Let me begin by paraphrasing a daughter's letter to her father, of June 1326:

I haven't wanted to upset you, but I am suffering from an illness of the head that will totally deprive me of sight, though I may be able to tell light from dark. Can you please find someone there who can help me better than the doctors here? They are calling this illness “cataract”, and apparently it can be cured if mine is the right kind, though no one here says it is.

The frightened daughter is the 25-year-old Princess Isabel, wife of Frederick of Austria, writing from Graz to her father James II of Aragon. Her anxious father wrote back immediately, asking her what her doctors said her symptoms were and what they thought had caused the illness, so that he could summon his own medical experts to see if they knew how she could be treated. Unfortunately, as it happened, James himself died the next year, before he could do anything further.