Imagine you have someone sent back in time without warning. He is smart and has a good generalist grasp of science and technology, but is not a master of any particular field. He wants to use a crude microscope, for instance to demonstrate the presence of germs and thus germ theory.
At what point in history would our mechanical and glass shaping skills be sufficiently advanced to build a rudimentary microscope given only a general explanation of concave mirrors 'stacked' on each other? Assuming that they had sufficient funding from an interested source to cover reasonable expenses and time to try a few failed attempts before the final product? Could this be managed during the middle ages?