| http://www.w3.org/ns/prov#value | - No. 819,277 20 Claims. (Cl. 250-219) I This invention relates to apparatus for measuring the distance of a surface of an object from a reference position or positions and more particularly relates to apparatus for measuring the spacing between the surface of a material and a reference position or positions by means of electromagnetic radiant energy such as light, either visible, ultra-violet or in
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