(a) The slab of empty space is optically cancelled by a slab of negative-index material in . (b) The same cancellation effect works with an object in , if the negative-index slab contains an anti-object. (c) A spherical shell is optically cancelled by a negative-index shell . If the core is optically equivalent to a sphere of radius , then this device is invisible. (d) The same as (c) but with an object in the cancelled shell . The object is cloaked: both it and the cloaking sphere are invisible.