The speaker has a glass eye, and the poem turns that fact into a strange kind of optimism. A glass eye can't really see, so the title, "Second sight," points two ways at once: a replacement for lost vision, and the older idea of seeing what ordinary eyes miss. The speaker's hope is that this odd, artificial eye might lead them to something that is not "dull" or "old-fashioned."
The last stanza is the twist. They hope to find a thing "that nobody could unsee," something so striking that once noticed it can't be forgotten. There's a dry joke in that, since the speaker's own eye can't see at all. The tone feels wry and quietly curious rather than bitter. Judging by the language, the speaker isn't mourning a loss but treating it as an unusual vantage point, a chance to look at the world differently. The mood is light, a little playful, and open to surprise.