I think the original choice of lead as a substance that he can't see through has to do with his "X-ray" vision. Lead is the most common substance used as shielding for X-rays.
I think we've all agreed that his vision gizmo isn't actually the same type of X-rays. But I think the lead thing has just continued as part of cannon.
There certainly are other things that he shouldn't be able to see through based on that theory...plenty of other things that CAN be used as shielding from radiation/X-rays.
In general, the thickness of a material to attenuate (shield) x-ray radiation is dependent upon the energy of the x rays, the material's chemical composition, and the material's density. Which explains why tin (specifically) doesn't work - the density is much different from lead...but something like tungsten (which is actually better at shielding from radiation - just a lot more expensive) should also block his vision. Actually, ironically, steel is also attenuating for radiation - not as well as lead, would just require more material...but under that theory, Clark shouldn't be able to see through really thick stteel either.
I think the original lead theory was based off actual X-rays and then just kept as a single substance for simplicity.
Interesting thing to think about and nitpick. <g>
Jill
