1. An adjustable fluid-filled lens or mirror assembly comprising a fluid-filled envelope and a supporting structure therefor; the fluid-filled envelope being constituted by a first wall which is formed of a distensible elastic membrane having an exterior optical surface of variable curvature, a second wall which is spaced from the first wall on a z-axis, and a collapsible peripheral side wall which extends between the first and second walls, and being filled with a substantially incompressible fluid; a membrane holding structure which is attached to a peripheral edge of the membrane for holding the membrane under tension; and one or more selectively operable actuator assemblies for moving one or more corresponding regions of the peripheral edge of the membrane on the z-axis towards or away from the second wall for controlling the profile of the peripheral edge of the membrane; wherein the or each actuator assembly comprises a connecting member that is attached to the membrane or membrane holding structure at a respective actuation point, a linear actuator which is mounted to the supporting structure and a linkage which is connected between the connecting member and an actuation point on the linear actuator; wherein the linear actuator is a linear SMA actuator comprising an array of SMA wires which are coupled together to work in parallel and which extend transversely of the z-axis, and the linkage is configured for converting linear motion of the actuation point of the SMA actuator in a direction transverse the z-axis to linear motion of the connecting member on the z-axis, thereby to move the corresponding region of the peripheral edge of the membrane towards or away from the second wall, and wherein the individual SMA wires are accommodated within a plurality of comb-like structures that are spaced longitudinally along the SMA wire array.