That it is less perfect in peripheral than in central vision, in monocular than in binocular vision, in brief than in prolonged vision; that it is attenuated at a great distance; that it varies with individuals according to the richness of their perceptual world, and finally that it is less perfect in coloured lighting, which cancels out the superficial structure of objects, and brings the reﬂecting potentialities of different surfaces to a common level, than in colourless lighting which leaves these structural differences intact.