In his later years he learned much from the British mathematician Roger Penrose and used this knowledge to design many "impossible" etchings such as Waterfall or Up and down.
Sa kanyang mga taon mamaya siya natutunan marami mula sa British dalubbilang Roger Penrose at ginagamit ito sa kaalaman sa disenyo ng mga "imposibleng" etchings tulad ng mga talon o Up at down.