Our heads is illuminated by a circular light fitting about 4 inches in diameter and dating from the mid eighties. It's turned on by rotating the body of the unit. We have owned the boat for about 5 years and fortunately in that time we have never had to change the (tungsten) bulb. However, when that time comes how do we remove the cover? Apart from the on and off positions there seems to be a third position but no pushing, shoving, cursing or twisting seems to have any effect. Several years ago on a charter boat I remember changing the bulb in a similar unit but, for the life of me, I can't remember how to do it. Can anyone help please?