I was messing around with mine, and one of the kids must have been turning knobs. I tried to focus the microscope and "crack"... there went my counting chamber. I focused right through the slide (and they aren't cheap slides). I thought that I might mention that (at least if you have kids around) you should start at the lowest magnification, find the area you are looking for, and then increase the magnification one objective at a time until you are up at 400X.
I work with microscopes every day, and never thought I would make this mistake at home, but I'm back shopping for a new counting chamber.