Theory of the Flicker Photometer. 315 



time and use, that this particular alternating method o£ obser- 

 vation was abandoned. Instead, complementary opaque 

 disks wore used as described to interrupt the view of a 

 stationary surface of the best attainable uniformity. 



Fio-. 12. 



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Illustrating spurious critical frequency- log I relations obtainable with 

 mechanically imperfect rotating disks. 30° data obtained with opaque 

 disk of 30° opening exposing fixed white surface. 330° data from 330° 

 white disk having surface irregularities. True characteristics of these 

 openings shown in fig. 6. 



The explanation we believe to lie in a combination of the 

 phenomena of visual acuity with those of perception of 

 flicker. The disk in its imperfectly smoked state had upon 

 its surface various small specks, brush marks and other 

 irregularities. At high illuminations these defects are most 

 visible, calling for a considerable speed to cause their dis- 

 appearance. With decreasing illumination and the falling 

 off of visual acuity these defects became less visible and the 

 speed called for approximates that demanded by uniform 

 disks. Whether or not this is the complete explanation, the 

 remedy suggested by it succeeds, and suggests that the 

 method of experimenting with illuminated rotating white 



