"Nathaniel had a cast in his eye; and perhaps these two
circumstances, taken together, prevented his seeing the matter in its proper light."

A cast in eye means one eye deviates. I haven't been able to find definition to tell whether it means isophoria or exophoria. Strabismus does not indicate which deviation is present:
Alternate Names : Crossed Eyes, Esotropia, Exotropia, Squint, Walleye

Definition
Strabismus involves deviation of the alignment of one eye in relation to the other.
Thoughtless people can really wound the sufferer by using vulgar terms "cockeye" which usually means one eye is turned in, and "wall eye" meaning one deviates laterally.
I used to have a "latent squint" which gave me a lot of trouble until my ophthalmologist added corrective prisms to my prescription. He didn't think I could manage intensive reading required by college. He was almost right.