Theory of the Flicker Photometer. 



305 



Details of the method of observation were quite similar to 

 those employed in an earlier investigation to which reference 

 may be made*. 



The first point to be studied was the shape of the curve 

 connecting critical frequency and angular opening. This 

 was investigated by obtaining the critical speeds for a series 

 of opaque disks whose openings were 30, 120, 180, 240, 300, 

 330, and 345 degrees. The experimental values are shown 

 in fig. 4 along with the computed ones. Two sets are 



Fig. 6. 





















X 



A 









/% 









30° 



' X330 







// 









p 





























I03I 



Critical frequency-log I data for 30° and 330° degree white disks, 

 showing absence of symmetry at all illuminations. 



shown, one set, for a comparatively high illumination, exactly 

 as obtained, a second set for a lower illumination, derived 

 from the others from critical frequency -log I data to be 

 discussed presently. 



It will be seen that the computed and observed data are, 

 in general, similar. The chief difference between the ex- 

 pression used by Porter to represent this case and the one 



* " Spectral Luminosity Curves obtained by the Method of Critical 

 Frequency," Ives, Phil. Mag. Sept. 1912, p. 352. 



Phil." Mag. S. 6. Vol. 31. No. 184. April 1916. Y 



