You can easily see one millionth of an inch. During my training as a Mechanical Engineer we did some work at a metrology lab. We used what in the US is called a

You can easily see one millionth of an inch. During my training as a Mechanical Engineer we did some work at a metrology lab. We used what in the US is called a Gauge block http://en.wikipedia.org/wiki/Gauge_block http://en.wikipedia.org/wiki/Gauge_block . If you set two gauge blocks by one millionth difference you can see the light between them. Also don't forget the human eye can see as far as a few light years - think stars! By the way the Wikipedia article on Gauge blocks is also very inspirational for any aspiring inventors/enterpreneurs.