The idea is that the meniscus of a drop of water will work as a lens. This works because of surface tension and by controlling the attraction of the water to the surface, you can actually form convex and concave surfaces.
Yes, our forefathers back in the 1600's would not have had plastic, but would have used anything else they could make a hole in e.g. thin wood, a leaf, etc.
It is amazing to think that things were actually discovered back then, using devices like this, such as the first observations of bacteria and protozoa.
See This DIY Microscope Design Is All Wet
[Robert Murray-Smith] wanted to recreate how some ancient microscopes worked: with a drop of water as a lens. The idea is that the meniscus of a drop of water will work as a lens. This works becaus…