User blog comment:D2r/Unanswered Questions of Redwall/@comment-3518397-20110411191411/@comment-1427879-20110412112125

Same way people usually do. Just try the types of lenses until you can see clearly. They may not know exactly what's wrong with them but...For glasses, they have windows don't they? They can use whatever glass they did for that, alter it a bit, add some metal so it will hook over their ears and stay there and presto: glasses.