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B a u s c h & L o ni b Optica 1 Co m p a n y The lens for projecting the image of the opaque object is mounted on top of the dark chamber, together with a first-surface mirror which directs the light toward the screen and causes the picture to appear in correct position from left to right. The lens is either a 4-inch diameter, of 15 or 18 inches focus, or a 25-inch focus, with a diameter of 5 inches, the selection depending on the projection distance and the size of picture desired (see table, page 18). The mirror supplied with the 15 and 18-inch lenses is fixed above the lens in an adjustable mount, which permits the images from both opaque object and lantern slide to be brought into coincidence on the screen. The 25-inch lens is mounted in the horizontal position with the mirror behind it and enclosed in the mounting. No adjustment of the mirror is required with this lens. A lens is supplied for lantern slide projection which is of the proper focal length to give approximately the same size picture as that secured with the projection lens for opaque objects. For English Classes The projection lantern has recently been used to great advantage in English classes to project on the screen the students’ compositions. In this way the work can be read and criticized by the entire class. To meet such a requirement with this instrument we have cut the regular opening for opaque objects in a plate which can be taken out by removing four screws. An opening 6x7 inches is thus rendered available for projecting composition sheets. The plate is supplied in all instruments of this model and may be removed whenever the need arises. Since this lantern is also an excellent one for Departments of History and Language, as well as the auditorium, and is easily taken from one room to another, the instrument should find almost continuous use. Specifications Base —Consists of heavy sheet metal supports, front and rear, 10/4 in. wide, and carrying dark chamber at height of 5X in.; extreme length, 23>4 in.; each foot is provided with screw hole for fastening Balopticon to table, if desired. Lamp House and Dark Chamber —One continuous with the other; measure 9/4 in. wide by 15 in. long; lamp house is 17 in. high; dark chamber, 12^-in.; light-tight and freely ventilated, constructed of double sheet metal walls, with an air space one inch in thickness between the two walls; roof of lamp house is fitted with our special, patented ventilator; dark chamber provided with large, light-tight door on side; convenient handle on outside of dark chamber at upper right corner controls shield by which light is cut off from opaque object lens or lantern slide lens, as desired. Illuminant —1,000-watt, concentrated filament, gas-filled, Mazda stereopticon lamp, of special design; provided with ground and polished glass reflector, 8 in. in diam., corrected to throw a parallel beam of light upon the specimen. 52