102 



Mr. H. E. Ives on Hue Difference 



this type, the deviations being no greater than can be ex- 

 plained by the fact that he moved the hue of the mixture 

 continuously from one end of the mixture line to the other,, 

 thus probably encountering differences in the value of the 

 just noticeable distance along this line. 



Fig. 1. 



15 r~ 



'1000 



200O 3000 



2C 



4000 



500O 



Relationship between speed! ^ j and hue difference I — ), as calculated 



from theory. 



In order to apply the theoretical work just outlined to the 

 spectrum, it is necessary to know the position of the spectrum 

 and any comparison light used in the complete colour- 

 mixture diagram, namely, the colour triangle, and also the 

 distances corresponding to equal hue steps in the triangle. 

 It is important to note, moreover, that the colour triangle to 

 be used is the equal luminosity one, and not the equal 

 sensation-sum triangle usually plotted. 



Assuming that we have such an equal luminosity triangle 

 available with the equal hue intervals for all parts and all 

 directions determined, the process to be gone through may 

 be outlined as follows : — First, the distance between the 

 spectrum colour and the comparison light is measured ; the 

 mid-point of the line joining them, that is, where the equal 

 luminosity mixture occurs, is then found; the length of a 

 just distinguishable hue difference at this point is read off, 

 and the ratio of this length to half the whole distance gives 



