VARIOUS TYPES OF Pno.IECrloN APPARATUS.
A remarkably large number of different devices and mechanical movements have been proposed from time to time for projecting the film. In many cases, practically the same mechanism is used for both taking and pro jection, or the two functions are sometimes served by the one apparatus. The object to be attained, generally speaking, is that the film, travelling at a rapid rate, shall have periodical intervals of rest for the purpose of exhibiting each picture as it comes before the lens ; while during the period of motion, as the next picture is brought into position, the light is cut off by means of a shutter. All this has to be done so quickly that no interval is perceived between the pictures, nor any inter ruption of illumination. This, at least, is the principle employed in the majority of modern cinematograph apparatus, although many at tempts have been made to dispense with the shutter, so securing a greater percentage of light, or to avoid the intermittent movement of the film. It will be remembered that in the Praxinoseope (.see p. 6G4), by means of mirrors,
revolving in unison with the band of pictures, an uninterrupted moving image was viewed without the intervention of any slit or shutter, as in the Zoetrope. The same idea has been adapted in several ways for projection purposes, although, at present, such devices are un necessarily complex and involve the use of too many mirrors, with consequent loss of light.
A battery of projection lenses revolving in unison with the film has also been employed, but is evidently too expensive an expedient for general use. In some patterns of apparatus, however, the shutter is dispensed with by giving a comparatively long interval of rest to the film, coupled with an extremely rapid changing movement. This is satisfactory with the average film, but if the latter happens to be specially light or transparent, sufficient illum ination is passed during the period of change from one picture to another to cause a "rainy" appearance, or flickering.