Abstract: A gap in an optical guideway is occupied, when the switch is in its diverting condition, by a quantity of air or other gas. To change the switch to its through condition, an actuator forces a column of liquid against the gas to compress (and thereby displace) the gas in the gap with the liquid. The actuator includes a preferably wide reservoir of the liquid and a diaphragm which is flexed to force the liquid up the column against the gas. When the actuator is deactivated the compressed gas forces the column of liquid out of the gap to return the switch to the diverting condition.