  The ratio of the nucleon $F_2$ structure functions, $F_2^n/F_2^p$, is
determined by the MARATHON experiment from measurements of deep inelastic
scattering of electrons from $^3$H and $^3$He nuclei. The experiment was
performed in the Hall A Facility of Jefferson Lab and used two high resolution
spectrometers for electron detection, and a cryogenic target system which
included a low-activity tritium cell. The data analysis used a novel technique
exploiting the mirror symmetry of the two nuclei, which essentially eliminates
many theoretical uncertainties in the extraction of the ratio. The results,
which cover the Bjorken scaling variable range $0.19 < x < 0.83$, represent a
significant improvement compared to previous SLAC and Jefferson Lab
measurements for the ratio. They are compared to recent theoretical
calculations and empirical determinations of the $F_2^n/F_2^p$ ratio.
