The proton and deuteron structure functions $F_2^p$ and $F_2^d$ are measured in inelastic muon scattering with an average beam energy of 470 GeV. The data were taken at Fermilab experiment E665 during 1991 and 1992 using liquid hydrogen and deuterium targets. The $F_2$ measurements are reported in the range $0.0008<x<\mathrm{}0.6\mathrm{}$ and $0.2<\mathrm{}{Q}^2<75\mathrm{}$ Ge${\mathrm{V}}^2$. These are the first precise measurements of $F_2$ in the low $x$ and $Q^2$ range of the data. In the high $x$ range of the data where they overlap in $x$ and $Q^2$ with the measurements from NMC, the two measurements are in agreement. The E665 data also overlap in $x$ with the DESY HERA data, and there is a smooth connection in $Q^2$ between the two data sets. At high $Q^2$ the E665 measurements are consistent with QCD-evolved leading twist structure function models. The data are qualitatively described by structure function models incorporating the hadronic nature of the photon at low $Q^2$. The $Q^2$ and the $W$ dependence of the data measure the transition in the nature of the photon between a pointlike probe at high $Q^2$ and a hadronic object at low $Q^2$.

• Received 9 February 1996

DOI:https://doi.org/10.1103/PhysRevD.54.3006