What is a loss or decay of sight, from loss of power in the optic nerve, without any perceptible external change in the eye; -- called also gutta serena, the "drop serene" of Milton called?

amaurosis — A loss or decay of sight, from loss of power in the optic nerve, without any perceptible external change in the eye; -- called also gutta serena, the "drop serene" of Milton. (Webster's 1913.)