118 J. C. DeVOSS and rose ganson 



tables to be given in this paper, we shall designate a few of the 

 colors by the numbers of both of the adjacent grays. 



In our search for grays which would be confused with the 

 stimulus color we were finally driven to make tests with a number 

 of gray cambrics. These also can be accurately defined by their 

 flicker equivalents, and by no other method which we can dis- 

 cover. Of course the cambrics were used in both double and 

 triple thicknesses, to prevent their transmitting light, both in 

 experiments with the cats and in ascertaining their flicker values. 



Quite aside from disputes as to the cause of the phenomena of 

 flicker or what it measures, it makes colored and gray surfaces 

 comparable, accurately definable and hence reproducible. It 

 permits any series of colors or grays to be interpolated with 

 any other since rotation obliterates differences of texture. It 

 is accurate because very slight amounts of flicker are readily 

 perceptible. Consequently we regard it as necessary to define 

 our colors by their flicker equivalents, though our work is explor- 

 atory and qualitative. 



While we have used flicker values merely for the sake of 

 definition, Ives (p. 852) says of flicker photometry: 



1. "It surpasses all other photometric methods in sensibility 

 and reproducibility in the presence of color difference." 



2. "It agrees at high illuminations with the equality of 

 brightness method, when the latter is freed from the psychol- 

 ogical uncertainties inherent in its use." 



3 . It measures at high illuminations what may fairly be termed 

 the true brightness." 



4. Brightnesses measuring equal to the same measure equal 

 to each other and the sum of the measurements of the parts 

 is equal to the measurement of the whole." 



While these conclusions are drawn for the conditions of Ives's 

 experiments they at least do not diminish the probability that 

 our own flicker equivalents are at least rough approximations 

 to measurements of brightness, though we have used them for 

 the purpose of defining our papers. 



In the following tables and description we shall use the initials 

 F. E. for flicker equivalent and except where otherwise stated 

 we shall not regard it as a measure of any property of the colored 

 or gray papers but merely as the most accurate method of identi- 

 fying any one of them. 



