PHILADELPHIA — Jason Scott-Warren recalled the intense emotions he felt when he started to suspect that the scribbled notes in the margins of a copy of Shakespeare’s First Folio owned by the Philadelphia Free Library were written by none other than John Milton.

One of the most prized and scrutinized books in the world, the First Folio, published in 1623, was the first edition of Shakespeare’s collected plays. And here, in one of the 234 copies known to survive, were previously unrecognized traces of the greatest English poet being read by the man widely seen as the second greatest.

“It’s a combination of elation and fear, a certain kind of terror,” Dr. S cott-Warren , a lecturer at Cambridge University, said Thursday in an interview, describing his feelings.

“As a scholar, you get a sense of the fixed landmarks,” he said. “Suddenly to have a new landmark to come right up through the ground is quite disconcerting; there’s something alarming about that.”