'5) In This Two Part Painting By Albott Thayer, You Can Immediately See The Bird Part At Left Side. But It Is Practically Impossible To Get a Better Partial Fit To Any Bird At Right, The Camouflage Is So Successful!
By Abbot Thayer - Thayer G.H 1908 The concealing colouration of animals: new light on an old subject. Century Magazine. LXXVI, 249–261., Public Domain, https://commons.wikimedia.org/w/index.php?curid=22561557
File = Painting2ViewsOneInvisible=800px-Abbott thayer countershading