On Feb. 22, 1322, the Norman crossing tower of the Cathedral at Ely in Cambridgeshire collapsed. To this somewhat disastrous event, in a cathedral that was by then considered to be largely complete, we owe one of the great inventions of the late Gothic style, Ely's octagonal crossing, crowned by its octagonal lantern of timber - and also, by long extension into the late 18th century, one of J.M.W. Turner's finest early watercolors.
Although Turner was, at the age of 21 or 22, still working in the ''topographical'' tradition of 18th-century watercolor, producing accurate, detailed views of picturesque places or famous buildings - in response to a steady demand for such drawings - there is already evidence of some of the ambitious and imaginative traits that were gradually to predominate in his work as it developed and expanded through the first half of the 19th century.
No one at the time of his ''Interior of Ely Cathedral,'' for instance, was producing watercolors as dimensionally large as Turner's, or as monumental and sublime in scale. His Ely interior makes dramatic use of the man-dwarfing height of the Gothic octagon, emphasizing its upper reaches, which almost float ethereally above the heads of the diminutive congregation.
He was greatly helped in this effect, of course, by the facts of the subject itself: The palmlike ribs of the vault, springing upward to support the high, eight-sided lantern, are exaltation in architectural form. These ribs (and for some reason Turner chose only to show four to each side of the lantern instead of the actual five) seem to hold up this central construction with a strength not proportionate to their slender elegance. (In fact, the lantern is not as heavy as one might suppose, and it depends on a strong wooden framework hidden behind the stone vault.)
But the misty quality Turner achieves as the eye moves from the shadowy arches below to the light-filled lantern above is virtually a prophecy of his later, overwhelming fascination for ''aerial perspective,'' for the way distant objects become less distinct in space. The whole interior is shown as an enclosure with endless, indeterminate extensions dissolving into light and intangibility or disappearing into hidden recesses of the building.
We know that the artist was commissioned to make paintings of Ely Cathedral by Dr. Yorke, the bishop, but he may have made this large watercolor because of his own stimulated enthusiasm for its architecture. It was exhibited, historians now maintain, in 1796 at the Royal Academy, and everything about it suggests that the spaces and light of the octagon meant rather more to him than a mere job done for a patron.
Nikolaus Pevsner has described Ely's octagonal crossing with notable delight, saying that its design could only have been a ''deliberate attempt at breaking the 13th century's discipline of right angles.'' It allows ''the eye to wander into dim, incomprehensible distances . . . an effect of surprise and ambiguity.''
''The basic emotion,'' he further writes, ''created by the octagon as one approaches it along the nave is one of spaciousness, a relief, a deep breath after the oppressive narrowness of the Norman work. Then follows . . . the next moment's feeling, a feeling of surprise. Its immediate cause is that light falls in from large windows diagonally - a deviation unheard of in the church architecture of the West.''
Turner ''said'' very much the same things through the medium of watercolor, and he especially enjoys the sunlight penetrating the interior in angled shafts, using them as a device almost as ingenious as the architecture itself to cut across, display, and explore Ely Cathedral's unexpected vastness.