The young Samuel Palmer was enchanted by twilight. Many of his so-called ''visionary'' pictures of the 1820s are celebrations of this between-time when (in his words) ''trembling light/Glimmers behind the little hills.''
''The Valley Thick with Corn'' comes from a sextet of ''sepias'' with rich, varnished shadows and resonant light, all in the Ashmolean Museum, Oxford. Each of them seems to belong to some phase of twilight - the almost complete darkness of late twilight, the crystalline stillness of morning twilight, and in this case, that strangely crisp time on a late summer's evening when the hills begin to darken against the flat sky and every smallest detail of the landscape seems like a jewel, self-illumined and super-clear.
Palmer's imaginary world is both cosy and mysterious, the English countryside , cottagey and picturesque, transmuted not only by a revitalized pastoralism but also by a fierce religious devotion. Palmer found an aesthetic source for such feelings in the art of the ''grand old men,'' the old masters he knew through prints. Durer's woodcuts, the landscape backgrounds of Raphael, the early Netherlandish pictures, were his artistic hunting ground and stylistic inspiration. He taught himself deliberately to draw with severity and exactness by emulating these prints. They fed his need for excess and intensity. And he looked at the hills of Dulwich - where he would go and sketch from his home in London in 1824 and '25 - through the eyes of these early artists. He tells himself, ''Look for Van Leydenish qualities in real landscape. . . .''
The results are splendidly apparent in ''The Valley Thick with Corn.'' Here he follows his own instructions, making a picture that is ''most elaborate and full of matter'' - filled with details and forms - ''yet simple of style, and . . . what would have pleased men in the early ages.'' As in the prints, the hills , however distant, have ''clear force of line.''
Perhaps the most extraordinary thing about Palmer's visionary work is that it is entirely his own and yet it is overflowing with youthful enthusiasm, not only for other artists and for actual landscape, but for poetry and religion. It is so rich that it can accommodate a tremendous range of influence and reference without losing its originality.
''The Valley Thick with Corn'' is crammed with the fruitfulness of nature and the opulent results of man's cooperation with it; and it is also without doubt intended as a symbol of ''that peaceful country where there is no sorrow and no night.'' Everything in it is tranquil and satisifed. The large harvest moon - an image of completeness - rises: Palmer most certainly saw it as the ''lesser light'' of Genesis which God created ''to rule the night.''
The poetry of Milton (his vision of paradise), of the mystics, of Virgil, and the prose of Bunyan's Pilgrim's Progess are all part of Palmer's atmosphere; but it is the Bible which he particularly relates to ''The Valley Thick with Corn.''
He quoted a favourite passage from Psalm 65 on the back of this picture's mount. Interestingly he chose an English translation of these words earlier than the one in the King James Version. His quotation comes from the ''Great Bible'' of 1539: ''Thou crownest the year with Thy goodness: and Thy clouds drop fatness. They shall drop upon the dwellings of the wilderness; and the little hills shall rejoice on every side. The folds shall be full of sheep, the valleys also shall stand so thick with corn that they shall laugh and sing.''