Some notes on the metallurgical photomicroscope
Transactions of the Faraday Society
The technique of the photomicrography of metals has advanced very much during the last ten or twenty years, but there are still very marked evidences that many who take up microscopic work in connection with metallurgy appear to study the microscope itself either not at all or only to a very small extent. The consequence is that statements are made about the structures of various metals which are not correct; the presence in sections of minute particles or membranes of constituents other than
... tuents other than those stated to be there has been missed simply because the operator did not know how to use his microscope properly. Again, photographs are published which have only a slight resemblance to the structures photographed, in some cases the definition is so bad that the reproductions are not worth the paper they are printed upon. One has only to look through the Journals of, for example, the Iron and Steel Institute to see how true this is. Even when a metallurgist has devoted a considerable time to the study of the microscope, mistakes may arise in the interpretation of structures. For example, it has been stated that iron carbide (cementite) is not attacked by sodium picrate when its thickness is less than 0.001 mm. (this statement is repeated in one of the most recently published treatises on metallography). This is quite incorrect. Not only are the carbide laminae of pearlite attacked when considerably thinner than this (certainly not more than one-tenth of the thickness mentioned), but also the minute granules in sorbite, produced on tempering hardened steel at about 600° C. Possibly the reason the above misstatement was originally made was either that the aperture of the objective used was not sufficiently high or that the resolving power was much reduced by the use of a prism illuminator or both.