''The art of leaving out'' was one of John Sell Cotman's descriptions of watercolour painting. ''Greta Bridge'' shows this English artist at what many nowadays consider the height of his powers, though he was only 23 or 24 when he painted it. Certainly for that date (1805 or '06) it was an inspired extension of the watercolour medium. Cotman valued its transparency and ease, using it as a superbly spontaneous vehicle for capturing the breadth and atmosphere of landscape, but also applied it as an immaculate means to rationalize nature's structure and lucidity.
Nothing in ''Greta Bridge'' seems left out, but in this tranquil arrangement of intervals, a great many incidental details have clearly been eliminated. Instead of the complicated leafage of the trees, Cotman painted their larger shapes: he sees them as mass rather than minutiae. In the rocks, bridge, farmhouse and stilled water a classic simplicity prevails, undisturbed by any ruffled flicker of texture or light. It is as if he had chosen, with careful judgment, to re-present the entire scene in the calm terms of reflections in water. Placidity and flatness are recognised as never before to be the unique capacity of watercolour, and most of Cotman's watercolours of this time have the same extraordinary quality of economy and consistency.
Sometimes this painter has been praised as a master of ''pattern,'' as though he were an artificial styliser, a designer who merely relates shapes to one another two-dimensionally. Looking at ''Greta Bridge,'' however, is not just to enjoy a design. It is true that order, balance and placement - the answering of one block of colour or tone to another, or the contrast of clear shapes in light with others in shadows - are dominant traits. But this interlocking of mass with gap, the awareness of the felicity of objects in harmony with the ''empty'' areas between them, is powerfully spatial in effect. The landscape, with a gentle logic, is perceived in planes, and the eye is led from foreground to the distant glimpse of hill and the unreachable peace of the sky beyond, with perfect conviction at every stage. There is no hint of working solely on the paper surface. The leap from near and far could hardly be more acutely realised.
This painting and the others Cotman produced in a similar style were not liked in his own day. One recent commentator has suggested that this may have been because of a generalising tendency in his work: some contemporaries criticised a later etching, for example, ''because it might be anywhere.'' The friend who relayed the criticism to the artist added: ''Two-thirds of mankind, you know, mind more what is represented than how it is done.'' It is true that Cotman came through to a kind of universality which was rather more than a record of a picturesque place visited. Nevertheless, ''Greta Bridge'' was made during (or, possibly from a sketch, just after) a period of six weeks he spent immersed in the special beauties of late summer in a very particular place in Yorkshire, Rokeby Park. His host at Rokeby was John Morritt. Morritt's father had built this bridge over the River Greta in the 18th century, just outside the park gates, on the Roman road from York to Carlisle. The elegant slenderness and graceful arch of this structure obviously inspired Cotman. It might almost have been the ideal center for a composition belonging more to the Age of Reason than to the coming Romantic era.
Although Cotman's home, both literally and stylistically, was Norfolk in East Anglia, the northern county of Yorkshire seems to have brought something new and vital to his art at this time, as it also did for Turner's and Girtin's. ''Greta Bridge'' is, unquestionably, the response of an artist to the feel, perhaps even the strangeness, of a place. To anyone who knows Yorkshire, it somehow seems instantly familiar.
Interestingly, Cotman wrote in a letter that while staying at Rokeby (the visit also included a side trip to Durham) his ''chief study'' was ''Colouring from Nature, many of which (studies) are close copies of that fickle Dame, consequently valuable on that account.'' What he brought to that ''Dame'' was a considerable degree of order - though in the process he by no means robbed her of her charms.