232 RECENT PROGRESS OF THE THEORY OF VISION. 



alone is the expression of a constant property of the object in 

 question. 



Considered theoretically, the task of judging of the colour 

 of a body under changing illumination would seem to be im- 

 possible ; but in practice we soon find t'lat we are able to j udge 

 of local colour without the least uncertainty or hesitation, and 

 under the most different conditions. For instance, white paper 

 in full moonlight is darker than black satin in daylight, but we 

 never find any difficulty in recognising the paper as white and 

 the satin as black. Indeed, it is much more difficult to satisfy 

 ourselves that a dark object with the sun shining on it reflects 

 light of exactly the same colour, and perhaps the same bright- 

 ness, as a white object in shadow, than that the proper colour 

 of a white paper in shadow is the same as that of a sheet of the 

 same kind lying close to it in the sunlight. Grey seems to us 

 something altogether different from white, and so it is, regarded 

 as a proper colour ; ' for anything which only reflects half the 

 light it receives must have a different surface from one which 

 reflects it all. And yet the impression upon the retina of a grey 

 surface under illumination may be absolutely identical with that 

 of a white surface in the shade. Every painter represents a 

 white object in shadow by means of grey pigment, and if he has 

 correctly imitated nature, it appears pure white. In order to 

 convince one's self of the identity in this respect i.e. as illumi- 

 nation colours of grey and white, the following experiment 

 may be tried. Cut out a circle in grey paper, and concentrate 

 a strong beam of light upon it with a lens, so that the limits of 

 the illumination exactly correspond with those of the grey circle. 

 It will then be impossible to tell that there is any artificial il- 

 lumination at all. The grey looks white. 2 



1 The local or proper colour of an object (Korperfarbe} is that which it 

 shows in common white light, while the 'illumination-colour,' as I have 

 translated Lichtfarbe, is that which is produced by coloured light. Thus the 

 red of some sandstone rocks seen by common white light is their proper colour, 

 that of a snow mountain in the rays of the setting sun is an illumination- 

 colour. TR. 



2 The demonstration is more striking if the grey disk is placed on a sheet 

 of white paper in diffused light. TR. 



