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this distribution at a specified voltage can be obtained from the Bureau of Standards, Washington.

Other methods of producing artificial daylight involve the use of blue glasses or gelatine filters before standard illuminants. The most available system of this sort at the present time consists of a No. 78 Wratten photometric filter (97) manufactured by the Eastman Kodak Company and a cylindrical acetylene flame (41) produced by a standard burner (also obtainable from the same

Fig. 8. Spectral Transmission of Tungsten-to-Daylight Filter (Wratten No. 78)

(According to the Wratten Light Filter Booklet.)

company). This combination yields a white closely approximating average noon sunlight, and gives a very satisfactory standard white for practical purposes. Figure 8 shows the spectral transmission of the original No. 78 Wratten filter. It is planned to make the “78” and “86” series of filters in the future a quite accurate means of converting one color temperature to others, the original filters having been only approximate means to this end. The color temperature of the standard acetylene flame is 2360 degrees K., which corresponds with that of a vacuum tungsten filament burned