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FILMS, AND THEIR PRODUCTION 189 the printed film, and gives a better gradation of tone. It is estimated that such machines will print about 1,000 feet of film per hour from an ordinary negative. For printing special positives, for example, in which the pictures may have to be printed in a special order, the printing machines have to be specially modified for the particular purposes. Perforating the Positive. —Owing to the fact that in the camera the film is fed by means of perforations, the negative film must be perforated before exposure. In the operations of developing the negative there is a certain shrinkage in the film. Consequently the spacing of the perforations in the finished negative are slightly smaller than in the undeveloped negative. Where intermittent- moving-film printing machines are used, the positive must of necessity be perforated before printing, in order to obtain simultaneous intermittent feed. The perforation gauge for the positive film should, strictly speaking, be very slightly less than the gauge used for the negative film, by just the amount due to shrinkage in the finished negative. When the positive is developed and finished, there will then be an additional slight shrinkage. Where continuous printing machines are used, there is no method of positioning the pictures to the perforations, and it is thus necessary to perforate the film after it is printed and finished. In all such cases, and especially where there is any likelihood of any variation in the spacing between the pictures, as is found to be the case with bichromate printing methods, the provision of means for adjusting the spacing of the perforation is very valuable, and in a still further degree the provision of means for visually inspecting the film as perforation proceeds, so as to be able to adjust the spacing. DEVELOPING THE POSITIVE. The development of the positive film is conducted in the same manner as for the negative film, but the different