ORIENTATING DEVICES 
143 
microscopes intended for the study of metals. Here, however, we 
are dealing with opaque objects, and needing reflected light only, 
the orientating device can be constructed entirely of metal. A 
good example of this style of construction is found in Robin’s 
metallograph.^ 
In this instrument the stage is attached. Fig. 73, to the micro¬ 
scope stand by a ball-and-socket joint as shown, making it pos¬ 
sible to focus upon any given area of very irregular specimens. 
To facilitate the examination of crystals with reference to their 
different behavior toward polarized light according to the direc¬ 
tion through them that the light is sent, Schroeder van der Kolk 
Fig. 73. Robin Ball-and-socket Stage 
for Metallurgical Microscopes. 
Fig. 74. ten Siethoff glass Hemi¬ 
sphere. 
suggested fastening the specimens to a small glass hemisphere. 
This idea was later eleborated by E. ten Siethoff,2 who combined 
the hemisphere with a system of condensing lenses, thus permit¬ 
ting not only the orientation of a crystal and its study under the 
influence of plane polarized light sent through in the directions 
of the different axes of vibration, but also permitting observations 
with strongly converging polarized light in different positions. 
The apparatus consists of a condenser which is laid upon the stage 
of the microscope, the diameter of its mounting being such as to 
fit into the stage opening. The construction is shown in Fig. 74. 
The crystal fragment is laid upon the flat surface of the glass 
hemisphere S. 
1 Robin, Trait6 de Metallographie, p. 50, Paris, 1912. 
^ Central, f. Min., 1903, 657. 
