How do they work?
A plano-concave and a plano-convex lens with equal curvatures are contacted together by a drop of transparent lubricant.
When the lenses are centered, a transmitted light beam is undeviated.
Capillary forces hold the lenses closely together, but allow one of the lenses to be rocked about their common center of curvature.
This rocking motion turns the two plane surfaces into an "adjustable optical wedge" or prism that deviates (steers) transmitted light in two dimensions.
The refractive index (n) of the transparent lubricant matches the refractive index of the lenses, thus eliminating reflective loses at the curved surfaces.