266 COLOUR VISION 



field the coloured sector for eliminating initial contrast was kept con- 

 stant and the white gradually increased from zero until the contrast 

 colour was eliminated. It was found that with constant colour valency 

 of the exciting field, combined with increase in the white valency (and 

 therefore increase of black induction due to it), the same amount of 

 colour contrast is caused by a simply proportional increase of white 

 valency in the responding field. Increase of colourless contrast there- 

 fore proportionally diminishes the colour contrast. 



(3) In the contrast- ex citing field the coloured sector and the white 

 sector were increased in such a manner that the colour valency and the 

 white valency increased in the same proportions. In the colour- 

 responding field the coloured sector for eliminating initial contrast 

 was kept constant and the white sector gradually increased from zero 

 until the contrast colour was eliminated. It was found that with 

 proportional increase of colour valency and white valency in the exciting 

 field there was generally no increase in contrast, but in some experiments 

 the contrast effect increased nearly proportionally to the intensity of 

 the contrast-exciting mixture up to a certain optimum value. 



With regard to colour blindness Hering 1 in 1885 examined a series 

 of cases of anomalous trichromats as to their colour mixtures with rota- 

 ting discs. He found that they could be divided into two groups, one 

 group making the luminosity match of spectral red (660 JU./A) to spectral 

 blue (447 /x/z) as 1-15 : 1, the other as 7 : 1. From his observations he 

 called the first group, i.e. partial deuteranopes, relatively yellow-sighted ; 

 the second group, i.e. partial protanopes, relatively blue-sighted. Since 

 colour equations are purely relative no exception can be taken to this 

 nomenclature. He found the position of his fundamental green in the 

 spectrum to differ in the two classes, for the first a relatively longer 

 wave-length, for the seond a relatively shorter. For colourless mixtures 

 of spectral red and blue-green, or yellow-green and violet, or to a less 

 extent yellow and blue, the relatively yellow-sighted required a larger 

 amount of the short wave-length component. Similarly they required 

 more green for a match of spectral yellow with a red and yellow-green 

 mixture, or of greenish-yellow with an orange and yellow-green mixture. 

 He found the results inconclusive with the blue-sighted owing to too 

 great differences of saturation. There was no typical difference in the 

 matches of blue with a green and violet mixture. Similar, though less 

 marked, differences of the same nature were found in peripheral matches 

 by Biedermann (yellow-sighted) and Singer (blue-sighted). The 



1 Lotos, N. F. vi. 1885. 



