Remarks like these, and the further quotation given below, make it seem particularly tragic that Ruskin apparently had no knowledge of Goethe's Farbenlehre. This is the more remarkable in view of the significance which Turner, with whom Ruskin stood in such close connexion, ascribed to it from the standpoint of the artist. For the way in which Ruskin in his Modern Painters speaks of the effect of the modern scientific concept of colours upon the ethical-religious feeling of man, shows that he deplores the lack of just what Goethe had long since achieved in his Farbenlehre where, starting with purely physical observations, he had been able to develop from them a 'physical-moral' theory of colour.

Ruskin's alertness to the effect on ethical life of a scientific world-picture empty of all qualitative values led him to write:

'It is in raising us from the first state of inactive reverie to the second of useful thought, that scientific pursuits are to be chiefly praised. But in restraining us at this second stage, and checking the impulses towards higher contemplation, they are to be feared or blamed. They may in certain minds be consistent with such contemplation, but only by an effort; in their nature they are always adverse to it, having a tendency to chill and subdue the feelings, and to resolve all things into atoms and numbers. For most men, an ignorant enjoyment is better than an informed one, it is better to conceive the sky as a blue dome than a dark cavity, and the cloud as a golden throne than a sleety mist. I much question whether anyone who knows optics, however religious he may be, can feel in equal degree the pleasure and reverence an unlettered peasant may feel at the sight of a rainbow.'

What Ruskin did not guess was that the rudiments of the 'moral theory of light' for which he craved, as this passage indicates, had been established by Goethe long before.

In the section of his Farbenlehre dealing with 'physiological colours', Goethe devotes by far the most space to the so-called 'afterimages' which appear in the eye as the result of stimulation by external light, and persist for some little time. To create such an afterimage in a simple way, one need only gaze at a brightly lit window and then at a faintly lit wall of the room. The picture of the window appears there, but with the light-values reversed: the dark cross-bar appears as light, and the bright panes as dark.

In describing this phenomenon Goethe first gives the usual explanation, that the part of the retina which was exposed to the light from the window-panes gets tired, and is therefore blunted for further impressions, whereas the part on which the image of the dark frame fell is rested, and so is more sensitive to the uniform impression of the wall. Goethe, however, at once adds that although this explanation may seem adequate for this special instance, there are other phenomena which can be accounted for only if they are held to derive from a 'higher source'. Goethe means experiences with coloured after-images. This will be confirmed by our own discussion of the subject.

What we first need, however, is a closer insight into the physiological process in the eye which causes the after-images as such. Wherever Goethe speaks of a simple activity of the retina, we are in fact concerned with a co-operation of the retina with other parts of our organ of sight. In order to make this clear, let us consider how the eye adapts itself to varying conditions of light and darkness.

It is well known that if the eye has become adjusted to darkness it is dazzled if suddenly exposed to light, even though the light be of no more than quite ordinary brightness. Here we enter a border region where the seeing process begins to pass over into a pathological condition.⁴ A 'secret' of the effect of light on the eye is here revealed which remains hidden in ordinary vision, for normally the different forces working together in the eye hold each other in balance, so that none is able to manifest separately. This equilibrium is disturbed, however, when we suddenly expose the eye to light while it is adapted to darkness. The light then acts on the eye in its usual way, but without the immediate counter-action which normally restores the balance. Under these conditions we notice that the sudden dazzling has a painful influence on the eye - that is, an influence in some way destructive. This will not seem surprising if we remember that when light strikes on the background of the eye, consciousness is quickened, and this, as we know, presupposes a breaking down of substance in some part of the nervous system. Such a process does in fact occur in the retina, the nerve-part of the eye, when external light falls upon it. If the eye were solely a structure of nerves, it would be so far destroyed by the impact of light that it could not be restored even by sleep, as are the more inward parts of the nervous system. But the eye receives also a flow of blood, and we know that throughout the threefold human organism the blood supplies the nervous system with building-up forces, polarically opposite to the destructive ones. In sleep, as we have already seen, the interruption of consciousness allows the blood to inundate the nervous system, as it were, with its healing, building-up activity. It is not necessary, however, for the whole of the body to pass into a condition of sleep before this activity can occur. It functions to some extent also in the waking state, especially in those parts of the organism which, like the eye, serve in the highest degree the unfolding of consciousness.

Having established this, we have a basis for an understanding of the complete process of vision. We see that it is by no means solely the nerve part of the eye which is responsible for vision, as the spectator-physiology was bound to imagine. The very fact that the place where the optic nerve enters the eye is blind indicates that the function of mediating sight cannot be ascribed to the nerve alone. What we call 'seeing' is far more the result of an interplay between the retina carrying the nerves, and the choroid carrying the blood-vessels. In this interplay the nerves are the passive, receptive organ for the inworking of external light, while the blood-activity comes to meet the nerve-process with a precisely correlated action. In this action we find what Goethe called the 'inner light'.

The process involved in adaptation now becomes comprehensible. The cause of the dazzling effect of light of normal intensity on an eye adapted to the dark, is that in such an eye the blood is in a state of rest, and this prevents it from exercising quickly enough the necessary counter-action to the influence of the light. A corresponding effect occurs when one suddenly exposes to darkness the eye adapted to light. One can easily observe what goes on then, if, after looking for a time at an undifferentiated light surface such as the evenly luminous sky, one covers the opened eyes with the hollowed hands. It will then be found that the space before the eyes is filled by a sort of white light, and by paying close attention one recognizes that it streams from the eyes out into the hollowed space. It may even be several minutes before the field of vision really appears black, that is, before the activity of the inner light in the choroid has so far died away that equilibrium prevails between the non-stimulated nerves and the non-stimulated blood.