When Meg Murry, Charles Wallace, and Calvin visit her, she uses the seeing glass to look into Meg's future. She sees a black thing and a bright light, but she doesn't understand what these things mean. She tells them, "The seeing glass shows me many things, but sometimes the things it shows me are so strange I don't understand them. It's as though I'm looking at the world in a different language."
The "black thing" she sees likely represents Meg's fear and doubt (especially about her father's disappearance), while the "bright light" represents the possibility of hope and rescue. This vision hints at the dangers and challenges Meg will face, but also points towards a brighter future.