Knob on rear view mirror?

[macromegarage418]

I have a 2000 model year sovereign with auto lights and auto wipers, the rear view mirror is auto dimmming , there is a small rotary knob on the back. what is it for? Many thanks

[BrianG]

From memory I think it allows adjustment of the delay time for auto lights to come on, so that they don't come on immediately, for example, just because you've driven under a few trees in daylight. It only has an effect when the light switch is in the 'auto' position.
On the replacement mirror I had to get for my X308 and the later X350, this function was removed from the auto-dimming mirror.