268 COLOUR VISION 



not think that this theory applies to the differences of luminosity 

 observed in various colourless mixtures in passing from parafoveal to 

 peripheral vision or on changing the condition of adaptation of the eye. 

 He regards these changes as functions of the physical stimuli or wave- 

 lengths of the light, not of their physiological valencies. He found that 

 in the normal photopic eye the white valencies of different lights change 

 unequally in passing from the centre to the periphery, as well as at the 

 same site with dark adaptation. He therefore concludes that the white 

 valencies of all lights change under different conditions. They increase 

 for the normal and the colour-blind eye in passing from light to dark 

 adaptation. In the normal this change is unequal, in that the white 

 valencies of the long wave-length lights increase relatively less than those 

 of the short wave-length lights. With relative yellow-sightedness this 

 adaptative change of the white valencies is more pronounced than with 

 relative blue-sightedness. The two types of red-green blind persons 

 show analogous changes. In typical totally colour-blind people there 

 is a change in the absolute white valencies, but none in their relative 

 values. 



These and the regional changes Tschermak attributes to changes 

 " in the photo-chemical stimulus-intermediaries in the absorption 

 apparatus of the visual organ." One of the photo-chemical white 

 substances may be the visual purple and its accumulation during 

 protection of the eye from light may underlie the increased excitability 

 of peripheral parts of the retina. Thus, change in concentration of 

 an elective absorbing substance alters the absorption equivalents, and 

 might be expected to act in the opposite sense to the Purkinje pheno- 

 menon, a view which has been adopted by v. Kries 1 (v. p. 206). 



Tschermak 2 indeed considers the theory of the specific brightness 

 of colours insecurely founded (nicht hinldnglich begrundet), chiefly on 

 the grounds of the supposed unequal summation of luminosities in 

 mixtures. Thus Hering 3 mixed the lights from a red and a green glass 

 of the same subjective brightness, and found that half the quantity of 

 the mixed light was darker than either of the components alone. Similar 

 results were obtained with complementary colours, thus agreeing with 

 Ewald and Kiihne's results 4 . Bonders 5 obtained similar results with 



1 Ztsch. f. PsycJiol. u. Physiol. d. Sinnesorg. xxix. 81, 1902. 



2 Eryeb. d. Physiol. i. 2, 797, 1902. 



3 Lotos, N. F. n. 31, 1882 ; vi. 57, 1885. 



4 Untersuch. a. d. physiol. Inst. zu Heidelberg, I. 153, 208, 1878. 



5 Arch f. Ophth. xxx. 1, 15, 1884. 



