Collect the lenses. You will need four convex lenses; two (L1 and L4) with a focal length f1, and two (L2 and L3) with a focal length f2. Note that f1 should be larger than f2.
Arrangement 1.png Begin by positioning L1 and L2. They should be a distance t1 apart, where t1 = f1 + f2.
Arrangement 2.png Position L3 and L4. They should also be a distance t1 apart.
Position the two lens assemblies. There should be a distance t2 between lenses L2 and L3, and all four lenses should be in a straight line. t2 = 2*f2*t1/(f1-f2)
Position an object between L2 and L3. If you look along the central axis of the lenses, this object should be invisible. WARNING: the object itself should not block the central axis (it should have a hole in the middle), or it will not cloak.
Congratulations, you've built a simple cloaking device. The main advantage of this type of cloaking device is that it can be scaled up indefinitely, so long as you can find large enough lenses. The main disadvantages are that the object needs a hole through the centre, and that it can only cloak in a particular direction. In experiments, it could cloak an object when viewed up to 15 degrees off the central axis.
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