(gutta serena). An old name for amaurosis. It was at one time thought that a transparent, watery humour, distilling on the optic nerve, would produce blindness without changing the appearance of the eye.
“So thick a `drop serene' hath quenched these orbs.”Milton: Paradise Lost, iii. 25.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894