One of the initiatives to provide a new international standard for the kilogram is the International Avogadro Project. Essentially this involves producing extremely perfect spheres of silicon atoms. There are currently two of these, perfected by a master lens-maker called Achim Leistner. The Wikipedia article references his ability to feel imperfections at an atomic scale, and I have seen the same claim made in a documentary on the subject.
Is it actually possible for humans to feel bumps or imperfections that might only be a few atoms 'high'?