104 Dr. W. Pole on some unpublished 



therefore special study. They belong, as already stated, to 

 seven species. 



5. An infinite variety of impressions may be given by 

 diluting the fundamental yellow with different proportions of 

 white. The painters have a practical name for these, namely 

 tints ; but we want, for quantitative purposes, another form 

 of expression. The freedom of a colour from white is usually 

 called, in this country, "purity;" but it will suit our present 

 object better to adopt the equivalent of the German term 

 Sattigung. Helmholtz, for example, repeatedly uses such 

 expressions as " hochste Sattigung," or " bei sehr gesattigten 

 Farben ;" and I propose to define these mixtures as degrees of 

 saturation. If to y parts of yellow we add w parts of white, 



then the resulting " degree of saturation" is = ^ , the full 

 00 y -j- ur 



saturation of the yellow colour being =1. 



6. Then we have an infinite variety of impressions of a 

 different kind, by darkening the fundamental yellow with 

 black. These modifications painters call shades, and we 

 may term them degrees of brightness or degrees of lumi- 

 nosity. Thus, if to y parts of yellow we add b parts of black, 



the degree of luminosity will be expressed by —j~r, the 



original luminosity of the yellow colour being = 1. 



7. But there is another infinite variety of impressions 

 obtainable by mixing yellow with different proportions of both 

 ivhite and black, by which its saturation and luminosity are 

 both reduced, and the character of the impression is materially 

 changed. If y parts of yellow are mixed with w parts of 

 white, and b parts of black, the degree of saturation will be 



= —~ ; and the decree of luminosity = — 7 — =-• 



y + b + w ° J y+w+b 



8. 9 , 10 are corresponding varieties of impression for the 

 blue colour. 



11. Then, finally, there is an infinite variety of impressions 

 of " grey," formed by mixing white and black in different 

 proportions. The luminosity of any such mixture will be 

 w 



w + b 



The mixtures of yellow with blue produce no new impres- 

 sions — none but what are included in th^ above classes. 



It is worth while also to define another quantitative function 

 of the dichromic mixtures, *. e. the strength or power of the 

 impression of the colour proper, in relation to the full power 

 of the hue from which it is derived. I propose to call this the 



