The after-image left in the memory by some of Thomas Gainsborough's last full-length portrait paintings - and it is emphatically the case with ''The Morning Walk,'' as it has come to be known - is of figures elevated into an extraordinary refinement and tallness. The elegance and fashionable assurance of his wealthy clients is only a part of this. What is involved is a style that does not merely match its subject, but actually invests it with a kind of stately self-confidence. William Hallett and his wife, Elizabeth, have almost become the pretext for Gainsborough's expression of artistic dignity and command: they are figures in his dream landscape, strolling like royalty (or like beautifully groomed thoroughbred racehorses) through a wooded landscape that belongs to a world of high art or the theatre.
It is significant, perhaps, that this marriage portrait, painted shortly after the couple were married in 1785, has earned itself a popular title showing how people associate it with something more than mere portraiture. It lifts the business of making likenesses into an idea realm of the imagination, identifying attitudes of class and status, but also touching on a kind of experience more universal than self-importance or family pride.
Gainsborough was engaged in a painting (now in the Frick Collection, New York) called ''The Mall'' at about the same time that he was painting ''The Morning Walk.'' ''The Mall'' is exactly like a stage set, with fashionable women strolling among trees, seeing and being seen. It is to a considerable degree a fantasy scene, conscious of the example of the French painter Watteau's treatment of the beau mondem as a kind of indoor-outdoor fancy-dress party.
Romantic airs and graces are transformed into visual chamber music, the delicate rustling of silks imaginatively mingling with the gentle, feathery sound of foliage wafted by a light breeze, as though the shimmer of paint and brushwork could convey sound.
Gainsborough had close friendships with musicians and people in the theatre, and his achievements in paintings like ''The Morning Walk'' (which might almost be two of the figures strolling in ''The Mall,'' lifted into glorified close-up to tower graciously above the viewer) can be appreciated as to some extent analogous to the artificial world of music and scene-painting. It is the naturalness of an artist in marvellous control of his art, like a musician at one with his instrument. Words a friend used to describe Gainsborough's letters and conversation might equally apply to his art: ''. . . Gay, lively - fluttering round subjects which he just touched, and away to another.''
''The Morning Walk'' seems to be full of air and liveliness because it is full of the quickness and lightness of Gainsborough's touch. Its compelling ''reality'' results from the artist's persuasive feel for light, which unifies figures and setting so that there is never any doubt that the refinement of fashion is in perfect keeping with the refinement of nature. It is only when one begins to analyze the picture coolly that its splendid artificiality, and the skill of its contrivances, become at all evident. But as viewers we remain as undisturbed by this as the Halletts apparently are by weather or mud or insects, or by the slightest awareness of the absurdity of their dress for a walk in the country. We willingly accept that country as the English parkland it purports to be: and yet it belongs to the theatrical and heroic world of Van Dyck and his portraits of Charles the First. It is really a delightful and impressive backdrop.
But in the completeness and confidence of Gainsborough's mature vision, and his supreme ease of manner, he arrives at a picture so cohesive that it seems a world observed rather than artificially composed. His contemporary, Joshua Reynolds, though very different in character, spoke admiringly of Gainsborough's ability to form ''all parts of his picture together: the whole going on at the same time, in the same manner as nature creates her works.'' He did not use an assistant to paint the draperies of his portraits (or even the dogs in them), as was the contemporary practice. Obviously portraiture became to him a form of expression that was far from being a routine of studio techniques. It engaged his total attention and involved all his admiration of other artist's work, of music and of poetry. The images that resulted have a unique blend of elegance and exultancy.