What do the Jurors think is wrong with the woman's testimony?

In Act II, 4th Juror begins a discussion by reintroducing what he considers to be the most compelling piece of evidence, the testimony of the woman across the street, who claims to have heard a scream and then to have seen him stab his father through the windows of the elevated train passing by. While he is speaking, he rubs his nose where his spectacles have made indentations. This causes 9th Juror to realize that the woman also had those same marks on her nose and must have worn glasses, despite the fact that she didn’t wear them in court, presumably for her own vanity. This causes all of the jurors to question the eyesight of the woman, who may have witnessed the murder without her glasses.