AMSTERDAM — Everyone knows that Vincent van Gogh cut off his left ear. But since that fateful event nearly 128 years ago, there has been continuing debate among scholars about the severity of that mutilation, which took place in Arles, France, in December 1888. Did he simply slice off a little chunk of his ear, or did he lop off the entire ear?

The author and amateur historian Bernadette Murphy, while researching the last period of that Dutch Post Impressionist’s life for a new book, discovered a document in an American archive that may help resolve the issue. A note written by Félix Rey, a doctor who treated van Gogh at the Arles hospital, contains a drawing of the mangled ear showing that the artist indeed cut off the whole thing.

The letter and drawing will be displayed for the first time at the Van Gogh Museum’s exhibition “On the Verge of Insanity,” which opens here on Friday and runs through Sept. 25, along with previously unexhibited documents and artifacts that try to provide more detailed evidence about van Gogh’s mental illness.

The exhibition will also include about 25 paintings and other objects, like a corroded revolver that van Gogh may have used to kill himself, museum officials say. These will try to explore, in particular, the final stretch of his life while his troubles escalated, from the ear-cutting incident to July 29, 1890, when he apparently committed suicide in Auvers-sur-Oise, France.