Stefan Hell, at the Max Planck Institute for Biophysical Chemistry, proposes in 1994 a technique — called STED (stimulated emission depletion) — that breaks what for more than a century had been considered an absolute, insurmountable physical limit of optics: the diffraction limit, theoretically established by Ernst Abbe in 1873, according to which no conventional optical microscope can distinguish details smaller than roughly half the wavelength of the light used. Hell manages to surpass this limit by using a second laser beam that selectively switches off fluorescence around an extremely small point, allowing much smaller regions than diffraction would otherwise permit to be illuminated. Eric Betzig, together with William Moerner — who had demonstrated in 1989 the possibility of detecting the fluorescence of a single individual molecule — develops an alternative approach, known as single-molecule localization microscopy, which randomly activates only a few scattered fluorescent molecules at a time, determines their exact position with great precision, and repeats the process thousands of times to reconstruct a complete image of far higher resolution than conventional diffraction allows. Both approaches, grouped under the term "nanoscopy", allow, for the first time, direct observation of cellular structures at nanometer scale using relatively conventional optical microscopes, revealing details of living cells' internal organization previously accessible only through electron microscopy, which cannot observe living samples.