Under the greenwood tree

PAINTING portraits ''in little,'' which produced such vivid records of prominent figures in Elizabethan and Stuart England, had originated under Henry VIII as a sideline to the painting of illuminated manuscripts. Perhaps its two greatest practitioners, according to Edward Norgate's 17th-century treatise, were ''Mr. Hilliard and his rare disciple, Isaac Oliver.''

Oliver, however, as Sir Roy Strong argues in his 1983 book ''The English Renaissance Miniature,'' cannot be described simply as Hilliard's disciple: '' . . . any examination of his work quickly establishes that apart from . . . technical expertise . . . Oliver makes no use of Hilliard at all.''

Strong sees Oliver, in fact, as a complex and ''major artistic personality'' whose drawings - works of art in their own right - belong to the emergence of a new era in English painting. But ''with Hilliard,'' he writes, ''we are touching the end of a tradition.''

Very little is known about Oliver from documents (he seems to have been the son of a Huguenot, a goldsmith from Rouen, who came to England to escape religious persecution), so his art itself is almost the only means of understanding his character and significance. Strong concludes that Oliver is ''by far the greatest painter to work in England between Holbein and Van Dyck.''

Certainly he seems to have been trained in an aesthetic much more ambitious, and closer to the achievements of the Italian Renaissance and the ''mannerism'' that grew out of it, than to the tradition of medieval craftsmanship from which Hilliard's miniatures emerged.

Oliver must have turned to miniature portraiture out of necessity: English patronage demanded it. It seems that his sitters' tastes could limit his ambition throughout his career. Even in its last years - as in the case of the sparkling, brightly colored ''Lord Herbert of Cherbury'' of 1613-14 - his abilities found expression in a manner he had really outgrown. At the time he portrayed this chivalrous, ''melancholic'' young dreamer under a greenwood tree, other paintings by him show a tendency toward the ''mannerism'' of contemporary artists on the Continent. Sir Roy is even aware of the early works of Rubens and Hals. And he was influenced by the fantasy and classicism introduced into English court art by the Italianizing architect and designer of masques, Inigo Jones.

But ''Lord Herbert of Cherbury,'' in its jewel-like detail and its brushwork, looks back to Hilliard's emblematic style. In fact, this is one of its charms. Respecting Lord Herbert's conventionality, Oliver has produced what Strong calls ''a Romantic Hilliardesque vision.''

Hilliard, however, could never have painted such a convincing landscape space , receding through layers of fresh green foliage, painted with relish, to a blue distance; or given the reclining, self-conscious aristocrat such an air of naturalism and actuality. As with some of the lyric poetry of the early 17th century, this painting is fascinatingly caught between two styles: a new attitude instills an old language.

As for Lord Herbert himself, much more is known about him than about Oliver. He was a gentleman, soldier, poet, statesman, and philosopher. He has been called the ''father of English deism.'' Not long after Oliver painted him, he was James I's ambassador to the French court (1619-24). His poems are still sometimes reprinted under the banner of 17th-century ''metaphysical poetry,'' though his younger brother, George Herbert, is the supreme poet of the family. Lord Herbert counted John Donne and Ben Jonson among his literary friends.

He was keen on having his portrait painted. An earlier, stiffly formal painting by Robert Peake shows him wearing robes in which he was made Knight of the Bath. But Oliver's version of him is much more enchanting.

When Lord Herbert was over 60 years old he wrote his autobiography. Remembering the vigor and success of his youth, he must have looked at this small picture with a feeling of gratified nostalgia. Probably it was even an exercise in nostalgia when it was painted: It is quite theatrical in its mixture of old-fashioned chivalry and new-fashioned ''melancholy.''

Melancholy was considered a sign of imagination and ''wit.'' Only about seven years later Robert Burton was to write in his ''Anatomy of Melancholy'' (see ''Loose-leaf library'') that ''most pleasant it is . . ., to such as are melancholy given, to lie in bed whole days . . . to walk alone in some solitary Grove, betwixt Wood and Water, by a Brook side, to meditate upon some delightsome and pleasant Subject. . . .''

It might almost be a text for Oliver's ''Lord Herbert of Cherbury.''

Under the greenwood tree Who loves to lie with me, And turn his merry note Unto the sweet bird's throat, Come hither, come hither, come hither:

Here shall he see

No enemy But winter and rough weather. Shakespeare