Nystagmus: Rapid involuntary movements of the eyes.
Geoffrey shook in fear, his eyes flicking from side to side rapidly.
“Be still, child!” said Father Simpkins, flicking holy water into Timothy’s face. Timothy blinked; the priest was just a restless shape before him.
“Begone, foul demon!”
After many more exhortations, the priest turned to his parents and said, “I am sorry, but the devil’s influence is too strong. The boy will have to be taken to a sanctuary for further measures.”
It was a few centuries too early for the priest to know the condition had a name – nystagmus, and it was only made worse by stressful situations.