Typical mode patterns observed from the laser operating near threshold are shown in Figs. 8, 9, 10, and 11. These figures show the evolution of the mode structure as a function of the cavity parameter Pp. Figure 8 was obtained using fresh Brewster-angle windows. It shows that HG-type modes are often obtained from Brewster-angle-window gas lasers, even when a circular intracavity aperture is fitted, because of the large astigmatism contributed by the windows. Empirically,we find that the HG description will be good for PO, 50.