Such inflammation may come from eye-strain, sometimes from glare like furnace heat, or the reflection of the sun on the snow.

Snow-blindness is sometimes painful.

Why?
Iritis.
"In any case, a chronic irritation came into your case some time.
Little by little there came a heavy exudation around the edges of the inflamed iris.

It was so heavy that we call it a 'plastic' exudation.
Now, that was what was the technical trouble of your eye--plastic exudation.
"This exudation, or growth, as we might call it, went on from the edges of the iris until it met in the middle of the pupil.

Then there was spread across the aperture of your lens an opaque granulated curtain through which light could not pass.