Here in the uk, its just not polite to have a mirror "assume" the correct brightness; we need to be able to command the wretched device to stop at the touch of a button. It comes from generations and generations of slaves that worked for our empire. Back in the days when we had horse & carriages , our rear view "man" (often a slave) would stand in front just to the side of the driver holding up the family's finest mirror from the traveling silverware case. If he didn't hold it still, dazzled you, or you couldn't get a decent reflection on what was going on behind you, you were legally allowed to smack him on the head with the butt of your flintlock.