No Pictures Move

Dominion, Volume 33, Issue 282, 23 August 1940, Page 6

No Pictures Move

How The Entertaining Optical Illusion Is Caused

It may surprise many people to know that such a thing as a, "moving picture” does not exist. .What is actually seen on the screen is the result of an optical illusion, or to put in even more familiar terms, the result of a trick. The basic principle of the motion picture, sound or silent, is this: it is established that if any object be looked at, the image of the object remains in the brain, and is "seen” for about one-twentieth of a second, after the object has been removed from sight. It is because of this that the ‘‘moving picture” is possible .

'THE film with its small photographs is passed between a powerful light and the aperture of a system of lenses at a speed of 24 complete pictures a second. Each little photograph remains on the screen for one-twenty-fourth of a second. It is then shut off by a revolving shutter as it moves away from the aperture, and the following one takes its place. The shutter prevents any changing of the picture from showing on the screen. The mechanism of the morion picture projector is a piece of precision machinery made up of spool boxes, a gate, automatic cut-off, the sprockets, intermittent action —which is the heart of the motoin picture machine —and the shutter. Film’s Jerky Movement.

The shutter exposes each one of the small photographs of the film for a fraction of a second to the rays coming from a lamp house, it cuts the light off from the screen during the changing of the picture, and turns the light on again while the picture is at rest in the gate. The shutter may be either a rotary disc or the barrel tvpe. The gate is the aperture in the direct path of the light emitted from the condenser lens. It is here, as the intermittently moving film passes, that tlie picture is illuminated and the film held fiat, so that the image on the screen may be focussed. The sprockets are toothed wheels. The top one leads the film from the upper spool box to the gate. An intermittent sprocket pulls the film through the gate, and the bottom sprocket carries the film to the lower spool box. The intermittent action for securing the correct “pull down” of the film to the aperture is the most vital part of the mechanism. The shutters is divided into two or three sectors, only one being really essential. The gearing of the machine is so arranged that during the interval of time in which the shutter passes before the gate and cuts off the light, the intermittent action simultaneously jerks one picture away and the succeeding one into its place. Therefore, no movement of the film reaches the screen. In the lamp house is generated the light that is focused through the film on to the screen, and is the actual picture seen. The lamp house might be either an arc type or an incandescent lamp type. The incandescent lamp is used mostly for portable work, being much cheaper, ■ lighter in weight and suitable for small auditoriums. Its great advantage is its efficiency with lower power. The incandescent lamps vary between 100 and 1000 watts.

The arc lamp is universally used iu all permanent installations in theatres seating about 1000 persons and over. Essentially the arc lamp consists of two carbon rods about 12in. long and Siu. diameter through which a very heavy current passes. A carbon vapour now fills the space between the tips and burns and the tips of the carbons become the source of light. It is reflected from a spherical mirror mounted behind it through on to the gate. We now come to the sound head. This unit, usually built separately, is mounted underneath the intermittent action. Once the film enters the sound

head the following sequence of events takes place. As the film passes down from the sound gate, a beam of light from a special lamp is concentrated by an optical lens system, and an aperture containing a slit which brings the light to focus as a fine line, across the sound trade. It is essential here that the film should move at a uniform speed to avoid what might be called “flutter’' of the sound. This is accomplished as follows: As the film leaves the intermittent movement, the intermittent action is cancelled out by a power-driven sprocket. In the sound head are more sprockets and tension rollers, which are carefully balanced by fly-wheels, weights and springs. Tims the film moves at a perfectly steady rate through the sound head.

As the sound gate is ID J pictures below the picture gate, the sound recordings are purposely printed on the film 19J pictures in advance of the corresponding picture, so that .the sound track and picture will reach their respective gates at the same time, and the sound will be heard at the same time as the picture appears.

The last link is the screen. In certain theatres, as also for amateur use where loudspeakers are not mounted directly behind the screen, the type of screen most favoured is the glassbeaded reflective type. The beaded screen surface is composed of a layer of small glass beads. These have been sprayed evenly over a sheet of fabric that'produces high reflection without glare, thus avoiding eye-strain. A general type of sound screen iu use consists of a loosely-woven and surfaced fabric with openings of uniform spacing. Various types of metals have been tried, but these have proved very unsatisfactory, the usual fault being rapid discolouration. White-Dot Signal. A complete film today may consist of 10 or more 1000-foot reels, and as these cannot all be accommodated on one projector, two or more are used, and frequent “change-overs” are necessary. The films are usually wound on to 2000 foot spools, and when changing over from one projector to another the aim naturally is to make the change unnoticeable to the spectators. About 11 feet from the end of the spool a white dot appears in the top, righthand corner of the screen. The audience as well as the operator may see it. This is the cue for the operator to start up the second projector, in which the second part of the film has been threaded. It has 11 feet of leader strip, or blank film, and while running through this gives the projector motor a chance to attain correct running speed. „ , „ . At the end of the 11 feet of the first reel another dot appears, and this is the signal for the operator to shut down number one and start up number two. This is done by two synchronized shutters that shut out the light from number one and open the light for number two. At the same the operator swings a “fader” round > to change the sound input to the main amplifier. The shutter changeover is usually done mechanically. I