Figure 1: (a) The slab of empty space $0 is optically cancelled by a slab of negative-index material in $-L. (b) The same cancellation effect works with an object in $0, if the negative-index slab contains an anti-object. (c) A spherical shell $b is optically cancelled by a negative-index shell $a. If the core $r is optically equivalent to a sphere of radius $c$, then this device is invisible. (d) The same as (c) but with an object in the cancelled shell $b. The object is cloaked: both it and the cloaking sphere are invisible.