CHAPTER IV. 

 VERTICAL ILLUMINATORS, METALLURGICAL MICROSCOPES. 



The study of opaque objects with ordinary compound micro- 

 scopes requires that the illumination 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 1 illu- 

 minate 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. 



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 

 the insertion of an objective into their lower opening. Mounted 

 in the axis of the adapter, or a little to one side, is a reflecting 

 device which receives light projected upon it through an aperture 

 in the walls of the cell and reflects the rays downward through 

 the objective upon the preparation on the stage. 



The reflecting device consists of a totally reflecting prism or 

 a thin disk of glass or mica or a tiny mirror or a half disk mirror. 



77 



