302 Messrs. H. E. Ives and E. F. Kingsbury on the 



symbols (m and p) there used, we finally obtain for the 

 complete expression, 



I 4 A 2 



I log — hlog sin 7T(j> — log <f> — log & j 

 — log 2- log s -J o^* 1 ***). da) 



where a and 5 are the experimentally found constants for 



a 180-degree disk under an illumination I, (The quantity 



outside the brackets should, of course, reduce to unity for the 



•case of 180 degrees opening. The outstanding error is one 



of the consequences of using the approximate two-term 



4 

 expression. The term log — has been dropped in most of 



the calculations below.) 7r 



In fig. 4 are plotted curves showing the values of co as 



Fiff. 4. 







10 



20 



30 



4 





< 



^ i 



















O^ 



H,i ! 



















"^ 





^ 

















^^! ' 

















\ 











[S 













fe ^ 







360 

 300 













\ 









X 









"~°^ - -. 



3 





240 







»!80 







,120 





C -X 



\ 



6* 







o^ 









\ 

















\ 





f 







1) 







60 







\ 





J 







/) 









\ 



^ 









20 



Cycles per Second. 



30 



40 



Lower diagram. — Full lines, calculated critical frequencies for differerr 

 angular openings, a, high illumination ; b, medium illumination ; a, low 

 illumination for blue light. Circles and crosses give experimental 

 values. 



Upper diagram. — Calculated critical frequency-log I lines for three 

 angular openings. Dotted lines indicate characteristics for blue light at 

 low intensities. All calculated data are on a basis of S='001. 



