With all the vibrancy of life

''A beautiful picture by this artist is the act of an honest man.'' Zola's often-quoted definition of Pissarro's talent was prompted by this actual painting when (with another of the same rural area, northwest of Paris, at Pontoise) it was exhibited, a year after he painted it, at the Salon of 1868. Its ''honesty'' is evident in its boldness and, more than that, in the extraordinary truth of its tone. Pissarro has found a way of echoing in paint the intensely dark silhouette of shapes against the brightness of the sky, and this veracity is continued from the horizon down the hillside into the deep shade of the trees, the strong forms of the houses, the walls, and finally the foreground mass of bushes.

His eye told him that the leaves of these bushes were scarcely different in tone from the most distant objects. It also told him that the summer evening light, stretching lazily over the land, merged everything into a unified wholeness, absorbed a multitude of lesser details, and presented him with a broad simplicity of light and dark. And it told him that the road and grass-verge, as they came forward under his feet, actually blurred and grew less specific to the gaze. The painting concentrates attention instead on the middle distance and, spectacularly, on the skyline, which are given far clearer definition and importance.

All these characteristics of ''La Cote du Jallais, Pontoise'' seem unquestionable enough, but at the time they were like acts of rebellion. They made even favorable critics refer to Pissarro's ''brutality''; yet he was painting tranquil evocations of countryside bathed in sunlight and peace.

At the time he painted ''Jallais'' Pissarro had not begun to work alongside Monet and the other artists who were all soon to be nicknamed the ''Impressionists.'' His vigorously developing art still looked toward Courbet's strong landscapes, to his thickly applied paint, his heavy contrasts, his directness, and his relentless love of bright, fierce green. Years later, Gauguin was to refer to ''Pissarro green,'' and this Pontoise landscape is early evidence of his unadulterated transference of the vivid color of grass and leaf to oil paint on canvas.Initially this emphatic hue must partly be a response to Courbet's pictures as well as the rich fertility of Pontoise. Pissarro also looked toward the containing ''envelope'' of light in the landscapes of Corot and to their classical order and balanced construction: ''Jallais'' is a robustly composed picture. And he was further impressed by Daubigny, who lived near Pontoise and had painted in precisely the same neighborhood. It was Daubigny's support which persuaded the Salon jury in 1868 to accept Pissarro's two pictures.

But Pissarro was plainly already his own man. One can see in this picture the first fresh inklings of a landscape painted in terms of planes; the passing phenomena of light on hillside, roof and road is ordered into a syntax of unmistakable horizontal, vertical and diagonal surfaces. Probably the divisions of the particular area of cultivated countryside stimulated this geometrical approach, but equally it could have been these land-divisions which had, in the first place, attracted Pissarro to paint there. Either way, the resulting style is a positive and original move in the direction which his friend Cezanne was later to investigate with consistent determination. Pissarro himself went on to lighter and gentler things.

It seems that it was the generality of Pissarro's early paintings which most offended his contemporaries. One critic informed him that he needed ''more clarity in the outline of branches and the trunks of trees.'' Another complained that ''the air becomes heavy at his touch,'' and that ''his melancholy figures are treated in the same way as trees, plants, walls and houses.'' The problem was that unlike the typical academically painted views of the day, the wholeness of a landscape had become more important for Pissarro than its parts. His scene was no longer an arrangement of distinct, separately identified objects. He was more concerned to paint the totality of what he saw than to pick out the parts in an attempt at greater ''realism.''

With hindsight it is easy enough to understand that he was, in fact, working in line with a long classical tradition - though newly revitalized by the excitment of painting in the open air, right in front of the motif. There are sketches by Poussin and Claude in the 17th century which, in their chiarascuro and massing of shapes, as well as their surprising spontaneity, are not dissimilar to this picture by Pissarro. But his offense against accepted practices came from making an enlarged and finished work retain the vigor and roughness of a sketch. That Zola championed such a daring breakage of the 19 th-century rules says much for his perception and courage. He must have been aware how provoking he would be by stating that Pissarro ''possesses solidity and broadness of execution, he paints generously, following the traditions, like the masters. . . . ''

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