THE correct functioning of many pieces of apparatus depends not only upon dimensional accuracy, but also upon the surface finish of contacting surfaces. Practically all engineering establishments have available a microscope suitable for magnifications up to at least 200 diameters and, in any event, such microscopes, even under present circumstances, are in reasonably good supply. By means of such a microscope and readily available accessories, it is possible accurately to measure and control certain types of finish by a method that has recently been perfected. At the outset it will be as well clearly to define the scope of this method. It is only applicable to finely finished convex or flat reflecting surfaces; with increase in scratch depth or roughness the more difficult does it become to interpret the results and, in general, the method is not applicable if the average height of the surface above the mean line exceeds about 20 micro inches.
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