View source for Flat surface

'''To check if a surface is flat:'''

It only works for a polished slab<br/>
This works rather well as a quick check.<br/>
Find a straight line some distance away that you can see at an oblique angle reflected on the slab.<br/>
Its remarkably sensitive even to slight bow Move your view point to make that line traverse the length and width of the slab.<br/>
Any imperfections in flatness show up as a kink in the straight line.<br/>
Its remarkably sensitive even to slight bow.<br/>