A dark disk surrounded by a light ring appears darker as the ring luminance is increased. A recent computational model of darkness induction (Rudd, 2001; Rudd & Arrington, 2001) postulates that neural signals associated with separate borders combine additively to influence disk lightness. We extended this model to include both lightness and darkness induction signals. According to the extended model, the inner edge of the ring (dark inside) acts to darken the disk and the outer edge of the ring (light inside) acts to lighten the disk. The effect of changing the ring luminance thus depends on the relative magnitudes of the two separate edge induction effects. Two disks were presented side-by-side on an Apple flat-panel display. Each disk was surrounded by a light ring. The luminance of the ring on the right was varied to modulate the lightness of the right disk. The luminance of the left ring was fixed and the subject adjusted the intensity of the left disk to achieve a lightness match to the right disk. For each of four observers, the luminance of the matching disk varied inversely as a power of its surround ring luminance, as predicted by the model. We also found that the lightness induction produced by the outer edge is weakened when the edge is moved farther away from the disk. The magnitude of the lightness illusion is then what one would expect from the local luminance contrast at the disk/ring border alone. Finally, we demonstrate a similar distance effect with real surfaces (papers of different gray levels).