That marriage of portraiture and genre painting - the English 18-century ''conversation piece,'' or family portrait - is sometimes described as ''informal.'' Looking at the works of Arthur Devis - today considered one of the notable exponents of this type of picture - the word seems strangely inappropriate.
It is clear what is meant: Compared with the heroic baroque portraits of Van Dyck and his 17th-century followers, such modest paintings of middle-class patrons, carefully arranged in house or country settings, do not seem pretentious. But informal?
Devis and the conversation piece have received increased attention recently. In 1980 the Yale Center for British Art made him the central figure of an exhibition on the conversation piece.
Now he is almost the exclusive subject of a major show, with loans from Yale and private and public collections in Britain: ''Polite Society by Arthur Devis.'' The exhibition recently completed a showing at the Harris Art Gallery and Museum here in Preston and now will go to the National Portrait Gallery, London, Nov. 25 through Jan. 29, 1984.
The phrase ''polite society'' summarizes Devis's paintings. Too, his world seems altogether remote from ours. In certain ways, however, as the show's catalog makes evident, they cannot be taken as accurate representations even of the society they apparently depict.
Steven V. Sartin, in an essay on his life and art, argues that ''there is a degree of dissimulation (absence of exact imaging) in the paintings,'' but adds that ''this was perfectly well understood and accepted by whoever commissioned them at the time,'' although over two centuries it ''has been forgotten.''
Obviously we cannot see Devis's art in the same light as did his contemporaries. It is apparent, however, that he was not especially innovative and not an artist of burning ambition.
A comparison with the previous generation's genius, Hogarth, indicates the unadventurous range of Devis's subject matter. In fact, he worked within such restraints that it is hard not to see his style as a kind of instant convention - almost as formula painting. Next to contemporary conversation painters like Hayman or Highmore, or even Gainsborough's master, the French artist Gravelot, Devis's figures look stilted and his compositions contrived. Interestingly, it is to early Gainsborough, in his portraits of Suffolk gentry, that he comes closest. But unlike that astonishing artist, Devis developed very little: He may have stretched the boundaries of his style, but he never radically broke through them.
As for ''dissimulation,'' he was not, for instance, above such devices as clothing unconnected sitters in different pictures in the same dress, or providing them with very similar postures.
On occasion they might be shown seated or standing in identical houses, though living at opposite ends of the country. He had recourse to manuals of ''genteel behaviour'' to suggest suitable poses, and to architectural pattern books for Palladian interiors which were, at times, more imposing or fashionable than his clients' actual homes. Clearly he was catering to their aspirations.
Also apparently quite acceptable was his use of the lay figure, the doll employed as a regular part of a portraitist's equipment to substitute for absent sitters. Two examples are shown at Preston. But Devis not only made little attempt to disguise his dependence on it, but seems to have enjoyed presenting his family groups as virtual doll-gatherings. This miniaturization, and its dreamlike naivete, clearly adds to their delight in 20th-century eyes. The intimacy is delicate and exact, the scale meticulous.
But all this hardly justifies the tag ''informal.'' The domesticity of context may have convinced his patrons there was something natural or casual about Devis's conversation pieces, but their style, the way he painted them, is formality itself. Except for one or two fascinating exceptions in this show, the people he portrayed, though on the same stage, scarcely acknowledge one another's presence, and certainly don't converse. Positioned with scrupulous balance and propriety, they are only linked by an unemphatic clarity of composition.
Devis's world is essentially one of formal gentility translated into a patiently stilled, but surprisingly alive, kind of picturemaking. His color intuition can be astonishingly subtle. It is remarkable, for instance, the way he mutes a background so that the lovely or vivid color (and texture) of a costume might sing out. He relishes bright, straight colors.
And he was magically inventive in exploiting the precise, elegant line. It recurs everywhere - as two fishing rods making an incisive triangle against the sky, as the taut curve of a riding crop contrasted with the relaxed loop of a horse's reins. It reappears as the vertical stem of a sapling or the light or the slender legs of a chair.
Throughout the 19th century, Devis was discounted or forgotten, and even by the time of his death in 1787 he had been relegated to obscurity by changing fashions. It seems likely, however, that even during his most successful period , in the 1740s and '50s, he was always seen as no more than a competent provincial.
This comparatively minor role may have been a matter of patronage almost more than limited skill. His connections with the exclusive ruling group of Tories in his hometown of Preston seems to have meant a particular clientele. The majority of his portraits are of landed gentry in Lancashire, Cheshire, and Derbyshire. Comparatively few are of people in London.
These seem to us the epitome of a certain kind of 18th-century living. The dignified spaciousness in them, their order and lucidity, even their spareness, display a pure pleasure in correctness. Devis's sense of spacing and interval, the openness of his world, keeps surprising us by its daring economy. Only at first sight do his paintings look ''all of a sort,'' as one of his contemporaries critically put it.
His perception of character may certainly be restricted, and confined to the faces alone: But it is there, distinctly, nevertheless. As a painter of a felicitous kind of social and period atmosphere, however - not to mention that invigorating but invisible commodity, country air - Devis is often both expansive and unexpectedly original.