90 ELEMENTARY CHEMICAL MICROSCOPY 



face of the specimen will be in a plane normal to the optic axis 

 of the microscope. 



Metallurgical Microscopes. — The extraordinary interest in 

 the microscopic study of metals and alloys within the last ten 

 years and the astonishing development of theories relative to 

 their constitution and structure, followed by the application of 

 this information to the mechanic arts, has led to the design of 

 special forms of microscopes to facilitate the study of the many 

 different problems arising in the metallurgical industries. In 

 all these special types of microscopes we have to deal with 

 compound microscopes, having permanently attached, between 

 ocular and objective, a vertical illuminator, usually of the prism 

 type. 



Since the etched surfaces of metals ordinarily yield images of 

 such intricacy that notebook sketches become impracticable, 

 recourse must be had to photography. Most metallurgical micro- 

 scopes therefore include as an integral part of the instrument 

 a photographic camera, and when thus provided they are often 

 known as metallographic microscopes or metallographs. 



In order that the structure of an alloy may be studied it is 

 essential: (i) that a small area shall be ground to a plane sur- 

 face polished and etched; (2) that this plane surface shall lie 

 normal to the optic axis of the microscope; (3) that the area of 

 this plane shall be so situated with reference to surrounding 

 parts that the objective may be brought sufficiently close to it 

 to be focused. 



Were the preparation to be laid upon the stage of an ordinary 

 microscope it would have to be thin and to have another sur- 

 face ground parallel to the etched surface. To avoid these dif- 

 ficulties and further to permit the examination of fragments 

 of moderate size, the microscope is more conveniently inverted, 

 i.e., constructed with the objective lying below the stage. The 

 alloy can thus be laid upon the stage, polished surface down over 

 the stage opening. It will thus meet the requirements that its 

 etched surface shall lie in a plane normal to the optic axis. 

 Coarse adjustment focusing is accomplished by displacing the 

 stage up or down, the tube of the microscope remaining in a 



