I hate to tell you this, but it seems that one of their uses was to focus light for 'oblique illumination of opaque objects'The very expensive gadget you are looking for in order to focus lamp light on a mirror is probably a bullseye bench condenser:
.. they were turned , bullseye down , and the light was sent upwards and bounced off the back of them and down onto the stage... Beck says that they could get light under a 4mm objective.