Motography (Jan-Jun 1913)

226 MOTOGRAPHY Vol. IX, No. 7 the film, for example, were perfectly black, it would obstruct all of the light in its path so that its shadow would be produced at D, on the screen. In the same way the outlines of the lighter objects on the same film would be shown in proportion to their density. In passing from the film to the objective lens P, the converging lines of light cross the optical center Fig. 8.— The Barrel Type Shutter. line {X-X) at a point midway between the two lenses that form the objective P. From the point of intersection, the lines of light again spread out or diverge, but in such a way that the lines that were formerly at the top of the image on the film are now thrown on the bottom of the screen. The reason of the inversion of the image on the film will be seen if the line reaching from the top of the arrow B is followed to the bottom of the screen, and the lower end of the arrow D. Since the angle of the light rays reaching from B to the center of P is the same as that reaching from P to D it is evident that D is as much larger than P as it is farther from the point of intersection. It will be noted that the center of the condenser is on the same center line as the center of the objective lens. It is evident that the film must stop centrally on the center line of the lens X-X if the image is to be equally distributed on the screen S, without distortion. The film mechanism is arranged so that each picture is centered on the optical center line at the moment that the shutter is opened. To prevent light from passing around the edges of the picture on the film and to steady the film, a small plate with an opening equal to the size of the picture is placed centrally on the line X-X at G. This is known as the film "gate." As the area of the condenser lens is much less than the superficial area of the sphere of light surrounding the arc P, about 90 per cent of the light is lost by absorbtion by the walls of the lamp house, hence only 10 per cent is effective at the condenser lens for projection. In the future the projector will, no doubt, be provided with a reflector that will concentrate and throw the light on the condenser where it belongs. When this is accomplished, it will be possible to secure brighter pictures with less expenditure of current. THE MOTION HEAD. The part of the projector that contains the film mechanism is known as the "motion head" and is entirely independent of the optical system except that the head sometimes affords a support for the objective lens. In the casing of the "head" is the shutter, the intermittent film feed, the two film magazines, and the safety shutter. At the side of the housing is the operating crank for driving the machine. The principal elements of the motion head are shown in their usual relation by Fig. 7. The light from the condenser C passes through the film F, the gate G, the objective lens O, and the shutter D to the screen at E. The sprocket 6" engages with the perforations in the film and draws the fresh film from the reel R. A roller V keeps the film in engagement with the sprocket and controls a loose loop L known as the "takeup" loop. This loop prevents excessive strain from being thrown into the film by the intermittent feed. From the loop, the film is drawn into the film gate G where it is straightened out and put under slight tension by the friction of the tension plate K. The film is drawn through the gate by the sprocket T which is driven by the intermittent movement, the film being kept in contact with the sprocket by the roller U. From the sprocket the film is wound on the reel R. The light passing through the film is periodically interrupted by the shutter D at the time when the film is being pulled through the gate. THE SHUTTER. In nearly all machines the film remains stationary in the gate for four-fifths of the one-sixteenth second taken to pass one picture. One fifth of the total time is taken for the shifting of the film, and therefore represents the time the lens would be covered by the shutter, were the action of the shutter instantaneous. Since a considerable length of time is required for the shutter edge to traverse the width of the light beam, the actual period of darkness is greater than the theoretical time, which of course reduces the value of the screen illumination. To obtain a maximum shutter opening and illumination it is evident that the shutter should close promptly and open promptly at the instant that the picture comes to rest in the gate and when it again starts to move out of the field of the lens. The problem of quick shutter action has led to the development of a multitude of devices, only three of •Fig. 10. — The Geneva Intermittent Movement Used for Feeding the Film. which have survived the tests of practical usage. All of the three shutters, the disc, barrel type, and multiple disc types, are rotary and revolve continuously in fixed relation to the movement of the film feed mechanism. The barrel shutter used in some machines now on the market consists of two parallel blades, that revolve together about a common center, the blades being segments of the surface of a cylinder whose center coincides with the center of rotation as shown by Fig. 8. When the blades A and B are in the position shown, the converging light rays C-C and D-D pass between them to the screen. On rotating the cylinder in the direction