def air2vac(air):
    """Convert from in-air wavelengths to vacuum wavelengths.  Based on Allen's
    Astrophysical Quantities.

    :param air:
        The in-air wavelengths.

    :returns vac:
        The corresponding vacuum wavelengths.
    """
    ss = 1e4 / air
    vac = air * (1 + 6.4328e-5 + 2.94981e-2 / (146 - ss**2) +
                 2.5540e-4 / (41 - ss**2))
    return vac