Projection Apparatus (1917)

Bausch & Lomb Optical Company We use only the 90° type of arc lamp, i. e. with the carbons at right angles to each other, and most of our lamps are so mounted that one carbon (the positive in case of direct current) is in alignment with the optical axis of the apparatus. This is the only form of arc lamp that can be used where accurate centering is absolutely essential to satisfactory results, since it keeps the horizontal crater always in the optical axis, while with other styles of arc lamp the position of the crater with reference to the optical axis would be constantly shifting as the carbons burned Arc lamps operate best on about 50 volts across the arc. Consequently it is necessary to use a resistance, or rheostat, in series with the arc to reduce the line voltage (either 110 or 220 volts) to that required. Rapid strides in illuminating engineering have been made in Gas_FUIed another direction during the past year, with the result that the Mazda Lamps ^ gas .fiU e d, Mazda stereopticon lamps with concentrated filament will replace the arc lamp in many Balopticons designed only for the pro- jection of lantern slides and opaque objects, both on account of simplicity of operation and high degree of efficiency per amount of current consumed. This is particularly true where only alternating current is available, since an arc lamp (especially of high amperage) is not very satisfatory to operate on alternating current and is very low in efficiency compared to one operating on direct current. These new Mazda lamps have an efficiency of from 0.8 watt per candle, in the lower wattage, to 0.55 in the higher. For either lantern slides or opaque objects they are far more effective than the alternating current arc lamp, using approximately one-third the amount of current to produce equal illumination. The Mazdas are, of course, entirely automatic and silent, burning with the stead- iness that characterizes the ordinary incandescent lamp. Furthermore, the light is very pleasing to the eye and gives a very good rendition of color values. These lamps are made in sizes from 250 up to 1,000 watts and are used wit either a portion of the globe silvered or in conjunction with an optically corrected, crlass reflector The reflector is recommended because the unsilvered lamp is Tess expensive to renew. In order to secure the full efficiency of the reflector for lantern slide projection it should be set at a fixed point with reference to the filament and should be mounted to move with the lamp when adjusting the latter with reference to the condensers. See pp. 104-105. The length of life of these lamps cannot be positively stated. They are very carefully made, however, of a much heavier filament than that used in the ordinary illuminating Mazda lamp and should give an average life, according to the manu- facturer, of approximately 200 hours. Even with this as the maximum life of each lamp, however, it will be found upon investigation that the replacement of Mazda lamps is less expensive than the operation of an arc lamp, when the con- sumption of current and carbons is taken into consideration. The acetylene burner, while not equal to some other forms Acetylene q{ non _ electric illuminants, is nevertheless the form generally Burner use( ^ w h e re electricity is not available. Its use has been rendered very convenient by the introduction of charged tanks of acetylene gas, such as 16