286 The Sensations of Vision [242, 243. parent coloured veil, seems to be spread over the field. The immediate impression is not that this colouration is absent where there is white, that is, it is not just a mere substitution of the complementary colour of the ground in place of the white ; but the idea seems to be that two new colours are substituted in the place of the white, namely, the colour of the ground and the complementary colour. The connection is clearest in the arrangement shown in Fig. 54, where the observer looks through the green glass inclined at an angle of 45°. He decides that the black spot on the horizontal surface is pink-red, but he also decides that this spot, as well as the entire surface with its pink-red colour is seen through the green glass, and that the green colour given by the glass extends uninterruptedly over the entire lower surface, and even over the dark spot. Thus he believes that he sees two colours together at this place, that is, green, which he attributes to the glass plate, and pink-red, which he attributes to the paper behind it; and the two of them together do, in fact, give the true colour of this place, that is, white. As a matter of fact, an object which, seen through a green glass, sends white light to the eye, as this spot does, would have to be pink-red. But when a white object of exactly the same appearance is placed above the plate of glass, every reason for resolving the colour of the object into two disappears; it looks white to us. It is the same way when coloured surfaces are covered with translu¬ cent paper. If the ground is green, the paper itself seems to be greenish. Now if the substance of the paper extends without a perceptible break over grey underneath, the observer thinks he sees an object shining through the greenish paper; and an object of this kind must, on the other hand, be pink-red in order to give white light. But if the white place is outlined as an independent object, and there is lack of con¬ tinuity between it and the greenish part of the surface, it is regarded as being a white object lying on this surface. In §20 above, it was stated that this sort of separation of two colours that are present in the same part of the visual field is a matter of judgment. We were confronted with this condition there as something that was an ob¬ stacle to the free realization of the sensation of a compound colour. A separation of this sort is a very frequent occurrence whenever the two colours are unevenly distributed. These phenomena were noticed first by Volkmann,1 and he describes the effect by saying that we seem to see one colour through the other. In the author’s opinion the faculty of making such a separation depends on the following circumstance. Colours have their greatest significance for us in so far as they are properties of bodies and can be used as marks of identification of bodies. 1 Müllers Archiv für Anat. und Physiol. 1838. S. 373.