I hope some of you go up to post #111 and look at some of the sizes the "engineer's chart"says objects are,in relation to the smaller than 40 microns we aren't supposed to be able to see. It is very strange that a spider web is listed as 2 to 3 when we aren't supposed to see smaller than 40 microns. My point is,you cannot just list a number and say we can't see it. The object being viewed may or may not be seen due to the background and/or lighting it is viewed against.
A metal lathe turning full of small scratches from being turned is not going to show a 1/250,000" scratch,but that scratch will show up in a polished,smooth surface. A .002" hair is easily visible under most conditions. So was the 1/2 thou. steel wool strand.
I just hate it when sweeping generalizations are made like that.