1882
Turner
William Cosmo Monkhouse (1840-1901) art critic, artist biographer and poet clearly heard this story about the young Wilkie from George Walter Thornbury (1828-1876). He was an author and contributor to Household Words and All The Year Round so Wilkie would undoubtedly have known him. Wilkie refers in his letters to 'varnishing days' at the Royal Academy when pictures were hung for the Summer Exhibition and the artists could varnish them - which really allowed a bit of last minute touching up.
Chapter VIII- Light and Darkness - 1840 to 1851
Turner was now sixty-five years old, and his decline as an artist was to be expected from failing health and stress of years. […]
Mr. Wilkie Collins informed Mr. Thornbury that, when a boy –
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    He used to attend his father on varnishing days and 
    remembers seeing Turner (not the more perfect in his balance for the brown 
    sherry at the Academy lunch) seated on the top of a flight of steps, astride 
    a box. There he sat, a shabby Bacchus, nodding like a Mandarin at his 
    picture, which he, with a pendulum motion, now touched with his brush and 
    now receded from. Yet, in spite of sherry, precarious seat, and old age, he 
    went on shaping in some wonderful dream of colour; every touch meaning 
    something, every pin’s head of colour being a note in the chromatic scale. [p121]  | 
  
From Turner by W. Cosmo Monkhouse, London: Sampson Low, Marston, Searle & Rivington, 1882