Play on words in Importance of Being Earnest?

It's a play on literal versus figurative understanding of "short-sighted." Her education has been very restricted, so she has a narrow or limited understanding of the world (was brought up, in other words, to be figuratively short-sighted), but (being the nitwit that she was raised to be) in this line she confuses that with literal short-sightedness (or near-sightedness, which can be corrected with gles).