Abstract: An adjustable liquid lens or mirror has a chamber delimited by a flexible membrane surface, which flexible membrane is supported in tension between a pair of resilient rings. A first fluid medium fills the chamber which, in the case of a lens, has a different refractive index from that of a second fluid medium contacting the other side of the flexible membrane. An annular support member for the flexible membrane comprises relatively movable first and second component parts, the first and second component parts of the support member being adjustably linked in a fluid-tight manner whereby the volume of the chamber is adjustable by moving one component part of the support member relative to the other in such wise as to vary the pressure in the first fluid medium and thereby to alter the shape of the said membrane surface.