`Post cards' from 18th-century Venice. Souvenir works helped tourists tell of their travels

NOWADAYS, when Uncle Harry and Aunt Mary set off on a 10-day, prepackaged tour of Europe, they tend to spend what little free time they have outside of guided bus tours writing post cards back home. This is partly to impress their neighbors and relatives, but also to show them ``all the pretty places we've seen.'' No matter that the Cologne Cathedral kind of gets confused with Paris's Sainte-Chapelle in Uncle Harry's and Aunt Mary's memory. The picture post cards will set it all straight when they get home. And why worry about mixing up the snapshots of La Grand'Palace in Brussels with the Mauritshuis in The Hague, because Aunt Mary has been keeping the exposed rolls of film identified with the date. Since the roll was shot on Tuesday, it must be La Grand'Palace in Belgium.

Just as with Uncle Harry and Aunt Mary, so in days long since gone, the rich, noble families of an earlier generation - who set off on what was called in the 18th- century ``Grand Tour'' - wanted to show their elegant neighbors and gossipy kinfolk back home the sights they were seeing, and impress them with accounts of the places they had visited, especially that most unique of all cities, Venice.

Fortunately, even though they didn't have access to Mr. Eastman's little invention or picture post cards, they had Canaletto. What better way to impress than with a painting of the Grand Canal, the Doge's Palace, Piazza San Marco? After all, a huge canvas over the fireplace is bound to attract attention and be the focal center for ice-breaking conversations at balls, parties, and banquets. Canaletto, the artist of Venice

Giovanni Antonio Canal, who assumed the professional name of Canaletto, was born in Venice in 1697. His father had been a much-respected theatrical scene painter, so it was quite natural for the 22-year-old Giovanni to go to Rome for a year to work on scenery. On his return to Venice, he started painting canvases of topographical views. These far exceeded the bounds of simple ``portraiture of places.'' Canaletto expressed in his Venetian scenes a genuine feeling for art, an understanding of cloud effects, color-filled shadows, and the light and atmosphere of daytime scenes.

These works, which pictured marketplaces, colorful regattas, and historic churches, became extremely popular with tourists, especially the English. It wasn't long before the British consul in Venice, Joseph Smith, a publisher and merchant by trade, became acquainted with Canaletto's work and started encouraging English visitors to commission ``souvenir'' canvases from the artist - typical Venetian scenes that could adorn the walls of their manor houses back in England. (And thus it is that the most comprehensive collection of Canaletto paintings can be found at the National Gallery in London.) Supply and demand

The demand for Canaletto's work became so great that he devised a system that enabled his pupils to participate in his work without interfering in the final result. Ultimately he was forced to work largely from drawings and even other artists' engravings rather than from nature. He also developed the use of the camera ottica, a device by which a lens threw onto a ground-glass screen the image of a view, which could be used as a basis for a drawing or painting.

Sometime around 1740, Joseph Smith encouraged Canaletto to create a series of 30 etchings, which he published in 1744. Since these could be reproduced in some quantity, they could help satisfy the tourist's desire for souvenir views of Venice. Smith later came up with another idea. He encouraged Antonio Visentini, a friend and colleague of Canaletto's, to create a series of etchings and engravings based on Canaletto's paintings of Venice. And, still more, Visentini finally began a series of his own, completely original engravings of Venetian scenes, at the urging of Smith.

A recent exhibit at the Ca' Pesaro here in Venice - ``Canaletto and Visentini/Venice and London'' - provided a marvelous opportunity to view the works of both artists, especially that of the almost unknown Visentini. (In his day he was a well-known painter, architect, engraver, and professor of painting.) The show also emphasized the close rapport that existed among Canaletto, Visentini, and Smith.

On display was a complete set in superb condition of the engravings by Canaletto, a very rare folio from the Gabinetto Nazionale delle Stampe di Roma. An almost complete series of the etchings and engravings by Visentini based on the paintings of Canaletto was also on display, a set that had once been the property of Joseph Smith and now belongs to the collection of Queen Elizabeth II. And the exhibit included many of Visentini's wholly original engravings - many created as illustrations for books - in addition to a large group of designs and original line drawings unpublished until now. There was also a notable selection of paintings by Visentini which came from both public and private collections. Etchings, engravings of most interest

The very essence of the show was the etchings and engravings, for these are the works by which both Canaletto and Visentini were best known in the days before photographic reproductions of oil paintings became possible.

While the exhibit contained two oil paintings by Canaletto, they were simply the point of departure for the show. (After all, almost every major museum in the Western world has its share of original Canalettos.) Likewise, the nine oils by Visentini on display were not the heart of the show, either. They simply provided an opportunity to compare Visentini's skill with the brush with Canaletto's mastery of the technique of topographic painting. (That Visentini demonstrates an exceptional talent for figure painting could be observed by a close inspection of the small area of each canvas devoted to folks gathered in some kind of activity: washing clothes, dancing, musicmaking.)

The collection of Canaletto's exceptionally skillful and sensitive engravings were of special interest, for they revealed a different side of the artist, a command of perspective and luminosity in this restricted medium of black lines superimposed on a white surface.

The Canaletto etchings, likewise, showed the skill and care the artist lavished on each scene, for in many cases there appeared, side by side, copies pulled from his first, second, and sometimes third proof of a single picture, each with a minute but all-important change from the earlier stage or stages. This could be seen, for example, in his ``Capriccio of a Pilgrim in Prayer'' - a view of a pilgrim at a roadside shrine in the foreground with a path across a wooden footbridge leading up a hill occupying the center of the picture. In the first proof, the slope of the hill and its path are not well defined; in the second, the path becomes more defined and prominent, and the effect of light and shade more prominent; in the third and final proof of the etching, a cross has been added to the top of the roadside shrine. These three stages of the engraving could then be compared with the original Canaletto etching that was the basis for this scene.

The Visentini engravings showed an entirely different technique, with each line precisely straight and the perspective resulting, not from a subjective sense of balance, but based precisely on a mathematical relationship. This presents a more realistic even if less artistic and personal view.

Also on display were engravings of Venetian churches which Visentini created as book illustrations, a set of decorative initial letters with Venetian scenes interwoven in the background, engraved vignettes for books, and engravings of architectural detail and perspective meant for teaching purposes.

The only thing missing from the exhibit at Ca' Pesaro were colored post cards to send to the folks back home showing them the exhibit I had just seen!