Polarization Flicker Photometer. 



375 



was read. Readings were made in both directions of rota- 

 tion, and half the difference taken as the least perceptible 

 rotation, which was then translated into brightness dis- 

 crimination fraction with the aid of the data of fig. 4. 

 The hue discrimination fraction was obtained in an exactly 

 similar manner, both lights being used in this case, and the 

 glasses being rotated until a difference of hue appeared 

 between the two halves of the bi-prism. 

 ,f These two steady observation discrimination fractions will 

 be denoted as Ab and Ah. Whether these fractions or the 

 mean error of setting most nearly compare with the 8's 

 need not be discussed here, for we shall merely use the ratio 

 of the two, which will be practically the same whichever 

 measure of perception difference is taken. 



The complete investigation of the various discrimination 

 fractions and their relationship consists of five sets of 

 measurements, which should be performed at one sitting 

 in view of the apparent variability of some of them. These 

 five measurements are as follows : — 



Determination of S B . 



Determination of equal brightness condition for two 



colours to be used for hue discrimination test. 

 Determination of Sh by critical speed measurements 



of two colours separately and together. 

 Determination of Ab. 

 Determination of Ah- 



Three complete sets as thus outlined were carried through : 

 two by the writer and a third by Mr. Kingsbury. The 

 results are tabulated below : — 





log £b. 



log Oh. 



CH 



ob 



A B . 



A H . 



Ah 

 Ab' 



A H 



Ab 



H.E.I 



-3-7 

 -4-2 

 -3-2 



-2-79 

 -3-09 

 -2-07 



8-1 

 13-0 

 135 



•034 

 •025 

 •044 



•034 

 03 



•08 



1-0 



1-2 



1-8 



84 



10-8 



7-4 



E. F. K 











Mean 





8-8 



The last column shows in a very striking manner to what 

 the flicker photometer owes its peculiar applicability to 

 colour difference work. In the confusion condition caused 

 by rapid alternation the appreciation of hue difference fails to 



