_From these three films we get all the lights and colours that we see in the daytime, with the single exception of_ =neutral grey=; and this appears to come, not from the eye at all, but from the brain. It may be seen even when the retina is quite blind, provided that the rest of the nervous apparatus is in working order; and it may be seen by night as well as by day; it is mixed, physiologically, with all our sensations of light and colour, though we cannot by psychological analysis pick it out from the lights and colours. Strange enough! but we shall understand better as we go on. The German physiologist Ewald Hering has shown that _the processes which take place in the films are, in all probability, chemical processes of an_ =antagonistic= _or_ =reversible= _kind_; that is why we never see a bluish-yellow, or a greenish-red; if we throw on the same part of the retina, at the same time, equal amounts of black and white, or of blue and yellow, or of purplish-red and bluish-green, the chemical processes go on in opposite directions and cancel each other, with the result that we see just nothing. This antagonism can be proved, under the right experimental conditions, for blue-yellow and for red-green; if these pairs are fittingly thrown together on the retina we see, in fact, only neutral grey; so that our seeing of the same grey, when black and white stimuli are acting together, does not necessarily mean that grey is a retinal mixture of black and white; the black and white may also cancel each other, and leave only the brain-grey to be seen. We have, then, the three films in each eyeball, and we have the brain-grey behind them. More than this: we have a night or =twilight eye=. When colours fade out, as twilight deepens, another retinal film comes into play; the lights that we still see come, not from the black-white film, but from a fourth film, of the same size, whose only sensation is _a slightly bluish-white_. Of course, this white is always mixed, physiologically, with the brain-grey; we never see it by itself; but we owe to it, among other things, the silvery look of blues in the twilight. The very centre of the twilight eye is totally blind; if on a moonless night you want to see a faint star or a distant street-lamp you must not look directly at it, but just to one side of it. Children’s fear of the dark is partly due to the fact that they cannot see what they turn their gaze upon; there had seemed to be something there, but when they looked at it, it eluded them; and if they think they see it again, and look in the new direction, again it is gone.