A simple model is presented for ionization-bounded PN which resolves the long-standing paradox of large limb brightening by taking into account the effect of inverse square dilution of the ionizing radiation from the central star. This greatly reduces emission from the ends of prolate nebulae, links the emission from different places in nebulae directly with the ionizing flux from the central star, and permits the calculation of reasonably accurate models, even when the density structure is not accurately known in detail. The appearance of a model taking the form of a hollow, prolate ellipsoidal shell, with an axial ratio of about 2.3, is calculated. Viewed from different angles, this structure can reproduce the shapes of many typical PN. In particular, it gives a better account of the structure of the Ring Nebula than any of the previously suggested models. The model is used to reinterpret previous measurements of optical emission lines across the face of the Ring Nebula. 20 refs.