Anamorphosis is a distorted projection or perspective requiring the viewer to use special devices or occupy a specific vantage point to reconstitute the image. The most famous example is Holbein's painting "The Ambassadors", with a memento mori in the foreground in the shape of a distorted skull:
http://upload.wikimedia.org/wikipedia/commons/thumb/9/9d/Holbein-ambassadors.jpg/608px-Holbein-ambassadors.jpg

Andrea Pozzo's 17th century ceiling for the Church of Sant'Ignazio in Rome combines anamorphosis with trompe l'oeil, to make the flat-roofed church look like it actually has a dome:
http://upload.wikimedia.org/wikipedia/commons/thumb/c/cb/Sant%27Ignazio_-_affresco_soffitto_-antmoose.jpg/787px-Sant%27Ignazio_-_affresco_soffitto_-antmoose.jpg

Some modern examples of anamorphosis by Julian Beever:
http://upload.wikimedia.org/wikipedia/commons/thumb/7/7e/Julian_Beever_Mountain_Dew_art_photo_by_David_Shankbone.jpg/800px-Julian_Beever_Mountain_Dew_art_photo_by_David_Shankbone.jpg

and Kurt Wenner:
http://upload.wikimedia.org/wikipedia/en/e/ec/Kurt_Wenner-Dies_Irae.jpg

Thought we'd have a bit of art history today, as I'm planning to hit the galleries at the weekend. What was the last good exhibition/installation/other you saw? And can anyone recommend stuff to see in London at the moment?