Daniel and Isaac roamed for the better part of an hour, ignoring the shouts and pleadings of the retailers on all sides, until finally Isaac stopped, alert, and sidestepped over to a small folding display-case-on-legs that a tall slender Jew in a black coat had set up. Daniel eyed this Son of Moses curiously – Cromwell had re-admitted these people to England only ten years previously, after they'd been excluded for centuries, and they were as exotic as giraffes. But Isaac was staring at a constellation of gemlike objects laid out on a square of black velvet. Noting his interest, the Kohan folded back the edges of the cloth to reveal many more: concave and convex lenses, flat disks of good glass for grinding your own, bottles of abrasive powder in several degrees of coarseness, and prisms.