264 RECENT PROGRESS OF THE THEORY OF VISION. 



Considered theoretically, the task of judging of the 

 colour of a body under changing illumination would seem 

 to be impossible ; but in practice we soon find that we 

 are able to judge of local colour without the least un- 

 certainty 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 brightness, as a white object in sha- 

 dow, than that the proper colour of a white paper in 

 shadow is the same as that of a sl:eet 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 ; l for anything which only 

 reflects half the light it receives must have a different 

 surface from one which reflects it all. And yet the im- 

 pression upon the retina of a grey surface under illumi- 

 nation 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 illumination 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 



1 The local or proper colour of an object (Kbrperfarbe) 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 illu 

 mination-colour. TB. 



