But any one who is interested in executive problems which demand concentrated attention and sustained investigation will find plenty to tax his fullest energies--problems of drawing, of colour and tone management, of imitative suggestion, and of technical application. As an example of a complex motive which would present a series of difficulties a picture might be imagined of the sea washing in among rocks, some of which are submerged while others stand up above the surface, the water clear and transparent and neither smooth nor much agitated. Through the water the objects beneath would be clearly seen and the surface would reflect the rocks above and catch gleams of light from the sky, and the movement of the small waves swinging towards the rocks and rebounding from them, and eddying over the shallow places, would make a pattern of lines and planes set at all sorts of angles. To realize such a subject adequately an almost perfect balance of observation would be needed. Too much attention given to the under-water details would destroy the suggestion of the surface; too much concentration on the surface lights and reflections would make the water seem opaque; exaggeration of the lines and planes of the ripples would diminish the breadth of effect and alter the character of the subject. The painter must perceive that this problem has many sides, and that each one must receive exactly its right amount of consideration if the pictorial solution is to be correct; if he has to make a compromise with reality the most subtle judgment will be required of him to create an illusion that will look like truth. To multiply such examples would be easy, for there is no phase of sea painting in which difficulties do not abound. It is difficult to paint a breaking wave, to preserve its architectural quality of design and its appearance of massive strength, and yet to show that it is a moving and momentary thing disappearing as quickly as it is formed. It is difficult to represent the confusion of a stormy sea, churned into foam and tossing in the wildest turmoil, and yet to make intelligible the order and regularity of its movement and the right sequence of its changing forms. It is as difficult to render the smoothness of calm, quiet water without making it look solid and opaque, dull and lifeless, as it is to suggest the liveliness of a breezy day without lapsing into meaningless repetition and restless pattern-making. Every successful sea picture is a difficulty overcome and a problem solved, and every successful sea painter is a man who has struggled earnestly with intractable material and has built his achievement on a foundation of laboriously acquired knowledge. Probably that is why there have been comparatively few great sea painters; it is certainly a reason why the few who can be accounted great should be regarded as masters of the highest rank with places of distinction in the history of art.