Among the observed variety of natural phenomena reflected in experimental results, there are not many physical characteristics that allow a particular part of the mosaic of physical laws to be seen entirely. Precisely one such vivid fundamental characteristic is the ratio of the inclusive cross section of electron--positron annihilation into hadrons to the Born cross section of muon pair production, referred to as the value of R. Experiments on measuring the ratio R (s) in the range of energies from 1.84 to 3.72 GeV are considered in this review.