58 COLOUR VISION 



deferred, though what has been said in the last sentence is hardly in- 

 telligible on any other assumption. 



Complete dark adaptation is only reached after prolonged exclusion 

 of light. Hence a moderate degree of scotopia is generally present in 

 every-day life, sufficient indeed to elicit the Purkinje phenomenon 

 merely by sudden diminution of the intensity of the stimulus. If after 

 remaining for a considerable time in a moderately lighted room the 

 illumination is suddenly diminished reds at once appear much darker 

 and blues much brighter 1 . 



The relative roles played by dark adaptation and by intensity of 

 stimulus may be stated somewhat as follows. Dark adaptation deter- 

 mines the degree of scotopia. This is shown by the fact that the colour 

 threshold or threshold value of photopic vision (Schwellenwert des 

 Tagessehens, v. Kries) remains constant whilst the general threshold 

 becomes lowered. Hence with good dark adaptation scotopic luminosity 

 values are high. As light adaptation increases the photopic values 

 increase and ultimately preponderate: hence Purkinje's phenomenon. 

 With still stronger light adaptation intensities of stimuli below the 

 photopic values are sub-liminal. 



As regards intensity of stimulus, as soon as it reaches the chromatic 

 threshold the luminosity values increase throughout the spectrum, but 

 the increase in the red is very slight as compared with that in the 

 blue. 



The older authors (Dove, Grailich, Helmholtz) directed their attention 

 almost entirely to the intensity, so that Konig lays stress upon the 

 alteration in colour matches as '' deviations from Newton's law of 

 colour mixtures 2 ." Hering 3 and his pupil Tschermak 4 , on the other 

 hand, regard adaptation as the sole cause. Hering 5 used two rooms 

 separated by a partition which had round holes in it filled with coloured 

 glass. Alteration in the illumination in one room varied the adaptation 

 and in that of the other the intensity of the light observed. He found 

 that with continued light adaptation, diminution of intensity of red and 

 blue caused equal change in the luminosity of the two colours. Red, 

 however, retained its colour slightly longer and appeared slightly 

 brighter. With dark adaptation the blue became whiter and brighter 



1 Hering, loc. cit. 



- Konig, pp. 108, 416 ; cf. Tonn, Ztsch. f. Psychol. u. Physiol. d. Sinnesorg. vii. 279, 

 1894. 



3 Lotos, vi. 1885 ; vii. 1886. 4 Arch. /. d. ges. Physiol. LXX. 297> 1898. 



5 Arch.f- d. ges. Physiol. LX. 519, 1895. 



