Mirror M2: the mirror M2 (also front coated) is capable of moving along the direction of the laser.
It is attached to the diaphragm of a speaker (figure 6(a)).
A dc supply (figure 6(b)) is connected to the speaker.
As the voltage to the speaker is changed, the diaphragm moves in one particular direction, thus moving the mirror in that direction.
The variable knob of the dc supply is a 10-turn potentiometer.