''By the Rainbow of Rubens,'' the English landscape painter John Constable said in a lecture in 1836, ''I do not allude to a particular picture, for Rubens often introduced it. I mean, indeed, more than the rainbow itself, I mean dewy light and freshness, the departing shower, with the exhilaration of the returning sun, effects which Rubens, more than any other painter, has perfected on canvas.'' No tribute to Rubens as landscape painter has outdone this one, largely because Constable was openly acknowledging his debt to the Flemish seventeenth-century artist and in the exact terms of his own artistic ideals. Landscape - its exciting variations of weather and light, its distances and skies, its dew and freshness - all these were the relished subject-matter of Constable's art.
He may not have been referring to a ''particular picture,'' but he was certainly familiar with the Wallace Collection's ''Rainbow Landscape'' and even noted his regret that it had become separated from its ''companion,'' a painting in the National Gallery known as 'The Chateau de Steen.'' One feels that ''Rainbow Landscape'' has all the qualities that Constable found inspiring in Rubens' work.
Constable devoted his entire life to landscape painting, while Rubens - that giant of the Baroque - had only come to it late in his career, and in ways which some have read as almost an afterthought. It is perhaps tempting to see Rubens' landscapes as the pleasurable enthusiasm for countryside of a retired man. Yet the whole of his art, his allegories and religious paintings and portraits, give off a similarly heightened enthusiasm and pleasure. Everything he did seems to have been charged with vitality and fullness, a celebration of the fruitfulness and inclusiveness and energy of life.
If earlier he had relegated the landscape backgrounds in his works to specialists and assistants, this may have been as much a practical measure in a studio flooded with commissions, as it was an attitude toward the painting of landscape as a minor art. He certainly didn't relax his immense powers as a painter when he turned to landscape. Although they are personal in their connection with his ownership of property in the country and with his posture as a country gentleman (Friedlander wrote that he ''watches the land like a country squire, like a huntsman - with optimistic vitality''), Rubens' landscapes are still impressive in size and public in statement. They can hardly have been produced for private family enjoyment alone, and they are finished works, as is shown by comparison with studies and drawings he made in the open air. Works like ''Rainbow Landscape'' are clearly studio paintings, carefully composed and arranged and finalized, not immediate sketches of a particular view.
What an extraordinary sense of scope, of extent, Rubens conveys in ''Rainbow Landscape.'' The drama of distance is given full play, the large foreground trees receding by means of powerfully deliberate perspective until they become faintly perceptible specks. Like Constable, Rubens would probably have acknowledged the inspiration of earlier landscapists, in his case such painters of rural vistas and panoramas as Pieter Brueghel and Altdorfer. Like them he presents the countryside as a broad arena stretching in all directions. One feels he was entirely unsatisfied by the limits of his panel, and aimed by whatever means he could to delimit his picture. There is evidence in another landscape that he started with a small detail and then kept enlarging and enlarging the scope of his scene by adding panels round it. It is as if he felt somewhat frustratedly that the sheer endlessness of the natural world, of that wide circle of the earth and the vast dome of the sky, were always beyond the grasp of painting. The arc of the rainbow in the Wallace Collection picture not only adds to the excitement of what Jennifer Fletcher calls ''the scarcely resolved conflict between storm and sunlight.'' It also extends the dimensions of the painting remarkably, perfectly depicting that intangible ambiguity of space peculiar to rainbows. He has even brought the low clouds forward toward the viewer so that we can believe that this sky is not merely a painted backdrop but an element which reaches everywhere and canopies the earth. The expectation and urgency of the rainbow instills the cattle and ducks, the horses and haymakers and peasant girls with buoyant spirits and exhilaration. A rich sense of humor is more than evident in such details as these girls' knowing self-awareness, the cows whose reflections in the stream seem to look at their originals in surprise, and the various antics of the cluster of ducks in the right foreground. The whole thing sings delighted praises to the vast playfulness of the world.