We have already noted that no closed-form solutions for the integrals in Eqs. (4) have been found for the Atype modes, even in the paraxial approximation. However, the results of experiments to be described have suggested that the ellipse given by Eq. (24) may be useful for the A-type modes, provided that we can find an equivalent B-type mode in each case.The A-type modes are specified by the semiaxes of the outer elliptical envelope W, and W,' and by the modeorder numbers NA and NA Experimentally, we can measure the semiaxes and the mode-order number NP directly. There is no direct way to find NA however. We might ask what would be the mode-order number NnB in this direction for a B-type mode having the same elliptical boundary? Such a B-type mode would have a half-width W, given by