Measurements of the Photon Total Cross Section on Protons from 18 to 185 GeV

The photon total cross section on protons has been measured with high precision in the Fermilab tagged-photon beam for photon energies from 18 to 185 GeV. The cross section decreases to a broad minimum near 40 GeV, and then rises by about 4 \ensuremath{\mu}b over the remainder of the range. A $\ensuremath{\rho}+\ensuremath{\omega}+\ensuremath{\phi}$ vector-dominance model (normalized to low-energy data) falls below the high-energy results by 2 to 6 \ensuremath{\mu}b, suggesting a contribution from charm-anticharm states.