The idea of the photographic ''detail'' of a painting might almost have been invented for the greater glory of those notable masters of the Venetian scene, Canaletto and Guardi. Their depictions of this city in the eighteenth-century were designed to augment memory by an exhaustive and meticulous accuracy of record.
British travellers in particular treasured Canaletto and took home his pictures of the magical city they had visited - pictures that seem to have been built up inch by inch and stone by stone to acquire the very face and feel of Venice and not just its remembered atmosphere. Making up the adequate composition of most of Canaletto's pictures is an extraordinary multiplicity of detail, and the viewer is invited to explore every busy aspect of the view just as if he were a tourist alert to countless visible items in the city's life which its inhabitants (themselves elements of the ''sight'') would hardly notice. Canaletto's paintings can be enjoyed in their components. They stand up admirably to such microscopic delight. They are sightseeing turned into art. They are the most painstaking and precise of holiday mementoes, the originals of a notion that later degenerated into the common post card.
A debated question is whether or not Francesco Guardi, Canaletto's junior by fifteen years, was a student of the older painter. He certainly owed, to Canaletto, a number of viewpoints of the city, though equally it might be argued that for any artist to paint the square of St. Mark's his position must be dictated quite firmly by the scene itself.
It is both to Guardi's advantage and disadvantage that he is inevitably discussed in comparison to Canaletto: it points up his borrowings, but it also shows what is new and personal in his painting. In a recent exhibition contrasting a Canaletto painting and this particular Guardi, both belonging to the National Gallery in London, Michael Helston writes of the Guardi: ''. . . (it) was painted with more of Canaletto's vision than his own. It is a tribute to both artists, who dominated an entire century of view painting in Venice, that their styles should have met in this way.''
While this is broadly factual, if one compares this painting with the actual Canaletto picture of St. Mark's Square on which Guardi based his composition, the differences of approach are tellingly apparent. These differences--as borne out by Guardi's later development--are more than a shift of emphasis; they are a shift of attitude. Guardi had already been a painter of religious pictures, and he turned to view-painting surprisingly late in his career. He brought with him a painterliness, a way of depicting the world, which is much less prosaic and literal than Canaletto's. In his ''Piazza San Marco,'' details such as the one shown here indicate how he used his brush to approximate rather than neatly delineate both buildings and people.
The people in the square, although cunningly characterized by stance and costume, are presented with a shimmering vitality (a mix of nervousness and boldness) which suggests rather than describes them. As he demonstrated in a later work, which shows a crowd of people rushing excitedly away from the viewer to watch the ascent of a balloon, Guardi, much more than Canaletto, saw the figures in his pictures as existing in groups, or even en masse, rather than as separate individuals some of whom just happen to be linked with each other. Not unconnected with this more general, organic approach to the world (which he sees as a whole of unified parts) is the absence of activity in the middle distance of his view of St. Mark's piazza. In Canaletto, the figures are dotted liberally throughout the space described. In the Guardi, there is a leap from close up to distant which is quite new. It is as if Guardi were himself part of the scene, standing on the same stones as the other Venetians quietly moving about in front of him. Canaletto seems to look at, rather than stand in, the scenes he pictures. He perhaps never really forgot his beginnings as a painter of theatrical scenery. Psychologically, there is a proscenium arch between him and his views; whereas Guardi, his successor in the art of Venetian view-painting, places himself (and by extension us, the ''viewers'') actually on stage.