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36 D. B. JOY AND E. R. GEIB [J. S. M. P. E. The lamps using this carbon ordinarily have a horizontal trim with the carbon tip facing the back of the lamp, in focus with an elliptical reflector that projects the light picked up from the carbon and arc to the aperture and film. These lamps are provided with a device that throws an image of the arc on a white card, on which the correct arc length and arc position with respect to the mirror are indicated. They also have a method of feeding and adjusting the arc that keeps the carbons in the proper position with respect to each other and the optical system, provided the lamp is operated properly and the projectionist pays a reasonable amount of attention to the arc image. The following illustrations show what happens to the light on the screen when the arc is not kept in the correct focal position, and indi- HALF SIZE OBJECTIVE LENS 6 '/z EFFECTIVE FOCUS FIG. 1. Diagram of optical arrangement used in the test. cate the leeway that the projectionist has in that respect in main- taining a reasonably uniform light on the projection screen. The curves were made by using one of the latest types of high-intensity a-c. projection lamps with an optical arrangement as shown in Fig. 1. If the magnification of the mirror, the size of the carbon, or the objective lens is changed, the shapes of the curves will remain essen- tially the same, but the relative values will be altered. Fig. 2 shows the effect on the screen light of keeping the arc length and the current constant but changing the position of the arc with respect to the mirror. As the arc is moved from the position of maximum screen light (C) toward the reflector, the total light on the screen decreases rapidly, the light at the corners of the screen in- creases in intensity, and the light at the center of the screen decreases in intensity; until, at the position A the light at the sides and corners