This is an illustration of extended Necker cubes. It is interesting that the two upper corners read as if we are looking up - the square comes forward (red view): and the two bottom corners read as if we are looking down (blue view) even though neither view is stressed in this simple drawing. You can see this reversed but it takes some work to make it happen, concentrating on a particular view and trying to see the extended cube as one long diagonal rectangle helps. Notice that your eye shifts views as you look around the rectangle. I think the same thing happens in the Boucher - I think artists understood this and used it to create discrete spaces - unified by the connecting geometry which gives the discrete spaces a sense of one space. Margaret Livingstone points out in her book "Vision and Art" that the eye can focus on only a very small area at a time about the size of a quarter. This means that while looking at a painting you experience it from moment to moment and piece by piece.
