‘But how astonished was I when the white wall seen through the prism remained white after as before. Only where something dark came against it a more or less decided colour was shown, and at last the window-bars appeared most vividly coloured, while on the light-grey sky outside no trace of colouring was to be seen. It did not need any long consideration for me to recognize that a boundary or edge is necessary to call forth the colours, and I immediately said aloud, as though by instinct, that the Newtonian doctrine is false.’