The Light Source
Reasonably monochromatic light is required for testing selected pieces of plate glass as well as the figured diagonal. This light may be obtained from any of the now rather common gas-tube lights; an inexpensive neon lamp is excellent, and argon is also good, but requires a darkened room.
Either type may be purchased at a radio supply store. The bulb
may be screwed into an ordinary light socket; an adjustable desk lamp is convenient. A thin piece of absorbent tissue wrapped around the bulb will provide satisfactory diffusion. The specimens
to be tested should rest on a piece of black cloth almost direct below the light. With these small specimens, the interference bands should be spread out, by pressing, until they are about 1/4" apart.
For precision testing of large flats (see Appendix B), the sharper bands to be had from the sodium, helium, or mercury-vapor lamps are much to be preferred. (Information on these lamps can be had from George W. Gates & Co., Inc., Franklin Square, N. Y.) A filter excluding all but the green line should be used with the mercury lamp.
With any of these lights a diffusing medium, such as tracing cloth, opal or ground glass, must be used. For accuracy, both light source and eye should be placed as near as possible to the normal to the surfaces under test. An arrangement that places both eye and illuminant normal to their centers.
Testing for Flatness
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