Ask Marilyn ® by Marilyn vos Savant is a column in Parade Magazine, published by PARADE, 711 Third Avenue, New York, NY 10017, USA. According to Parade, Marilyn vos Savant is listed in the "Guinness Book of World Records Hall of Fame" for "Highest IQ."
In her Parade Magazine column of December 10, 2006, a reader asked how the switch on rear view mirrors in cars dims headlight glare. Marilyn responded by describing one (of two) types of mirrors.
Marilyn could very well be correct in describing an "electrochromatic mirror," but the questioner could very well be asking about how an ordinary night-view lever works on a low-tech rearview mirror.
If the electrochromatic mirror detects glare from cars in the rear, it will automatically dim the mirror view, according to Marilyn. Why then would the questioner wait until she sees the glare before turning on this feature. The questioner does not mention the mirror automatically going back to normal, which Marilyn describes as a feature of her high-tech mirror.
The mirror described could well be the standard rearview mirror, which has a prism (or wedge) of glass in front of the silvered surface. There being an air gap between the wedge and the silvered surface, the wedge has both a front and rear surface canted in opposite directions, so that as the lever is flipped to point the silvered mirror at either the roof or seats inside one's own car, one or the other of the glass surface reflections will present a dim view of the rear.