CHAPTER V. 



THE EXAMINATION OF OPAQUE OBJECTS, VERTICAL 

 ILLUMINATORS, METALLURGICAL MICROSCOPES. 



The study of opaque objects with ordinary compound micro- 

 scopes requires that the illuminating rays shall fall upon the 

 preparations from a point situated above the stage of the instru- 

 ment. This may be accomplished in several ways: (i) the rays 

 from a radiant can be projected upon the surface of the object 

 by means of mirrors, or by means of a condensing lens; (2) a 

 plate of glass or a right-angled prism may be placed above the 

 objective in a tubular mounting so as to fall in the line of the 

 optic axis, and so inclined that any light rays striking the reflect- 

 ing surface will be directed down through the objective, thus 

 brightly illuminating the object. The devices of Group i 

 illuminate the preparation with oblique rays only; those of Group 

 2 reflect rays perpendicular to the surface of the object and are 

 usually termed vertical illuminators. In the vast majority of 

 cases the examination of opaque objects through illumination by 

 rays normal to the surface of the preparation is preferable to that 

 by means of oblique rays, since the images obtained are brighter, 

 etched figures are more easily interpreted and the finer striations 

 due to incomplete polishing are far less visible. Moreover, 

 photomicrographs are usually more easily obtained. 



Formerly parabolic reflectors of silvered glass or metal attached 

 to the objective were much employed; but inasmuch as such 

 devices can be used with only a very narrow range of objectives, 

 and with preparations of a certain size only, their usefulness is so 

 limited that chemists have quite generally abandoned them in 

 favor of vertical illuminators. 



Vertical Illuminators of simple construction consist of tubular 

 adapters or cells so threaded as to permit screwing their upper 

 end into the lower end of the body tube of the microscope, and 



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