ABSTRACT
A theory of constancy capable of explaining coloured shadows was first set out in Monge’s ‘Mémoire sur quelques phénomènes de la vision’ of 1789. This chapter explains how constancy worked more generally. An important foundation of the theory was another fact which Monge discussed at the start of his paper, that ‘red objects’ do not look red when viewed through a ‘red glass’ as physics would suggest but instead appear ‘white’ just as white objects do in these circumstances. There is a slightly different problem with Vallée’s attempt to make sense of Monge’s experiment with candlelight and daylight. In this case, he argued that people take the ‘slightly orangey white’ of lamp at twilight for ‘white’, with the result that the shadow it casts will shift towards its ‘complementary’ colour, ‘light blue’.
