How to Determine Contact
The first is a simple application of the axiom that only flat or spherical surfaces can be rubbed against each other and make contact at every point.
Wash and dry both tool and mirror and, with the palm of the hand or under side of the forearm, brush off any lint that may be left on the surfaces by the towel. Now short-stroke the mirror on the tool, going through the motions of grinding for about half a minute. On separating the disks, a fine dust, consisting of powdered glass, will be seen in those areas where the surface has been abraded. Or if not clearly visible, the dust can be found and picked up on the tip of the finger.
The second method is a matter of variable reflection. We know
that from a polished glass surface about four per cent of light of normal (perpendicular) incidence is reflected. As the surface is tilted,
increasing the angle of incidence (the angle made with the normal),
the percentage of light reflected increases, slowly at first, and amounts to total reflection as the angle of incidence approaches 90°
On an incompletely polished surface the amount of reflected light is lessened, and on a ground surface it may be nil (barring diffusion) except for very oblique rays, depending on the fineness of the grinding. And this is where use of a charge of No. 400 at the termination of work with No. 80, and the subsequent less-fine grades also, is beneficial.
how to determine contact part 3
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