re: #54 zombie

Another possibility I would focus on:
Lenses.

Renaissance techniques for grinding lenss involved moving an abrasive stone across the glass blank in a regular pattern of strokes. As the abrasive is moved across the glass, the material would be removed. The more often the stone passed over a given area of the glass, the more material would be removed. The patterns used resembled a multipoint star, with the glass in the middle. Different patterns would be used to create different curvatures. The result, if done carefully, would be a reasonably precise parabolic lens of a specified curvature or power. The interesting thing is the craftsman making the lens did not need to know anthing about algebra or geometry or parabolas. All he had to do was move his arm in the right pattern and a parabola would result.