Bechorot 38 - Eye Blemishes of Firstborn Animals
The first group of blemishes that can happen to a firstborn animal, leading it to loose its holiness and allow it to be slaughtered outside the Temple were blemishes of the ear . But how do we know the law of the blemishes altogether? The Torah said, " If an animal has a blemish " - include any blemish whatsoever. Then it continued "such as when it is crippled or blind" - to limit it only to blemishes that stop it from doing work. Then it generalized again, "any other blemish" to widen the definition, and include even permanent blemishes that don't stop it from doing work, such as the ones in the ear. And then the Torah limited it again, saying "serious" blemish, to exclude permanent blemishes that are not visible, such as in the gums inside the mouth. This method of Torah logic is called "amplification and limitation."
The blemishes in the eye include punctured, notched, or split eyelid, a cataract (which requires special expertise), unusual growths, the white of the eye entering the black (but not the black entering the white), white flecks - a condition leading to blindness, and watery eyes if this completely obstructs the vision.
Art: Francois Verwilt - A shepherd couple with goats and a cow nearby