NOTES ON THE CONSTRUCTION AND DESIGN OF 

 METALLURGICAL MICROSCOPES. 



By Prof. Cecil H. Desch, Glasgow. 



The use of the microscope in the examination of metals, first 

 introduced by Sorby more tJian 50 years ago, has become so wide- 

 spread* that a microscope is now an indispensable item in the equip- 

 ment of a metallurgical works, whilst the recognition of its 

 importance to engineering works and other places in which metals 

 are employed for constructional purposes is rapidly extending. It 

 is therefore essential to the conduct of these industries that instru- 

 ments should be available which will allow of the rapid and con- 

 venient examination of such metals as present themselves in the 

 course of routine testing, whilst it is obviously desirable that 

 elaborate and detailed investigation of specimens of special interest 

 should be possible. It is quite true that any ordinary microscope 

 of good construction may be used for metallographic work, provided 

 that the higher power objectives are duly corrected for uncovered 

 objects, but the increased convenience of a properly designed instru- 

 ment is so great as to justify its use, even for routine work. There 

 are now many patterns of metallurgical microscopes on the market, 

 and the following remarks are based on an experience of some 12 

 or 13 types of instrument, and the examination of the details of 

 many others. The writer has been reluctantly forced to the con- 

 clusion that, in spite of many excellent features in some of the 

 British microscopes, the German instruments have proved better in 

 use, and that their superiority is more marked, the longer the micro- 

 scopes are used. The British designs are often good, and the work- 

 manship, so far as the cutting of racks and screw-threads, etc., is 

 concerned, is often quite satisfactory, but in the course of prolonged 

 use the mechanical arrangements show defects, racks and screws 

 becoming loose, and the accurate focussing of high power objectives 

 becoming troublesome, to an extent which is not met with in the 

 German microscopes. The cause of this looseness after use appears 

 to be insufficient attention to the quality of the metal employed in 

 construction. A rack cut in soft brass, however accurate at first, 

 becomes loose through wear, and no compensation by means of 

 adjusting screws can be quite satisfactory. The fact that such 

 screws are provided seems to be a confession of weakness, since 

 the writer has used a Zeiss microscope, without such screws, for years 

 continuously without any sign of play in the mechanical movements. 

 Racks should be cut in hard, incorrodible metals or alloys instead 

 of in soft brass, whilst the pinions might also be of much harder 

 metal than is usually the case. It is probable that manufacturers 

 have been too much guided by tradition in the choice of the metals 

 to be used in the construction of scientific instruments, witness 

 the tendency, only now disappearing, to use highly polished brass 

 for heavy portions where cast iron would serve the purpose equally 

 well. 



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