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Bausch & Lomb Optical Company the large diameter is obvious when it is considered that the illumination a lens provides is directly proportional to the square of the ratio between its diameter and its focus. The lens for lantern slide projection is of 134-inch diameter and either 10 or 12-inch focus, as required to produce a picture from a lantern slide approximately equal in size to that projected from the opaque object. Both lenses are provided with a rack and pinion adjustment for focusing. For illuminant a 90-degree hand-feed arc lamp is provided, which can be used on either direct or alternating current. When using 25 to 35 amperes of electricity and operating on direct current, a picture up to 11 or 12 feet wide from an opaque object can be satisfactorily illuminated. With alternating current, a picture exceeding 8 or 9 feet wide should not be attempted. Specifications Base —Consists of heavy sheet metal supports, front and rear, 8 in. wide and carrying dark chamber at height of 9/4 in. Lamp House —Measures 1334 in. long, 15 in. high and 7/4 in. wide; light- tight and freely ventilated, constructed of double sheet metal walls, with an air space between the two walls and the roof fitted with our special patented ventilator; provided with large, light-tight spring door on the side and obser- vation windows on both sides; mounted between horizontal supports at front end and provided with handle at rear, permitting it to be easily tilted for por- jection of opaque objects and held rigidly in position by spring arm; conforms to the most rigorous requirements of Boards of Underwriters. Illuminant — Hand-feed arc lamp for direct or alternating current. Condensing System —Our regular triple system, with the two rear lenses mounted directly in front of the lamp house in our ventilated mount, giving a parallel beam of light in the dark chamber, and the front lens placed in front of the dark chamber immediately behind the slide carrier; diameter, 4/4 in. Dark Chamber —Of sheet metal, light-tight, measuring 12/4 x 11/4 x7 in.; has opening in bottom for projected objects 5 in. square; provided with hinged door and observation window on right side. Object Holder —Of square sheet metal mounted on arm controlled by two strong springs; will accommodate objects of varying thickness, and automat- ically bring object into proper plane of projection. *Slide Carrier— Our double carrier, No. 4430, with elevating device. Post Card Carrier —Two adjustable carriers with wooden backs and frame, which fit opaque object holder. *The quick-changing slide carrier, No. 4449, giving a dissolving effect, may be sub- stituted for $1.50 extra. 56