SCALLOPED POTATOES

8-10 medium potatoes, sliced
1/2 cup (1/2 lb) butter or margarine
1/4 cup flour
1 tablespoon salt
1 quart milk
1/2 cup finely chopped onion
1/2 cup finely chopped celery
1/2 teaspoon celery salt
2/3 lbs processed cheese
1/2 cup cracker crumbs (optional)