478 SCIENCE PROGRESS 



region until they meet in the centre, and a fresh colour green is 

 developed. In further development the red-green region is 

 replaced by yellow, the blue replaces the violet-green region, 

 then orange becomes distinguishable, and finally indigo. Every 

 fact points to this being how the evolution of the colour sense 

 has taken place, and there are various degrees of colour per- 

 ception corresponding to every stage in the process. 



V. Normal Colour Vision (19, 20, 21, 22) 



The theory of colour discrimination given accounts for the 

 facts of normal colour perception. When a spectral light is 

 diminished in intensity colours disappear in the order of their 

 development. Complementary colours are a necessity of the 

 theory. In the dichromic red and violet are complementary to 

 each other, and a mixture of red and violet is confused with 

 white and green. When the stage is reached that green is dis- 

 tinguished as a separate colour, vision assumes the trichromic 

 character which henceforth remains, and green now becomes 

 complementary to the other two colours. 



VI. Simultaneous Colour Contrast (23, 24) 



1. The colours seen by simultaneous contrast are due to the 

 exaggerated perception of a real objective relative difference 

 which exists in the light reflected from the two adjacent 

 surfaces. 



2. A certain difference of wave-length is necessary before 

 simultaneous contrast produces any effect. This varies with 

 different colours. 



3. A change of intensity of the light of one colour may 

 make evident a difference which is not perceptible when both 

 colours are of the same luminosity. 



4. Simultaneous contrast may cause the appearance of a 

 colour which is not perceptible without comparision. 



5. Both colours may be affected by simultaneous contrast, 

 each colour appearing as if moved farther from the other in the 

 spectral range. 



6. Only one colour may be affected by simultaneous contrast, 

 as when a colour of low saturation is compared with white. 



7. When a false estimation of the saturation or hue of a 

 colour has been made, the contrast colour is considered in 



