sick buff orpington chicken - looks blind

can anyone advise, we have a second chicken who has now developed a poorly eye.  not sure what has happened.  The one eye is sticky and weepy and it looks as if its disappeared .  we had a chicken a couple of weeks ago with the same thing and she was swollen around the side of the face by the eye and the eye looked as if it wasnt there.  they were both eating but balance and direction was a bit haywire, due to not being able to see.  we took the decision to put the first one out of its misery and now i cant believe its happening again.  the buff orpingtons are about 18 weeks old.    none of my other older chickens have any of these symptoms.  any ideas what this would be?