Until 1913 the filaments of all Mazda lamps operated in a vacuum. The vacuum serving the purpose of preventing oxidation and at the same time it reduced the energy loss to the least amount. It was found, however, under some conditions of construction that lamps filled with inert gas gave a higher efficiency and more satisfactory service than those of the vacuum type. In this construction, the filament is operated at a temperature much higher than that of the vacuum lamp and as a consequence gives light at a less cost per candlepower. Mazda vacuum lamps are now designated by the General Electric Co. as Mazda B lamps, Fig. 224, and those of the gas-filled variety, Fig. 225, are designated as Mazda C lamps. The filaments of the gas-filled lamps are intensely brilliant and where they come within the line of vision should be screened from the eyes. The high efficiency of these lamps permit the use of opal shades to produce a desired illumination at a rate of cost that compares favorably with the unscreened light of the vacuum lamps.