How to Determine Contact
Hold the clean, dry mirror face up, somewhat below a line between the eye and an unfrosted light bulb, as in Fig. 26a. A reflection of the filament will be seen on its surface. Now slowly
lower the mirror, centering the reflection, which will become reddish or copper colored and fainter as the reflection angle is decreased.
When it is quite dim, stop, and slowly slide the mirror to the right or left, allowing the reflection to pass across a radius, and off the edge. If the mirror is spherical, there should be just a slight and gradual diminution of the reflection from center to edge.
In the event of imperfect contact (lack of sphericity), high zones will be revealed by a brighter reflection, and low zones by a dimming of the reflection. Slight differences can be tolerated, but any abrupt change in the quality of the reflection should be remedied before proceeding to a finer grade. In making this analysis while working on No. 80, the mirror may have to be held as at a, and in the final stage of fine grinding, as at b, Fig. 26.
Cleaning Up
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