International projectionist (Oct 1931-Sept 1933)

10 INTERNATIONAL PROJECTIONIST December 1931 Cffoss-m/Rs FOC/9L .Point. TO SCREEN ^^ ^^^5S^^ Fig. 3. Sectional plan view, greatly exaggerated. Tilting the prism with respect to the optical axis moves the image laterally on the screen. Very little of this adjustment is required. Adjusting the prism with respect to the radial position around pivot screw moves the image up or down. These two adjustments suffice to make the prisms register with great precision prisms register with great precision when they are indexed. Each prism may be easily adjusted to make the image of cross-hairs register to within one-hundredth of an inch upon the screen, or even closer. In fact, this precision of the registration is only limited by the precision of the dividing head used for indexing. And assuming uniform motions in the prism wheels and in the film, this will also determine the precision of the registration of the successive images upon the screen. The really important thing, however, is that when the prisms are once indexed and fixed in place, the precision of their registration becomes permanent and cannot vary because of wear; otherwise such precision would be quickly lost. A word should here be added about film shrinkage. In every continuous cinematograph means for correcting or compensating its effects should be provided. In our rhomboidal-prism cinematograph this means is very simple and effective. A thin lens is interposed between the compensating prisms and the film; and by adjusting this lens the film can be magnified up to its proper size. The amount of magnification necessary is very little, for although film shrinkage is measurable in a foot of film, in one or two frames it is almost inappreciable. By using a variable speed drive for the film, it can be shown that the frames move downward on the screen if the velocity of the film be too slow, and upward if it be too fast. Synchronism of the film photographs with the compensating elements is, therefore, necessary to maintain the picture in frame. The factors that enter into the registration of the continuous cinematograph are four. And it can be proved that if there be synchronism, a uniform motion in the film, and uniform motions in the compensating elements, the registration of the successive images will depend entirely upon the precision with which the compensating elements were adjusted or indexed. The projectionist will now see why precision in motion picture projection has been advanced to a new high. It is a precision of so high an order that the factor of wear must not be allowed to enter into it, else it were futile and a waste of effort. But registration is now upon a much higher plane. It concerns itself only with uniform motions and no longer deals with wear. Wherever wear occurs in the mechanism, it is of no particular concern as long as it does not affect the uniformity of the motions. And, fortunately, the wear in a continuous cinematograph does not: for parts such as the spiral gears, which revolve the compensating wheels in opposite directions, may be worn thin without affecting the uniformity of the motions. The Gear Train These right and left-hand spiral gears constitute the heart of the mechanism. In the simplest form of the device, the sprocket is mounted on the shaft which carries the right and left-hand spiral pinions, and this reduces the mechanism to but three moving parts, all of which revolve at unusually low velocities. The wear on gear teeth depends largely upon the pressure exerted by them, or the amount of work they do; and it will be seen that the work in the continuous cinematograph is very light — turning over two freely revolving compensating wheels which, once set in motion, require but a minimum of effort to keep them revolving. The outstanding feature of this construction is, therefore, its extraordinary capacity for maintaining the precision. Wear is thus eliminated as a factor in the mechanism of the continuous cinematograph. The foregoing considerations are sufficient to show that if a uniform motion be imparted to the film, the very summit and peak of precision in the registration of the successive images can be attained — and, what is of more importance, maintained throughout a long period of time. In recording and reproducing sound, many of our technicians have become familiar with the advantages of a uniform motion in the film, and have directed their efforts toward obtaining it. Impedance rollers and heavy fly-wheels are responsible for a technic that approaches the theoretical limit of perfection in imparting a uniform motion to the film. And the thing to be noted is, that it is accomplished without the slightest dependence upon the accuracy of the perforations. Wear and inaccuracies are thus eliminated as factors in the perforations of the film. The perforations become of minor or secondary importance; and accuracy is no longer their predominant requirement. Their chief office is now to prevent the image from creeping out of frame. For this purpose only one row of perforations is necessary, and their size, shape, and number, may be varied greatly without affecting the results. It naturally follows that those technicians who have learned how to impart a uniform motion to the film, and who appreciate what this means in precision, will be eager to see the registration of the successive images in the camera, in the printing machine, and finally upon the screen, likewise placed upon this much more accurate and scientific principle. And when they have reached thi& stage, we hold they have become uniformmotion-minded, a harmless distemper that is destined to become epidemic. A Representative Opinion But with every new art, there is a long struggle to overcome the effects of prejudice or propaganda and obtain a foothold. And those of us who have endeavored for years to create an interest in the fascinating art of continuous cinematography know what this inertia amounts to; for the greater the mass of an industry, the greater becomes its inertia. Every new movement meets with inertia, however, and must overcome it. The following is an example of what has to be overcome in the average engineer, and it is given here because it represents very accurately the attitude of the motion picture industry itself with regard to the continuous cinematograph. About a year ago we happened to write to the secretary of a society of engineers upon the subject of continuous projectors, and he replied in part as follows: "Frankly I have no faith in a continuous projector, although this is merely my personal opinion, which is the result of having inspected quite a number of them .... Even if a continuous projector could be made perfect in all respects, what of it? What would it do or offer in the way of advantages to offset the high x--!-^ Fig. 4. The displacements have no effect upon the image