I bought a USB microscope for this exact reason, but it's useless for this as there's not enough room under it to be able to work under it when it's focused. I'm not home right now so I can't measure, but I think it was something like 10-20mm space between the scope and the desktop. This was one of the 200x microscopes that you can find all over the place. I bought mine from Dealextreme.

Stig