372 Mr. H. E. Ives on a 



e,qual brightness are alternated in the flicker photometer, the 

 ratio of R to G at one extreme phase is given by 



r (i+/Vi; v ") 



G = / x ^ ( 5 ) 



(i-iTm^) 



and at the other phase by the similar expression with the + 

 and — signs interchanged. 



Before detailed discussion of this it is desirable to digress 

 in order to develop the idea of a hue scale by which a 

 numerical value can be given to any hue, and in particular 

 the idea of a hue discrimination fraction will be developed 

 and used. 



Let us imagine the two juxtaposed fields of the mixture 

 photometer illuminated by two equally bright but differently 

 coloured light sources R and G. When the two opal glasses 

 stand at right angles to each other and normal to each light 

 source they are of the colour of these latter, and show their 

 maximum colour difference. As they are turned towards 

 parallelism they are illuminated by mixtures of the two 

 colours and become progressively more like in hue, until at 

 the position of exact parallelism they are the same. On 

 turning further they diverge in hue until at the end position 

 they show the same difference as at first, but in the opposite 

 direction. Now if we consider these hues as forming a con- 

 tinuous series, they may be described and given a numerical 

 value in terms of the proportion of one of the end colours 

 present in the mixture forming any hue. If, for instance, 

 we identify the hue by the amount of R in it, we have that 

 the hue, H, of the mixture in which the two colours are 

 present in the ratio Rh : G H is 



Rh 



Rh + G h ' 



A hue difference Hj — H 2 is then 



Rhi Rh2 



Rhi + Ghi Rh2 + Gh2 

 and this is the fractional difference 



4 

 2 



(6) 



H 1 + H/ ■ * • • • CO 



