The professor is a graduate of the London Transport School of Art Improvement. Apparently he learned his craft originally by painstakingly restoring details accidentally omitted from advertising posters on the underground.
Here, he felt that clumsy previous restorers had obscured some detail which the artist had originally intended to be there (See detail, right). "The composition's wrong. There's something missing".

A little turpentine, and the professor's amazing "ballpoint pen" technique soon revealed the artist's true intent.