"Black polish" has something of an air of mystery around it in fine horological circles. It is, however, something which is in principle simple (though difficult to achieve). Ordinarily, even materials which appear flat on a large scale are very bumpy on a micro-scale and thus reflect incident light in all directions; this is "diffuse" reflection. Most polished surfaces, however shiny, remain diffuse reflectors to a great degree because reflectivity and flatness are two different things. A sufficiently flat surface, however, will reflect incident light from one direction in only one direction; this is "specular" reflection. If such a surface is illuminated by a point-source of light, it will appear black from all but one viewpoint.
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. ..."
See also the Notebook on Heuristic Methods (For Measuring, Inspecting, and Testing using Hand Instruments) .
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