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. 1 



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, Traite de Metallographie, p. 50, Paris, 1912. 



2 Central, f. Min., 1903, 657. 



