The actuator (see figure 2) comprises a barrel 16 and a plunger 22 received within the barrel. A clutch mechanism 54 is operable between the plunger and the barrel. The clutch mechanism includes a driven member 60 and a fixed member 62 having respective castellations 86, 88. The driven member carries pins 80 which engage a three-start thread 38 on the plunger. The castellations 86, 88 are urged into engagement by three annular springs 70. 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 60. Two-handed or one-handed operation according to a user's choice is therefore possible. The actuator can be used in combination with a lens delivery device (see figures 1 and 2). Figure 7 shows an alternative form of clutch mechanism.
