DEVELOPMENT OF THE METALLURGICAL MICROSCOPE 

 AND ITS SUGGESTED APPLICATION TO SOME UN- 

 SOLVED PROBLEMS. 



By Herman A. Holz, New York. 



Every step forward in the development of apparatus for metal- 

 lurgical research work is followed by an increase of our knowledge 

 of the particular field of metallurgy for which the instrument serves. 

 In other words, suitable apparatus have to be developed to a high 

 stage of perfection before we can make accurate and reliable deter- 

 minations which enable us to gain valuable knowledge of certain 

 facts which were unknovt^n to us before or about which we were not 

 quite certain. 



The development of the thermo-electric pyrometer by Le Cha- 

 telier enabled us to find the transformation regions in steel, Sir 

 William Roberts-Austen's apparatus — making us© of thermo-elec- 

 tric forces — permitted us to determine these transformation regions 

 with great accuracy and in a convenient manner, resulting in 

 systematic research work which forms the basis of the art of heat- 

 treating steel. 



The most important apparatus which enable the metallographist 

 to find the way towards improvements in the structural details of 

 steel and to control the correct thermal treatment to which he 

 subjects the material, are the microscope and the permeameter. 

 The microstructure of steel can be observed and photographed by 

 means of the microscope, while it can be measured and expressed 

 in definite figures by means of the permeameter, thus permitting 

 quantitative determinations. The success gained in recent years in 

 obtaining higher efficiency from definite alloys, especially alloy 

 steels, and in developing steels and bronzes ot greater strength, has 

 been due to systematic metallographic, especially microscopic, 

 research work. 



As the methods of microscopic investigation have been improved 

 by the development of more efficient etching processes, so have the 

 design and construction of metallurgical microscopes been gradually 

 developed to a high stage, in regard to the quality of lenses, source 

 of light, vertical illuminators, etc., as well as to rigidity an.1 

 usefulness of mechanical arrangements. While the pioneer work 

 on the microstructure of metals was carried on by means of ordinary 

 (bacteriological) microscope stands, it was soon found that the 

 investigation of opaque substances, without using cover glasses over 

 the object, necessitated changes in the illuminating system and in 

 the grinding and mounting of the objectives. The vertical illumin- 

 ators now largely used for this purpose were developed in England 

 (45 degree plane glass reflector by Beck) and France (prism 

 reflector by Le Chatelier, first made by Pellin and Nachet). The 

 objectives used in connection with these illuminators have to be 

 mounted as short as possible; the nearer the reflecting surface stands 



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