of light observed through the first slide the rheostat would be adjusted to provide the level of light most comfortable for the viewer the adjustments to the rheostat in turn produce a reference signal 50 which is sent through switch 52 to either comparator 46 if in the 34 a 34 position or trigger mechanism 54 if in the 34 m 34 position as best illustrated in fig1 when the rheostat has been adjusted to regulate the intensity of light to the level most comfortable for the viewer reference signal 50 will be constant and proportional to that level of light if switch 52 is in the 34 a 34 or automatic position reference signal 50 is sent to comparator 46 where it is compared with signal 44 comparator 46 produces a comparison signal 56 which is sent to trigger mechanism 54 the trigger mechanism 54 is essentially a switch which opens and closes on a time basis to control illuminator 26 through power supply transformer 58 depending on the value of comparison signal 56 received from the comparator 46 trigger 54 will actuate to control the voltage available to the transformer 58 if for instance a fairly transparent specimen slide is replaced by a less transparent specimen slide the light passing through the more opaque slide will be proportionally less intense the photocell 38 detects the reduction of light and produces a.