Using high-resolution depletion spectroscopy, we have experimentally studied the physics of near-threshold low Rydberg states of all three stable isotopic variants of molecular hydrogen. The experiments were required to calibrate the absolute wavelength, including several transitions from the EF (v = 0) and EF (v = 6 or v = 9)to the same low-n Rydberg states. The measurements have been performed for several initial rotational levels in aH three stable isotopic variants. Transitions to very high vibvational levels of the B, B, and C states have been measured with accuracy 0. 002cm-1. The pulsed amplifier perturbations were also measured by optical heterodyne methods.