THE METALLURGICAL MICROSCOPE. 



By Walter Rosenhain, D.Sc, F.R.S. 

 (The National Physical Laboratory.) 



In a paper* presented to the Royal Microscopical Society in 1906, 

 the present author has described a Metallurgical Microscope in the 

 design and construction of which an effort has been made to apply 

 certain principles which he regards as fundamental for the construc- 

 tion of scientific instruments in general and of microscopes in par- 

 ticular. These principles have previously been discussed in a paperf 

 presented to the Optical Convention, 1905. For the purposes of the 

 present discussion, therefore, it will not be necessary to do more than 

 to summarise briefly some of the principal points affecting the metal- 

 lurgical microscope. 



In regard to mechanical design, the primary consideration i3 

 that of providing adequate strength and stiffness not only in the 

 base and limb, but also in the working joints, such as that upon 

 which the limb turns. The design of such an instrument should, in 

 fact, in the author's opinion, be based rather upon that of a machine 

 tool than on the unduly delicate, sometimes flimsy, and often un- 

 mechanical devices which are to be found in some scientific instru- 

 ments. One fruitful source of lack of rigidity may be found in 

 the presence of unnecessary movements ; for instance, it is now fairly 

 generally accepted as an essential feature of metallurgical microscopes 

 that the focussing movement, at all events so far as the coarse 

 adjustment is concerned, should be applied to the stage. The pro- 

 vision of a coarse focussing movement for the body tube as well, 

 therefore, constitutes an undesirable duplication. If the fine adjust- 

 ment is also applied to the stage, as has been done in the author's 

 design, then the body tube can be rigidly attached to the limb, 

 with a corresponding gain in rigidity. 



Another source of unsteadiness lies in the manner in which the 

 so-called vertical illuminator is frequently attached. Where this 

 fitting is screwed to the nose-end of the body tube and the objective 

 is screwed into the illuminator, a certain amount of play is liable 

 to occur. The author, therefore, very much prefers an arrangement 

 by which the objective is screwed direct to the body tube, and the 

 illuminator is inserted into the body tube, by means of a slide or 

 otherwise, through a lateral aperture. 



The apj^lication of the fine focussing adjustment to the stage 

 offers a further advantage which is of some importance, as by this 

 ai-raiigeiiieut the fine focussing movement can be placed in an axial 

 position. If this is done there is no overhang to magnify the slight 

 play which is unavoidable on all smooth running slides. This 



* "On an Improved Form of Metal lungical Microscope," Journal 

 Royal Microscopical Society, iqo6. 



t "The Mechanical Design of Instrumemts," Piroc. Optical Con- 

 vention, Vol. I, 1005. 



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