Ian Mackey brought his show and tell collection of rare and unusual sidewalk light prisms. You know... those purple glass squares you sometimes see embedded in sidewalks, typically in bigger cities. I always thought they were flat on both sides. But the side that faces down (underground) is really shapped as a prism, in order to capture and project the maximum amount of light from above, into the vault below. Many are embossed with manufacurer names and dates. Ian also brought a manhole cover with little glass "sky-lights" embedded in it. Wild stuff! Who knew?