  The recent values of the proton charge radius obtained by means of
muonic-hydrogen laser spectroscopy are about $4\%$ different from the electron
scattering data. It has been suggested that the proton radius is actually
measured in different frames and that, starting from a non relativistic quark
model calculation, the Lorentz transformation of the form factors accounts
properly for the discepancy. We shall show that the relation between the charge
radii measured in different frames can be determined on very general grounds by
taking into account the Lorentz transformation of the charge density. In this
way, the discrepancy between muonic-hydrogen laser spectroscopy and electron
scattering data can be removed.
