I invested in a couple of microscope lenses to use as diopters a while ago. I have finally found a way to successfully use them. I had originally intended to use them for water drop shots, but they have a very narrow depth of field, and tend to vignette with smaller apertures. And you need to be very close to the subject. Window ice (in the bad thermos...) turns out to be a perfect subject, because they are very flat.