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