1. An automatic rearview mirror system for a vehicle comprising: a reflective element actuable between at least two different reflective conditions; a forward light sensor to detect light forward of the mirror; a rear light sensor to detect light rearward of the mirror; a control circuit means coupled to said reflective element, said forward light sensor, and said rear light sensor for causing said reflective element to assume a reflective state dependent upon the output of said forward and rear light sensors, said control circuit means including a time-filter means for time filtering an ambient light signal which is at least partially dependent on the output of said forward sensor, said time-filter means providing faster time filtering of the signal during periods of increasing ambient light than during periods of decreasing ambient light, whereby said time-filter means and therefore said control circuit means approximates the adaptation of the human eye to changing ambient light levels.