corrected. The mask also determines the shape of the positive picture. It may be rectangular, oval, circular, or have rounded corners as desired. Imme- diately behind the film, in line with the exposing aperture, is a red screen, over which a shutter slides. When this shutter is opened, the operator can see the negative image through the positive film, and thus can ascertain that the position of the picture is correct in relation to the exposure aperture, and also that the perforations on the two films are in synchrony. The light by which the exposure is made is contained in a light-tight box, or may be placed on the outside of the wall of the printing room, an aperture being cut in the partition to admit the light to the printing apparatus, which is screwed to the wall. Between the light and the film there is a revolving shutter, as in the camera; which cuts off the light intermittently, enabling the suc- ceeding negative image and area of positive film to be brought into position before the exposure orifice. The film is drawn through the machine by a pair of fingers or claws which engage with the perforations of the two films and pull them downwards together. As the ; pictures are printed, the negative and positive films pass into boxes or other suitable receptacles to be re-wound on their respective spools. The machine can be operated either by hand or power according to the requirements of the establishment. As the sensitised surface of the positive film is slower than that of the negative, printing is carried out at a reduced speed, the average recommended with this apparatus being about five pictures per second. Another printing machine has been recently produced which is regarded as the simplest, most mechanically perfect, most compact, and self-contained apparatus yet devised for this work, and which compels attention as much from its efficiency as from the point of view of mechanical excellence. This is the Williamson printer. It is a complete unit, and can be moved from place to place with facility. It comprises a large rectangular box or cabinet, standing on one end. The front face carries the printing mechanism,