Borges had inherited from his father the disease...

"Borges had inherited from his father the disease that gradually, implacably weakened his sight, and the doctor had forbidden him to read in dim light. One day, on a train journey, he became so engrossed by a detective novel that he carried on reading, page after page, in the fading dusk. Shortly before his destination, the train entered a tunnel. When it emerged Borges could no longer see anything except a coloured haze, the “darkness visible” that Milton thought was hell. In that darkness Borges lived for the rest of his life, remembering or imagining stories, rebuilding in his mind the National Library of Buenos Aires or his own restricted library at home. In the light of the first half of his life, he wrote and read silently; in the gloom of the second, he dictated and had others read to him."

— Alberto Manguel, The Library At Night.