100 Mr. H. E. Ives on Hue Difference 



1 he OTi/. 



The greater part of the special theory necessary to handle 

 this question is contained in the next preceding paper, on 

 "A Polarization Flicker Photometer, &c." *, in the dis- 

 cussion o£ the brightness and hue discrimination fractions* 

 It is there shown (equation 14), that the critical speed at 

 the equality setting of two different colours is given by : 



«='-4^te , (i) 



where w M is the critical speed of the mixture, <w E and o> G are 

 the critical speeds for the two colours (R and G) separately,. 

 B B is the brightness discrimination fraction, and S H the hue 

 discrimination fraction. The latter is defined as the differ- 

 ence in the quantity of one of the colours in the mixtures at 

 the opposite phases, divided by the mean quantity, and it 

 was pointed out that this fraction, unlike the brightness- 

 discrimination fraction, varies with the size of the colour 

 difference. 



For the purposes of the present paper it is convenient to 

 consider this fraction in a slightly different light. Thus, 

 instead of identifying it with one colour of the mixture only, 

 it may be identified with both by considering it to repre- 

 sent a just distinguishable distance along a line of a colour- 

 mixture diagram, divided by (half) the length of the line. 

 It is thus twice the magnitude of the just distinguishable 

 fraction that would be most naturally derived if the definition 

 were developed solely from colour-mixture considerations. 

 Now this just distinguishable distance along the colour- 

 mixture line remains fixed, no matter how far in either 

 direction the line is extended, but the value of the fraction 

 decreases directly as the length of the line. Consequently,, 

 if we wish to lsarn the effect of increasing the colour 

 difference between the lights compared (confining ourselves 

 for the present to lights whose equal luminosity mixture is 

 always the same), it is only necessary to consider the value 

 of S H as varying inversely as this difference. 



If we call the distance apart of the two compared colours 



* Ives, he. cit. 



