[M]acromolecular assemblies. . ., which include viruses, are too large to study by conventional X-ray diffraction methods, but too small to be seen under a light microscope.
Unlike the most powerful electron microscopes—which can see down to the scale of individual molecules—or fluorescent-based microscopes—which rely on the use of coloured dyes—the new light microscope does not need to interfere in any way with the living material it is used to study.