STATEMENT OF THE THEORY 257 



scotopic luminosity of the spectrum. In dark adaptation the black- 

 white substance is raised to a condition of high potential, while the 

 chromatic substances are not appreciably affected. Hence with slight 

 stimulation the coloured components remain below the threshold, and 

 the curve of luminosity of the spectrum is that of the black-white sub- 

 stance. With increase of physical intensity the red and yellow add to 

 the brightness, the green and blue detract from it, and consequently 

 the point of maximum brightness shifts towards the red end of the 

 spectrum. 



This identification of the achromatic scotopic luminosity curve with 

 the activity of the black-white substance has led many to the erroneous 

 opinion that Hering regards the visual purple as being the black-white 

 substance. 



As Tschermak^, a pupil of Hering, says : " The theory of the specific 

 brightness of colours was propounded by Hering and Hillebrand to 

 explain the unequal changes in brightness of different coloured lights 

 in indirect vision and in dark adaptation (Purkinje's phenomenon), and 

 therewith also to explain the different distribution of subjective bright- 

 ness in the spectrum for the colour-seeing light-adapted eye as compared 

 with the non-colour-seeing dark-adapted or totally colour-blind eye. 

 The colour sensations, in general terms, may be said, according to 

 Hering's theory, to be compounded of (zusammengesetzt) or analysable 

 into (zerlegbar) a chromatic part and a non-chromatic grey part of 

 different nature (brightness) determining the nuance, and of different 

 relative magnitudes determining the saturation, so that so-called 

 coloured lights bring about a double excitatory effect in the light-adapted 

 eye, combining in addition to a colour valency also a white valency. 

 The latter — in conjunction with the continuous processes which produce 

 the intrinsic grey of the eye and with the contrast effects (increase of 

 blackness) which are due to reciprocal action from the surrounding 

 areas— determines the nature and relative magnitude of the non- 

 chromatic sensation constituent, the grey components. The bright- 

 ness of a colour sensation cannot be referred only to the brightness 

 of its grey components, but must also depend upon the colour- 

 components. Colour impressions, therefore, even with the greatest 

 possible saturation, i.e., with the greatest possible abstraction of the 

 grey components, possess also ' brightness ' — a quahty which cannot 

 be more closely defined and is only absent from absolute black." 



The tone-free sensation derived from the mixture of two 



1 Ergeb. d. Physiol, i. 792, 1902. 

 P. c.v. 17 



