International projectionist (Oct 1931-Sept 1933)

THE ART OF CONTINUOUS CINEMATOGRAPHY William C. Plank //. The Mechanics of the Continuous Projector MECHANICALLY, the continuous cinematograph is a simple device, there is nothing intricate about it whatsoever, and to project steady motion pictures upon this principle is easy. Steadiness becomes entirely a matter of workmanship, for the principle lends itself to great steadiness. But definition, or quality in the image, is entirely a matter of the optical principle employed. So continuous cinematographs may be very appropriately likened to objective lenses, in that it is a very simple and easy matter to obtain a mediocre image with them, but more difficult and involved to obtain an image having quality. The cardinal requirement in the continuous cinematograph is a rectilinear or straight-line displacement of all the conjugate points. The definition and flatness of field will always be found to depend upon how perfectly this requirement is fulfilled. In the continuous cinematograph devised by the writer (see illustration), the compensating elements are rhomboidal-shaped' prisms which possess the rare and peculiar virtue of satisfying the above condition. In the continuous camera (see Fig. 1), they give a straightline displacement to all the image points. These totally reflecting prisms are revolved in such a manner that their faces are always maintained perpendicular to the optical axis. The axial ray, therefore, always enters and emerges normal 1f-^^-^ ' A four-sided figure having its opposite sides equal, and its angles not right angles; lozengeshaped. Continuous cinematograph to these faces. And in its passage through the two prisms it undergoes total reflection four times, in a manner somewhat similar to that in prism binoculars and other prism instruments. The conditions for definition are, therefore, similar. The axial ray is displaced, not deflected, to compensate for the motion of the film. The advantage to be noted in this is that the compensating principle is independent of the objective lens. The moving film will appear stationary when viewed directly through the prisms, with Figure 1 out the lens in place. Hence, objective lenses of any focal length may be used. Matching the Reflectors Next in importance to the optical principle involved, is the matching of the plurality of compensating elements that go into a continuous cinematography, for this also has a great deal to do with the definition and quality of the image. Here, the reflective systems present an obvious advantage, for it is not difficult to match plane reflectors or optical flats. Any number of them can be matched so that the images reflected by them will match very accurately in size and composition. Needless to say, this is an important requirement in superposing them upon the screen without loss of definition in any part of the projected image. In a reflective element this virtue is not confined to a central portion only but extends to the very edge of the compensating element. Given a compensating principle that is sufficiently perfect to satisfy the requirements as to definition, and a plurality of accurately matched compensating elements, the only problem remaining is to mount the compensating elements and index them with precision. Here again, the rhomboidal prisms evince peculiar and unique advantages. If their faces be maintained perpendicular to the optical axis, they become insensible to every adjustment or movement but one — the adjustment with respect to the radius. This makes it easy to mount and index them. (See Fig. 4.) But a still greater advantage is that the precision or uniformity of the prisms cannot be affected readily in the mounting. The precision or uniformity of the optical intermittent movement depends upon the distance between the two parallel reflecting surfaces of the prisms, which may be ground and polished flat and parallel to within two wave-lengths. It is this precision that is not easily affected in the mounting of the prisms. Precision Registration Tilting a prism with respect to the optical axis (see Fig. 3), will move the projected image laterally upon the screen; and adjusting it with respect to the radical position will move the image up or down. It will be seen, therefore, that these two adjustments suffice to make the