submacroscopic
/ˌsʌbmakrə(ʊ)ˈskɒpɪk/adjective
Not large enough (or barely large enough) to be visible to the naked eye; visible without a microscope, using a simple magnifying lens.
Origin
Late 19th century; earliest use found in Bulletin of the United States Geological Survey. From sub- + macroscopic, probably after submicroscopic.