  Using semiclassical periodic orbit theory for a chaotic system in a weak
magnetic field, we obtain the form factor predicted by Pandey and Mehta's two
matrix model up to the third order. The third order contribution has a peculiar
term which exists only in the intermediate crossover domain between the GOE
(Gaussian Orthogonal Ensemble) and the GUE (Gaussian Unitary Ensemble)
universality classes. The exact expression is obtained by taking account of the
contribution from encounter regions where orbit loops are connected.
