Needless to say in the absence of a clear nomenclature and standardisation of colours the task of describing and recording colour relations is of great difficulty, but the unanimity of competent, that is, sensitive persons as to which colours are related in specific ways to which, is too great to be disregarded. It is as great as the unanimity among musicians as to the harmonic relations of notes to one another. The great differences between the two cases are not likely to be overlooked. The presence of physical laws in many cases connecting notes harmonically related and the absence of similar known physical laws connecting colours is a glaring difference. But it should not be forgotten that these physical laws are, as it were, an extra-musical piece of knowledge. What matters to the musician is not the physical connections between notes but the compatibilities and incompatibilities in the responses of emotion and attitude which they excite. The musical relations between the notes would be the same even though the physical relations between the stimuli which arouse them were quite different. Naturally enough the analogy with the harmonic relations of music has been the chief guide to those who have systematically investigated colour relations. Whatever may be the precise limits to which it may profitably be carried, for anyone who wishes to form a general conception of the emotional effects of colours in combination it is of very great value. Colour is of course primarily the cause and controlling factor of emotional response to painting, but, as we have said, it may, and commonly does, help to determine form. Parts of a picture which are through their colour out of all emotional connection with the rest of the picture, tend, other things being equal, to fall out of the picture altogether, appearing as patches accidentally adhering to the surface or as gaps through which something else irrelevant is seen. This is the extreme instance, but the influence of colour upon form through the emotional relations of colours to one another is all-pervading Sometimes colour strengthens and solidifies the structure, sometimes it fights against it, sometimes it turns into a commentary, as it were, the colour response modifying the form response and _vice versa_. The great complexity of the colour and form interactions needs no insistence. They are so various that no rule can possibly be laid down as to a _right_ relation for all cases. All depends upon what the whole response which the painter is seeking to record may be. As with attempts to define a universal proper relation of rhythm to thought in poetry (e.g. the assertion that rhythm should echo or correspond to thought, etc.), so with general remarks as to how form and colour should be related. All depends upon the purpose, the total response to which both form and colour are merely means. Mistakes between means and ends, glorifying particular techniques into inexplicable virtues are at least