No one has doubted for at least 100 years that Constable was one of the greatest British painters, but connoisseurs have not hitherto seriously rated his drawings - they tend to be tucked away in boxes at great museums and are difficult to see.
All that is about to change, by courtesy of a fanatical collector - David Thomson, eldest son of Lord Thomson of Fleet.
The 83 Constable drawings in the exhibition all come from his collection and reveal that Constable was a genius with the pencil. His hedges, houses, rivers, foliage, details and distances are caught with magic twists of the pencil.
Mr Thomson, 37, first fell in love with Constable's style as a child. His father had a group of oil sketches attributed to Constable - all of which have subsequently been reattributed to other artists. He himself bought his first misattributed Constable at 17 - Constable scholarship is a nightmare because the artist had so many imitators. He bought his first genuine drawing in 1976 at the age of 19.
His agent in the early days was Charles Leggatt, son of Sir Hugh Leggatt, who ran the former Leggatt Gallery in St James's. Mr Leggatt, who is now assistant curator of the Dulwich Gallery, suggested the exhibition. He also, back in 1976, introduced Mr Thomson to the Constable scholar, Ian Fleming-Williams, who became his close friend and collaborator.
Mr Fleming-Williams, 79, was joint organiser of the last two Constable exhibitions at the Tate and is the author of several ground-breaking books on the artist. The exhibition of drawings was his idea. He has firmly excluded watercolours and oils, arguing that their colour would distract the viewer from appreciating the artist's extraordinary skill in black and white. He has written the Dulwich catalogue and says it is his last Constable publication.
Mr Thomson, in contrast, underlines that the collection is unfinished - it is nothing like what it will be in another 30 years.
He already knows what other drawings he wants - it is just a question of teasing them away from their present owners. He is good at that. Besides buying from auctions, dealers and private collectors, he has even managed to extract drawings from institutions. The Mound of Old Sarum from the South-east came from the Royal Institute of Cornwall in exchange for 'an oil painting and quite a lot of money'; The Great Alder in Leadenhall Garden was promised to the National Gallery, Washington, but Mr Thomson persuaded them to swap.
The exhibition, extended by the addition of about 30 of Mr Thomson's Constable oils, is to be shown at the Frick Collection, New York, in November, and at the Art Gallery of Ontario, Toronto, next May.