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306 KUNZ, GOLDBERG, AND IVES Vol 42, No. 5 The maximum exposure obtainable with this system is about 30 times that given by a simple extended cage-filament lamp of equal wattage used in combination with a diffuser. A 50-w, 115-v projec- tion-type lamp, operating at about 100 v, used in this system pro- vides in 0.14 sec an exposure of 2500 m-c-s, which is sufficient for printing the very low-speed duplicating films. The illumination can be reduced by moving the lamp so as to enlarge the filament image to such a degree that part of it is masked off by the tunnel. To print the faster fine-grain release positive films without disturbing the ad- justment, it is necessary to reduce the illumination by decreasing the lamp current, or by inserting additional diffusers, diaphragms, or light-absorbing screens at the entrance to the tunnel. Relay System in the Bell and Howell Model D Printer.—A sphero-cylindrical relay system suitable for use in a Bell and Howell Model D printer is shown in Fig. 6. As in any relay system, the ob- jective lens should be located as close to the printing aperture as possible to obtain highest efficiency. However, in order to have the whole optical system in a simple lens barrel which can be inserted through an opening in the back of the lamp house, the objective is placed, in the present instance, directly back of the exposure-control aperture. The diameter of this objective is large enough to pass, without vignetting, all light beams necessary to fill the exposure- control aperture at the wide-open setting. The power of the cylindrical condenser lenses LiL 2 depends upon the type of light source used. It must be such that the final image of the lamp filament produced in the vertical meridian by the cylindrical condenser Z>iL 2 in combination with objective L^L& fills the printing aperture completely, with an extra margin for possible misalignment of the lens barrel in the printer. The focal length of the spherical lens Z/4 should be short enough to produce an image of the light source sufficiently large to fill the ex- posure-control aperture in the horizontal meridian, but at the same time long enough to allow the cylindrical lenses LiL 2 to form an image of the light source at D\. The lens L 3 is a weak cylindrical lens supplementing the power of L 4 in the vertical meridian to bring LiL 2 to focus at the exposure- control aperture. The lenses L]L 2 must be of such width that their image covers that aperture and the sizes of L 3 L 4 and D\ are calculated to fill the printing aperture similarly. As shown in Fig. 3, the diaphragm D\ is curved in the horizontal meridian to compensate