A steel sphere is pressed against the underside of a driven glass disc. The light from a reflected light microscope passes through the glass disc and the film of oil at the point of contact, It is then reflected from the surface of the steel sphere. The set-up is operated together with the basic module. The microscope is fitted to an adjustable x-y slide and has a focus adjustment. The thickness of the film of lubricant is determined visually from the colours of the interference rings produced.
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