The actuator comprises a barrel and a plunger received within the barrel. A clutch mechanism is operable between the plunger and the barrel. The clutch mechanism includes a driven member and a fixed member having respective castellations. The driven member carries pins which engage a three-start thread on the plunger. The castellations are urged into engagement by three annular springs. In the engaged condition of the clutch, the plunger is advanced by rotation relative to the barrel. An axial load on the plunger disengages the clutch mechanism, allowing the plunger to be advanced axially whilst rotating the driven member. Two-handed or one-handed operation according to a users choice is therefore possible. The actuator can be used in combination with a lens delivery device.
