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properly rolled up at the start or, at most, push in the bits of film sticking out, only in the case of a quite loose roll. Throughout all operations it should always be remembered that film is exceedingly sensitive material.
Now as to projection. The projector, like all machinery, needs most careful looking after. It must be properly oiled at the requisite intervals, not too much, or oil will drop on the film and spot it, nor too little, lest the bearings seize or the sprockets press too hard and damage the perforations. All worn parts, especially sprocket-teeth, must be at once renewed. If the film is spotted with oil, it may be cleaned with a leather rag dipped in carbon tetrachloride. The film must be very carefully put into the projector and the gates on either side the aperture plate must be of the right size, for if they are worn, the film may easily tear. Besides, they will strike against the box or other parts of the projector and scratch the film. Special attention must be paid to this point when using box projectors. The film must also be guided properly over the sprocket-teeth. If it is askew or if the teeth do not properly take the perforation, the latter may easily be worn so as to become of irregular size. Within the projector the film is carried between two gates. It is held firmly fixed within the aperture plate in order that the picture may be absolutely straight; the film is carried over sprocketteeth and thence conveyed in some projectors by the intermittent sprocket, in others by catches. At all these points the film is exposed to scratching. With standard film projectors the pressure in the gate aperture is so arranged that the pictures stand straight at the normal projection speed. If this speed is changed, the gate pressure must also be changed, by spring action. A special danger here is an excessive speed of projection; this results in excessive intermittent motion, whereby the film is unduly strained and the perforation especially suffers. If the title text is too short, the operator will often try by intermittent motion to reduce the speed of projection so as to project the titles for a longer time. Needless to say, this is extremely bad for the film. Substandard film is projected more slowly (16 mm. film moves at the rate of 7,3 metres per minute as compared with 17,3 metres per minute for 35 mm. film) and the question of gate-pressure regulation does not arise. Other causes of overstrain upon the film while being projected are undue friction in the winding process, dirt in the magazine and irregular feeding.
A film copy, when fresh, is soft, but gradually becomes harder with the hardening of the coating of gelatine, as this dries under the influence of heat from the lamp. In the absence of special precautions a perfectly fresh copy, when passed through a projector, would be seriously damaged by the metal rollers upon which the film runs. For this reason every fresh film should for the first few times be projected through an apparatus containing velvet rollers, before being projected normally. This is the only way of preventing bits of fresh film being broken off and becoming lodged in the rollers or in the gate aperture and so scratching the film after frequent pro