NOW YOU DON'T SEE IT, THEN YOU DO
Samuel Renshaw's real work was not with training objectivity, but with thresholds of perception.
Some years ago, a friend shared this picture clipped from a newspaper with me. If you don't know what it is, it's very difficult to say what it is. On the other hand, if you do know what it is, then it's impossible for you not to see it that way.
I was delighted to eventually discover that the picture was a tool of Renshaw's.