202 COLOUR VISION 



colours. Hering has therefore found it necessary to hypothecate a 

 white valency for the toned colours, thereby introducing a complication 

 which mars the symmetry of his design. 



The peculiar position of black and white in the gamut of colour 

 sensations early made itself felt. Woinow 1 distinguished between the 

 white which is a mere sensation of light and is due to stimulation of 

 the rods, the binary red-green and yellow-blue whites, and the quaternary 

 white which is a combination of the two binaries. Preyer 2 also dis- 

 tinguished between the whites due to stimulation of the chromatogenous 

 elements, the cones, and that due to the photogenous, the rods. 

 Konig 3 distinguished between the simple white of rod stimulation and 

 the trichromatic white of the cones, v. Kries expressly adopts this 

 view and thinks that " it would perhaps be more correct not to say that 

 the rods initiate colourless sensations, but that they initiate a sensation- 

 efl'ect which is variable in only one sense." 4 There is certainly a 

 peculiarity, which may be described as a bluish greyness, about the 

 scotopic colourless sensation which distinguishes it from photopic 

 " white." 



Evidence derived from the colour-blind led Young in his sole 

 reference to the subject 5 to ascribe Dalton's defect to absence or paralysis 

 of the fibres which were supposed to initiate the red sensation. Konig 6 

 adopted the explanation brought forward by Ad. Fick and Leber 7 that 

 so-called partial colour-blindness (dichromatism) is due to coincidence 

 of two of the elementary-sensation curves, v. Kries 8 adopted v. Helm- 

 holtz' reduction theory or congenital absence of one of the fundamental 

 components. Both Konig and v. Kries accepted Liesegang's view 9 

 that total colour-blindness (monochromatism) is due to rod- or visual 

 purple-vision. Hess and Hering 10 adduce against this view the complete 

 analogy in the normal and totally colour-blind dark-adapted eyes of 

 the low central and increasing peripheral threshold values. 



Later years have brought forth a crop of theories, mostly modifica- 

 tions of those already mentioned. The more important will be briefly 

 mentioned in the following pages. 



1 Arch. f. Ophth. xxi. 1, 223, 1875- 2 Arch. f. d. ges. Physiol. xxv. 31, 1881. 



3 Sitz. d. Akad. d. Wiss. Berlin, 1894. 



4 Ztsch. f. Psychol. u. Physiol. d. Sinnesorg. ix. 87 note, 1896. 

 6 Lectures on Natural Philosophy, 11. 315, 1807. 



6 Ztsch. f. Psychol. u. Physiol. d. Sinnesorg. iv. 241, 1892. 



7 Arch. f. Ophth. xv. 3, 26, 1869. 



8 Ztsch. f. Psychol. u. Physiol. d. Sinnesory. xm. 241, 473, 1897. 



9 Photogr. Arch. 1891. 10 Arch. /. d. ges. Physiol. LXXI. 105, 1898. 



