i've chosen the 50 watt mercury bulb because it's not self-ballasted, and it's sort of an antique and not too bright. (the minimum in a metal halide is 70 watts which is neither antique or in the process of being phased out, and it is much too bright). the fixture is a basic el cheapo (the one without the pull-chain) porcelain fixture which is held in place on the electrical box in the ceiling. inside the ceiling is not accessible to me so i would mount the ballast somehow (and HOW somehow is the question) on the wall. the self ballasted is minimum 160 watts and is too bright also. i also have an old lumiline (incandescent 60 watt tube) bulb which of course requires no ballast. strange hobbies for strange people. thanx for your help.