"In making the selection we mist recall the trout's view of the fly and the manner in which he sees it. Before it actually enters his window, he sees the impression it makes on the mirrored surface of the water. This is why a fish will often rush expectantly at a floating fly only to turn back at the last moment. The impression on the water shows him that an object is floating, but his window is so small when he is near the surface that he can't actually see what is on top until he swims under it."
"The Trout Fisherman's Bible" Dan Holland
Rick