The little ball on the end is the way to go. get a glass cutter with one of these for old glass. I have done some stained glass work in my time. For old glass get a new cutter - old glass is very brittle and you only get one pass with the cutter. Try and make 2 and you will crack it. Once you have scored, hold the glass and put a *very* slight stress on it the way you want it to crack, then start tapping lightly at one side (not the middle) you should start a crack fairly easily. If the glass doesn't instantly split along the score (it might) gently progress the crack by tapping. Don't be tempted to try breaking the glass over an edge. In my experience, this seldom works with small pieces of old glass. dipping the cutter in oil/turps/paraffin, light oil or even cooking oil works.