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 
