iii8 VISION. 



it follows that in the long-wave half of the spectrum, the white value of 

 a colour should be less than the brightness as determined by the 

 methods of heterochromatic photometry, while in the short-wave half the 

 reverse should be the case. This was shown to be true by Sachs, 1 who 

 used the method of pupil photometry ; and with the flicker method the 

 same relation between brightness and white value is found. 



The colourless sensation which occurs when two complementary 

 coloured lights are mixed, depends on the combined action of the coloured 

 lights on the black-white substance ; one of the two complementary lights 

 acts as a katabolic, the other as an anabolic, stimulus ; and with a suitable 

 intensity of each, these stimuli neutralise each other, and the chromatic 

 substance or substances remain in a condition of equilibrium. Hering 

 has repeatedly verified by experiment the proposition that the white value 

 of the grey produced by mixing two colours is equal to the sum of the 

 white values of those colours, the brightness of the grey depending wholly 

 on the action of the black-white substance. Abney and Eood have also 

 found that the brightness of a grey produced by mixing two colours is equal 

 to the sum of the brightness of those colours as determined by heterochro- 

 matic photometry. If both propositions are correct, it would mean that 

 the positive contribution made by the specific brightness of the warm colour 

 is exactly neutralised by the negative contribution of the cold colour. 



Peripheral retina and colour-blindness. — According to Hering, 

 the peripheral retina of the normal eye shows the same conditions 

 as are found in colour-blindness. In total colour-blindness, and in the 

 outermost zone of the retina, only the black- white substance is present ; 

 the middle zone of the retina is regarded as red-green blind, and the 

 smaller individual differences are of the same nature as those which 

 distinguish macular from extramacular vision. Since the curve of 

 luminosity of the spectrum at a low intensity depends, according to the 

 theory, on the black-white substance, it should correspond with that of 

 total colour-blindness, and Hering found this to be the case. He was 

 able in 1892, on the basis of his theory, to foretell the matches that would 

 be made by the case of total colour-blindness he was about to examine. 

 Hering has also supposed that the white values of coloured light were 

 the same for the extreme periphery of the retina as for total colour- 

 blindness, but according to v. Kries (p. 1084) this is not the case. 



Eed-green blindness and the vision of the middle zone of the peri- 

 pheral retina are ascribed by Hering to absence of the red-green 

 substance. 2 The investigations of Hess on the vision of the peripheral 

 retina and the nature of vision in acquired colour-blindness, and in 

 Hippel's unilateral case, are strong evidence in favour of Hering's view. 

 The chief difficulty in the way of the theory is the existence of two 

 distinct classes of this form of colour-vision. Hering's explanation is, 

 that these differences are of exactly the same order as the individual 

 variations first described by Eayleigh, and depend like them upon 

 differences of pigmentation of the macula and of the lens. A large 

 number of very careful observations were made by Hering 3 on Prof. 

 Biedermann and Dr. Singer, who were well-marked examples of indi- 

 vidual variation. He regarded the former as relatively yellow-sighted, 



1 Arch.f. d. ges. Physiol., Bonn, 1892, Bd. lii. S. 79. 



2 More exactly, the condition of the peripheral retina is regarded as one of feeble red- 

 green sense. 



3 Lotos, Prag, 1885, Bd. vi. S. 142. 



