24 COLOUR VISION 



subsidiary impressions, such as localisation, shape, dimensions, etc., when 

 comparing colours. Further, it is easy to show that the apparent 

 brightness and colour of objects can be altered within a wide range 

 without disabusing our minds of the opinion that the colours are 

 inherent properties of the objects. Thus, the paper of a book appears 

 white and the print black, whether we read it in the morning or at mid- 

 day or in the evening. Yet Hering has shown by accurate measurements 

 that the print may actually reflect more light at mid-day than the paper 

 did in the morning. Similarly the paper remains " white " and the 

 print " black " whether the book be read by daylight or gas light or 

 electric light or in the shadow of green trees. " The approximate 

 constancy of the colours of visual objects in spite of gross quantitative 

 and qualitative variations of the general illumination of the visual field 

 is one of the most remarkable and weightiest facts in the domain of 

 physiological optics " (Hering). We shall see that these gross variations 

 are compensated for by processes of physiological adaptation of the 

 visual nervous structures as a whole and that stimulation of retinal 

 areas by light arouses reciprocal activities in neighbouring areas. 



These complex processes should deter us from drawing too dogmatic 

 conclusions from the psychological analysis of colour sensations. 

 As McDougall^ says, mental activity consists in the process of 

 establishing in the mind relations between one thing and another. 

 This process in its best-defined form is apperception, " the process 

 by which a mental system appropriates a new element or otherwise 

 receives a fresh determination^." Each mental system is gradually 

 built up by a series of apperceptive processes, each such process con- 

 sisting in the presentation of some one aspect or feature of the whole 

 object through some sense-organ, and the bringing of this feature into 

 mental relation with various other aspects and features previously 

 apperceived and incorporated into the mental system. " In almost 

 every moment of waking life an apperceptive process is taking place ; 

 whenever an object is attended to the presentation of it is apperceived^." 

 Mental activity then consists essentially in the perpetual succession of 

 apperceptive processes, and the essence of apperception is the appro- 

 priation of the relatively novel presentation by the mental system built 

 up by previous apperceptions. At each apperception of any given 

 presentation of an object the appropriation of it by the mental system 

 is more ready and more complete, while the consciousness excited by 



1 Brain, xxiv. 605, 1901. ^ stout. Analytic Psychology, 1896. 



* Stout, loc. cil. II. p. 113. 



