Symbolism, Imagery, Allegory
Pro tip: As a general rule, issues with glasses and glass eyes are meant to signal vision changes in characters. Yes, literally, but also symbolically: They let us know people are seeing the world around them differently.
In "Good Country People," when Manley takes Hulga's glasses, she doesn't even notice; she just accepts her new vision of the scenery without question. But, something new is going on inside her. Check it out:
When her glasses got in his way, he took them off of her and slipped them into his pocket. (110)
It's subtle, but what happens is that Manley takes command of Hulga's vision—the way she sees the world around her in this moment is completely in his control. And you know a key part to how he got her to this moment in the loft in the first place? By telling her he likes girls with glasses (73). It's what he says to her right before he asks her on a picnic. The compliment is when he begins to direct Hulga's gaze his way, and pocketing her glasses is when he officially takes over the show.
Early in the story we are told that Hulga's "constant outrage had obliterated every expression from her face" and that she "would stare […] with the look of someone who had achieved blindness by an act of will and means to keep it" (5). This is your typical O'Connor paradox: Hulga must give up her glasses—devices which augment her vision—to gain a more creative, and perhaps more emotional, way of seeing the world.
Hulga's vision is, of course, literally quite altered--she sees lakes where there are actually hills. When she last sees Manley "successfully struggling over the green speckled lake" (142) her face is described as "churning;" Manley is gone, and she is sad and upset, but at least her face is no longer frozen and expressionless. By losing her literal vision, she is arguably regaining some emotional vision.
While this might sound like kind of a good thing, keep in mind that it also affects Hulga's ability to navigate the world. We don't see her stumble, but just like she needs her leg to get around, so, too, does she need her glasses. So Manley has altered her relationship with the world around her negatively as well. And when he admits to having stolen another girl's glass eye (142), his toying with Hulga's vision becomes all the darker seeming: This is a young man who likes to toy with the world.