32 ELEMENTARY CHEMICAL MICROSCOPY 



be in shadow. In a depression, on the other hand, the shadow 

 will be on the same side of the depression as the source of light. 

 When, however, we employ a compound microscope (without 

 erecting prisms) we obtain a reversed image of the preparation, 

 hence an elevation as seen in the microscope will have its shadows 

 on the same side as the source of light, and a depression will 

 have its shadows cast on the side away from the radiant. 

 Black specimens with more or less polished and therefore reflect- 

 ing surfaces which are marked by ridges or furrows are especi- 

 ally puzzling. Careful focusing up and down and changing 

 the direction of the illuminating rays will always eventually 

 yield images which can be rightly interpreted. 



It is obvious that the oblique illumination of opaque objects 

 can be employed with advantage only with low powers, since 

 the free working distance of high-power objectives is so small 

 that the path of any pencil of light which will strike the prepa- 

 ation at a point lying in the line of the optic axis of the micro- 

 scope must then be so oblique with reference to the optic axis 

 of the microscope as to be approximately parallel to the surface 

 of the preparation. 



Light rays reflected from the surfaces of anisotropic crystals 

 are polarized, but are not noticeably polarized if from isotropic 

 crystals. It therefore often proves of great value in qualitative 

 analysis to employ polarized light for the illumination of objects 

 to be studied by means of vertical illuminators. This method 

 of research has not yet received the attention it deserves, owing 

 to the difficulties of manipulation and interpretation. 1 



It is evident from the above discussion that for the critical 

 examination of most opaque objects the light thrown upon them 

 should be either strictly axial or very oblique, according to the 

 nature of the information desired. 



In the great majority of cases the examination of polished 

 and etched alloys by means of rays normal to the polished sur- 

 face is preferable to that by oblique rays, since the images are 

 brighter and clearer, etched figures more easily interpreted and 

 the fine striations which may not have been wholly removed 



1 See Wright, F. E. Proc. Inst. Min. Eng., Feb. 1920. 



