THEORIES OF COLOUR- VISION. 1 1 1 1 



between colour-blindness and the peripheral retina was made by Tick 1 

 the basis of a suggestion that, instead of assuming absence of a component, 

 it would be more in agreement with the correspondence in question, if it 

 were assumed that in these conditions there was an altered condition of 

 excitability of the physiological bases of the components ; that in the red- 

 green blind condition the red and green parts of the spectrum might 

 excite two of the processes in an equal degree, so that their curve of 

 excitability might nearly or altogether coincide with one another ; while 

 in the extreme periphery of the retina, and in total colour-blindness, 

 light of any wave-length would act equally well on all three substances. 



The chief advocates of the three-colour theory (Holmgren, v. Kries, 

 Konig) have now accepted the position, that the subjective sensations of 

 the red-green blind most probably correspond with the yellow and blue 

 of the normal eye ; while nearly all agree that the white of the colour- 

 blind agrees with the white of the normal eye ; and the old idea, that 

 colour-blindness depends simply on absence of one or more components, 

 has been almost entirely given up by advocates of the theory. 



In the second edition of the " Physiologisches Optik," S. 373, Helm- 

 holtz assumed that in the red-blind zone the substance sensitive to red has 

 become similar to that sensitive to green, while at the extreme periphery 

 all three substances have become like one another. As an alternative 

 hypothesis, Helmholtz suggested a psychological explanation, that, when 

 one of the fundamentals is absent, we interpret the colours which are 

 still perceptible, as the colours of a line which, in the colour triangle, 

 would pass through white parallel to the line joining the two funda- 

 mentals still present. According to Helmholtz, these would pass from 

 yellow through white to blue. 



Konig 2 has advanced the view, that the curves of the three elementary 

 sensations vary with the absolute intensity. These variations are held 

 to be the basis of Purkinje's phenomenon, and of the characteristic 

 spectrum of low intensity. He further supposes that the visual sub- 

 stances of the various forms of colour-blindness behave at ordinary 

 intensities in the same way as those of the normal eye at lower inten- 

 sities. He gives a series of luminosity curves of the spectrum at 

 different intensities, and shows their correspondence with a number of 

 curves in different individuals, some of whom had dichromatic, others 

 normal trichromatic, others anomalous trichromatic systems. 



The correspondence of the spectrum of low intensity with that of 

 total colour-blindness was explained by supposing that in the latter 

 condition the three substances behaved at all intensities in the same 

 way as in the normal eye at low intensities, i.e. all three substances were 

 acted on equally by light of any wave-length. 



v. Kries has supposed that, in colour-blindness, the three components 

 of the retinal process are present, and that the defect depends upon 

 abnormal action of the nerve paths, or nerve centres. 



Retinal basis for the theory. More recently, Konig 3 has en- 

 deavoured to bring the Young-Helmholtz theory into relation with 

 definite retinal substances. Konig explains Purkinje's phenomenon, and 



1 Verhandl. d. phys.-med. Gesellsch. in Wilrzburg, 1873 ; see also Arch. f. d. ges. 

 Physiol., Bonn, 1896, Bd. Ixiv. S. 313. Helmholtz had, however, previously made a similar 

 suggestion himself, " Handbuch d. physiol. Optik," 1867, S. 848. 



2 "Beitr. z. Psych, u. Phys. d. Sinn.," 1891, S. 311. 



3 Sitzungsb. d. k. ATcad. d. Wissensch., Berlin, 1894, S. 577. 



