Abstract: A combiner (10) has a glass reflector (11) with first and second curved surfaces (12, 13) each having an optical coating. A first prism (14) has a surface (15) which complements the surface (12) of the reflector (11) and a second prism (16) has a surface (17) which complements the surface (13) of the reflector (11). The surfaces (12 and 13) and the surfaces (15 and 17) are each primed and allowed to dry before being polished. The reflector (11) and prisms (14 and 16) are arranged with respect to one another such that surface (12) forms a well (18) with surface (15) and similarly surface (13) forms a well (19) with surface (17). The wells (18, 19) are completed with a seal (20) at a lower end (21) and bonding agent (22, 23) is injected into the wells (18, 19). The combiner (10) is placed in a vacuum chamber to remove trapped air bubbles and the agent (22, 23) is allowed to cool at a temperature which does not unduly stress the reflector (11) and prisms (14 and 16).