To ensure a good display of cinematograph pictures, many exacting conditions must be met and satisfied. Of course, given the combination of an intelligent operator, with perfect apparatus and films, only bad luck can mar the exhibition. Excluding the operator the main points are as follows. The lantern part of the projector can be considered simply as a projection lantern, i.e., a good, steady light must be maintained, no matter what the supply may be, and the pictures, etc., must be sharply focussed on the screen. The screen must be in good order and condition, also clean and hang vertically all over. All lenses in the apparatus must be bright and clean. The film must be in good order and condition, i.e., not torn, scratched, damaged or patched in any way, and must also have been properly exposed and developed when made. It should be understood that working of cinematograph displays require that each picture be got into its proper position in the lantern (while the light is prevented from reaching the screen), the light is then allowed to pass through it to the screen, the light is then intercepted by a shutter and that picture replaced by the next, and so on. The number of pictures shown per second is in ordinary work 16 to 18, this means that each picture has to arrive in its proper place, stop while the picture is shown, and move off to make room for the next picture, all in about one sixteenth of a second. It is readily seen that accurate work is called for. Now in practically all cinematograph projectors the film is carried downwards from a reel fixed above the centre of the projector, and it is fed downwards at a steady uniform rate out of the reel box. It is also wound, after exposure, into another reel box at a steady rate, but between these two reels there is of course the exposure position in the projector, and in that position the film motion is not uniformly steady, but, as explained above, it has to remain at rest while the light passes through it. The result of this requirement is that some arrangement had to be devised so that the film was not broken or jerked while it stopped, although the film flow out of one reel box and into another was constant. By reference to Fig. 39 it will be seen that this difficulty has been overcome by allowing a loop in the film both before it enters the exposure position, and after it leaves that position. These loops vary during the run of the film, within certain limits, but effect the desired result. The toothed roller which draws the film out of the reel revolves uniformly, while the roller which draws the film through the exposure position has an intermittent action. As the electric motor driving the whole mechanism, or the operator's hand (if so worked) has a uniform circular motion, some means had to be devised to introduce the required intermittent motion; one of the most commonly used means is termed the Maltese Cross Action, so named from the shape of one of the wheels concerned.

The mechanism consists practically of two wheels only, both set in the same plane. The driving wheel runs at constant speed, it has a piece cut out of its edge in one place only, and this cut away piece leaves a space in which the other wheel can just find room to revolve, when permitted. The two wheels are set apart at a less distance than the sum of their radia. Now in the driven wheel there are cut four slots at right angles, but not reaching to the centre. The metal at the edge of this wheel is cut away so that when the two wheels are in their working positions the driving wheel can still revolve. This driving wheel has a pin so fixed, just outside its circumference, which pin can slide into the slots cut in the second or driven wheel. The result is that every time the pin comes round it engages into a slot in the second wheel and turns it one quarter of a revolution. The pin is so placed that while it turns the second wheel, the cut out space in the driving wheel provides room for the driven-wheel to turn in. When the pin has done this, it proceeds with the driving wheel to make another revolution, and during this time the driven wheel cannot revolve since it is held by the edge of the driving wheel, fitting into the cut-away position of its circumference. The driven wheel from its shape has given this action its name. Now during the time that the driven or Maltese Cross wheel moves, the light must be cut off from the screen, and this is done by the revolving shutter somewhat like a fan which is revolved in step with the pictures. One other point necessary to watch is to ensure that the centre of the picture on the film itself is in the centre of the screen picture, it can happen that it may be anywhere on a straight line from the centre to the edge of the screen picture, means are provided on all projectors, to swing the picture, termed " centreing of picture to the mask." The following descriptions of the Empire Educational Cinematograph Projector, with operating instructions, together with Figs. of a picture theatre instrument showing the similarity in principles and main design, should thoroughly remove any difficulty that may be met in operating or overhauling this type of machine.

For standard size film and electric light (full Patents applied for). Specially constructed portable mode for schools, colleges, technical establishments and commercial purposes.