He used a square instead of a line. It’s not that Newton was wrong but rather that Newton was using a subset of Goethe’s findings. He recreated Newton by narrowing the box to a slit but when opened up revealed that the orientation of light-to-dark or dark-to-light matters.
More impressive than his theories though is Goethe’s methodology of observation. He was less on conclusions and heavier on observation and documentation. He didn’t try to use it to prove a theory, rather he observed and documented. When you follow his logic, it’s hard to not be convinced that he was dismissed too quickly especially considering an important fact that Newton missed and Goethe captured: Darkness reflects as well as light does. Newton thought reflection could not happen in the absence of light. [I thought so too because I was raised on Newton’s teachings from science classes] but Goethe was correct there.