When you read the meter you set the exposure scale to read the film and fstop recommendation. Gossen makes a converter that slips into the battery compartment so you can use a new non mercury. The Gossen has 2 metering modes, reflective and incident which the dome is for. When you use the reflective mode you stand by the camera and point the meter towards the subject you want to meter and read the scale. In the incident mode slide the dome over the cell, stand in front of the subject facing the meter and read the scale. Gossen's indicent mode is weak as the dome is not large enofe to really to do a good reading all of the light falling on the subject.