Dave Bell wrote: > Well, as best I can make it out, the spheres act as lenses with focal > length equal to their diameter. If they are embedded in a reflective > background, an incoming bundle of parallel rays is focussed to a point > (disregarding spherical aberration) on the rear surface, reflected back > (must be along complementary paths!), and recollimated into a parallel > bundle again - directly back along the entry path. From the diagram, a > small part of the light from the sun passes through the hole in the > silvered layer, reflects off the rear glass surface onto the beads, and > returns to the same point on the rear surface. Some of the return light is > reflected back towards the sun, and some penetrates the rear surface, to > the user's eye. He sees a diffuse, much dimmed virtual solar image, > aligned with the direction the major part of the reflected light takes, > towards the target.
Thanks for the explanation. That's about what I figured out in the meantime. The key is the combination of retroreflection from the beads with a (partial) reflection from any of the glass surfaces parallel to the mirror. Clever. --Art
