  A slight anomaly in optical spectra of the hydrogen atom led Willis E. Lamb
to the search for the proton size. As a result, he found the shift of the 2S1/2
level, the first experimental demonstration of quantum electrodynamics. In
return, a modern test of QED yielded a new value of the charge radius of the
proton. This sounds like Baron Muenchausens tale: to pull oneself out from the
marsh by seizing his own hair. An independent method was necessary. Muonic
hydrogen spectroscopy came to the aid. However, the high-precision result
significantly differed from the previous, electronic, values: this is the
proton radius puzzle. This puzzle produced a decade-long activity both in
experimental work and in theory. Even if the puzzle seems to be solved, the
precise determination of the proton charge radius requires further efforts in
the future.
