  The quantum spectra of hydrogen atoms in various magnetic fields have been
calculated with the closed orbit theory. The magnitude of the magnetic field
decreases from 5.96 T to 0.56T with a step of 0.6T. We demonstrate
schematically that the closed orbits disappear with the decrease of the
magnitude of the magnetic field when the corresponding finite resolution of
experiment is fixed. This may give us a good way to control the shape and the
number of the closed orbits in the system, and thus to control where a peak
should exist in the Fourier transformation of the quantum spectra.
