So... If I wanted to say "...further than the eye can see," is it farther or further?

'farther'
It is
' far' = at, or by a great distance:
"How far away can a shark smell blood? A shark can smell blood at a distance of half a kilometre."
comparative : ' farther '
superlative : 'farthest'

Use 'farther' and 'farthest' when talking about distance, and 'further' and 'furthest' for everything else.

"Move farther down the train so other people can board."
"A telescope helps us to see farther than the unaided human eye."