Robson's description
     

When we got back to Cambridge from Washington we made some larger prints to try out our idea for making measurements on people by getting them to draw in the boundary. Unfortunately, as we saw straight away, it doesn't really work. What you see depends so much on where you are looking, on eye movements themselves, on contrast adaptation produced by having the higher contrasts visible, and by the presence of the boundary line if it is actually drawn in. A pity, because it is such a nice idea.