64 



Prof. F. Allen on Four Transition Points in the 



The persistency curve obtained when the eve was fatigued 

 with the wave-length \ '620 fi is here reproduced in fig. 7 

 from a previous paper to which reference has already 

 been made. This curve is of the double red and green 

 elevation type, similar to the curves of the two figures 

 immediately preceding. 



Ke. 9. 



06 



X 



\ 









\ 

 \ 



\ 



\ 



Fatigue 

 Normal 

 Fatigue 



colour 560/ 



i 



•05 



•04 



03 



•02 

 •0! 





\ 









\ 



\\ 







X 



i 



\\ 

 \\ 





/ 

 / 



i 





X x 





. 



40/* 



•50 -60 



Wave Length. 



70 



Further consideration led to the conclusion that a second 

 transition point ought to exist between curves of the double 

 red and green elevation type and those of the single green 

 elevation type. Since the curve for the wave-length X'520/>t, 

 fig. 13, was known from a previous investigation, persistency 

 curves were successively determined with the fatiguing 

 colours X'560/x and V540 /x. The data for these are given 



