John Constable was a radical for his era: he actually observed nature!

The name of John Constable (1776-1837) must rank high on anyone's list of the world's greatest landscape painters. Not only did he produce some of the most memorable depictions of land, sea, and sky, he was also responsible for a number of the most truly astonishing oil sketches ever created. Together with the paintings and watercolors of Turner and the watercolors of Palmer and Blake - they are among the only truly great works of art produced by England during the 19th century.

As a matter of fact, Constable was both great and highly influential, and left his mark on the French painters of the Barbizon School, as well as the American landscape painters of the mid-19th century. Most important, however, his fascination with the effects produced by light and the weather at different times of the day paved the way for some of the open-air experiments of the early Impressionists.

For the first time, Americans wishing to see Constable at his best and in depth need not travel overseas. They can, instead, view a superb selection of his paintings and oil sketches at the Metropolitan Museum here. ''Constable's England'' is the first international loan exhibition of Constable's art to be shown in the United States. It consists of 64 works chosen from British and American public and private collections. Among them are several of his major paintings, including ''The Cornfield,'' ''View on the Stour Near Dedham,'' and ''Wivenhoe Park, Essex,'' as well as a number of his most stunning sketches.

Constable's career began traditionally enough with the usual formal schooling in London. By 1802, however, after three years of study, he became disillusioned. Rather than establishing styles of their own, he felt that he and his fellow students were merely copying the work of others so as to create pastiches of traditional devices and styles. Why not, he asked, go to nature itself and evolve a personal style through direct observation and the painting of ''laborious studies from nature''?

Obvious as that may sound, it was by no means common practice in those days. One could make outdoor sketches with pencil or wash, but they then had to be translated into painterly terms in the studio, and adapted to academic or traditional ideals.

Constable helped change all that. By taking his oil paints outdoors and engaging nature at its source in his oil sketches, he brought a new sense of immediacy and vivacity to landscape painting. Thanks to him, landscape painting became increasingly spontaneous (even though much of the final work was done in the studio), and the idea that a landscape could represent nothing more ''important'' than a particular quality of light, or a certain moment of the day , gradually began to take hold.

He did not, however, scrap art history altogether. Some of his most famous works owe something to the landscape paintings of those who preceded him. What he borrowed was peripheral, however, and in no way altered his original conceptions. Thosem always came from direct observation.

By the time he was 30, seeing and painting were almost a single act for him. His oil sketches give the impression that he could transcribe any view into paint in a matter of three or four minutes. And yet, no matter how swift the execution, it was deadly accurate. Everything essential, from trees and buildings to atmospheric effects, was given its proper emphasis and did its bit to contribute to the whole.

It's this sense of wholeness and authenticity that lies at the heart of Constable's genius and influence. No one before him had tried so hard to capture the full dimension of what the eye sees, experiences, and registers as it looks at and absorbs a particular view. In this he was a true revolutionary, whose work exerted an important influence on landscape painting as it was practiced during the rest of the 19th century.

This important and beautiful exhibition will remain open to the public at the Metropolitan Museum through Sept. 4. John Sloan etchings

John Sloan (1871-1951) was also a champion of direct observation, but not of fields and streams so much as of urban places and events - and of the people who could be found there. If Constable preferred open spaces and distant horizons, Sloan focused his attention upon city streets and teeming humanity. If the one depicted a natural world only slightly modified by man, the other represented a human environment only occasionally interrupted by a tree or a patch of sky.

People dominate most of Sloan's paintings and prints. It was not unusual for him to pack several dozen people into one composition, nor to depict a large crowd in an etching five or six inches high. Humanity fascinated him, and gave him ample opportunities to indulge his talents for characterization, social commentary, and satire.

For anyone not fully acquainted with the full range of Sloan's prints, I recommend a visit to the excellent exhibition of his etchings currently on view at the Mary Ryan Gallery here. I also recommend that he or she view it in a leisurely fashion, for the show's roughly 130 etchings depict so many facets of humanity that anyone going through it quickly will miss a good 70 percent of what Sloan was trying to say.

Sloan's prints are so richly detailed that it pays to study them closely. Some of his most revealing and delightful pictorial dramas occur in the midst of crowds, and derive much of their poignancy and humor from the precision with which each individual in the crowd is depicted as a very real and specific human being.

Sloan's compositional skills were exceptional, but never obvious. Since he preferred informality to formality, diversity to conformity, and art that didn't make an issue out of pattern or design, he worked very hard to make his compositions seem ''natural'' and spontaneous. His prints, as a result, give little indication of the planning and skill that went into them. His crowds, in particular, were shrewdly composed, and yet the people in them appear to relate to one another more as participants in a shared experience than as components of a rigidly controlled compositional scheme.

Sloan, in short, was more interested in drawing inspiration from life than from art. He saw life as a dense and rich tapestry, to be enjoyed and shared, rather than as raw material for the portrayal of a subjective state or the projection of a formal ideal. Man is central in Sloan's art; theme, design, and meaning come through his actions. And art comes about through direct observation of who man is and what he does.

This excellent exhibition - the largest showing of Sloan's prints ever assembled in New York - will remain on view at the Mary Ryan Gallery, 452 Columbus Avenue, through May 22.