Many viewers have noticed that, in George's reality, Mary does not wear glasses. In the alternate reality, however, she is shown as a spinster librarian wearing glasses. They point out that there's really no way that just knowing George could have affected Mary's eyes, so several theories have arisen to explain this conundrum. The most common explanation is that Mary spends so much time reading that she ended up straining her eyes. A mirror theory is that, in George's reality, Mary did not read a lot, so she didn't need glasses. Both theories are based on the idea that reading too much causes one to need glasses, an idea that was prevalent in the 1940s but has since been disproved. Another theory is that Mary needs glasses in both realities but doesn't wear them in George's reality in order to look 'pretty,' because it was a popular belief in the 1940s that 'boys don't make passes at girls who wear glasses.' A third theory is that something happened to Mary's eyes in the alternate reality, e.g., an illness that might not have happened had George been there. The most likely explanation is that the screenwriters were exploiting a common stereotype in 1940s in order to make Mary seem frumpy and unglamorous: that older, unmarried woman all wore glasses and dowdy clothes, had their hair in buns, and grew up to be librarians, teachers, or nurses. They also gave her a rather ungainly way of walking in this scene.