The projection machines are essentially of the same type as the camera ; that is, they involve an intermittent movement of the film during which the light is cut off by a shutter, but this inter mittent movement is usually accomplished by means of what is known as the Maltese cross or Geneva movement (see fig. 3). This moves intermittently when the operating wheel carrying the pin which enters the cross is moved continuously, and the move ment has the advantage over the claw and cam system that it is very robust and can be made to work immersed in oil.
The shutter usually has two blades, one of which is used to cut off the light during the movement of the film, the other interrupt ing the light, while the film is still, thus diminishing the flicker seen on the screen. Before the introduction of sound, films were projected at a faster rate than that at which they were taken in the camera. sound pictures are projected at the rate at which they are taken, i.e., 24 pictures a second or 9oft. a minute. The light sources used for projection are chiefly arc lamps. In modern theatres it is usual to employ arcs having carbons loaded with chem icals, in conjunction with a mir ror, the crater of the arc being turned away from the gate and the light from it thrown forward by a large concave reflector. For projection at a short distance or with small screens good results are obtained by the use of high intensity incandescent lamps.
A great deal of work has been done on continuous projectors in which the film moves through the gate continuously and the image is kept stationary upon the screen by means of some optical sys tem of rectification, such as a rotating ring of lenses which compensate for the movement of the film, or rotating prisms or mirrors which hold the image still. These machines have not come into general use though some of them appear to be satisfactory mechanically. The sound reproducing equillment is placed below the picture projection mechanism but on the same base with it. Volume controls are provided between the sound reproducer and the amplifiers so the volume can be controlled by the projection ist. This can also be extended to the auditorium and used by the manager.
The vacuum tube amplifiers have a very large power capacity. The loud speakers which are placed behind the screen are also very large, the latest type being i 2ft. square and approximately 4ft. deep. This equipment is required to fill a large auditorium with sufficient sound to give realism to scenes, particularly where they involve earthquakes and tornadoes, which must be very loud to be natural. A perforated picture screen is used in front of the loud speakers, the sound being projected through the perforations.
After projection, the film must be rewound, examined carefully, and all damaged or weak places repaired.
The apparatus used for amateur cinematography generally em ploys film narrower than the standard width. The widths in gen eral use are 8mm. and 16 millimetre. The Eastman Kodak Com pany introduced i6mm. film in 1923, and 8mm. film in 1932. They and other manufacturers have placed on the market cam eras and projectors adapted to take these small sizes of film. The narrow film is supplied in magazines or on spools for daylight load ing, and after exposure it is processed by means of a reversal process which transforms the negative directly into a positive ready for projection. The reversal process gives to the original prints very great freedom from grain, so that when projected they are entirely satisfactory upon the screen. Duplicates can be made from the original positives, standard size film can be reduced to the small size, and the small film can be enlarged to give pictures of standard size, although in doing this a certain amount of graininess in the resulting print is inevitable.
Only a very small amount of sub-standard film is developed to a negative which is used to make prints by contact in the way normal for standard film. In the case of i6mm. film the area of the pictures is only about one-sixth of that of standard film, and that of 8mm. film about one-twenty-fifth. The film for 8mm. pictures is made i6mm. wide and so can be developed on the same machines as are used for normal i6mm. pictures. In the camera, one-half of the film is used the first time it is run through. Then the spool of film is replaced in a reversed posi tion in the camera and the other half exposed. After processing, the film is slit up the middle and the two parts joined end to end to give a continuous film with a row of perforations down one side only.