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July, 1934] HIGH-INTENSITY ARC AND LIGHT the eye even without film in the machine. In other words, the more nearly identical in amount and wave-shape the light on the screen is during each shutter opening, the less discernible would be the fluctua- tions or beats in the light on the screen. It can be seen in Fig. 7 that the amount of light on the screen from one shutter opening to the next varies enormously in the case of th low-intensity a-c. arc. This is due directly to the fact that, as illus- trated in Fig. 6, during the half cycle that the carbon in focus is nega- tive the light remains at a comparatively low value. On the oth hand the light from the high-intensity a-c. arc, although of irregular wave-form, is much more nearly constant for each shutter opening and corresponds much more closely to the curve of the light obtained from the d-c. arc operating on rectified current with the shutter rui is the reason for the very decided improvement in steadiness over the low-intensity a-c. arc, and for the practical elimination, under favorable conditions, of noticeable fluctuation in the light on t screen from the high-intensity a-c. arc. In other words, the light- curve of the high-intensity a-c. arc, being of nearly the same inten- sity during each half of the current cycle, has practically the same fluctuation characteristic as that of the low-intensity d-c. arc oper- ated by a rectifier. The practical proof of this is obtained by observ- ing the projection of the same amount of light from the three types of arcs with the shutter running. This has been done in the laboratory with a very high degree of illumination on the screen and without film in the projector. Observation under such condi- tions agrees with the conclusions arrived at from the oscillograms : namely, that the light beat or fluctuation of the high-intensity a-c. arc is very much less than that of the low-intensity a-c. arc and of essentially the same magnitude as that of the low-intensity d-c. arc using neutral cored carbons and operated by a rectifier. From the standpoint of practical projection, the beat or fluctuation of light is so small that under reasonable conditions it can not be con- sidered detrimental to the quality of the picture. This was demon- strated at a recent meeting of the S. M. P. E. Projection Practice Com- mittee, when pictures were projected by a high-intensity a-c. arc. One member of the Committee remarked at the time that if he could not see the light-beat or fluctuation, it was not there so far as he was concerned. Factors other than the light source itself exert an influence on the