In a 2010 thread on a discussion forum on the Practical Machinist website, "Filing Flat" ( http://www.practicalmachinist.com/vb/antique-machinery-history/filing-flat-201685/), user "horomancer" describes a method of visual inspection for flatness:
"... I just look at the surface and watch for the tell-tale rolling of the light. A truly flat surface will flash when it reflects light towards your eye, while any deviation in the flat will let the light 'roll' across the surface. ..."
It is perhaps unsurprising that this user has chosen "horomancer" as his or her pen name, as this method would seem to rely on the characteristics of a horological "black" or specular polish in which a surface is flat enough to reflect light in one direction only.
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