VERTICAL ILLUMINATORS, METALLURGICAL MICROSCOPES 77 



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. These reflectors are mounted upon 

 small metal rods passing through the adapters at right angles to 

 the optic axis; a milled head at the end of the rod permits chang- 

 ing the angle of inclination of the reflecting surface. 



In several types the lateral opening for the incident light is 

 made variable in diameter either by means of an iris diaphragm 

 or a rotating collar provided with openings of different sizes. 



FIG. 34. Prism Vertical Illuminator. FIG. 35. Disk Vertical Illuminator. 



A typical prism illuminator is shown diagrammatically in Fig. 

 34. The reflecting device consists of a totally reflecting prism P 

 so mounted as to permit tipping slightly and thus changing the 

 direction of the reflected ray R. Incident light I is projected 

 upon the prism through the horizontal opening 0. A diaphragm 

 D extending not quite halfway across the aperture of the adapter 

 serves to screen the prism and to prevent interfering reflections 

 from blurring the image formed in the microscope. 



The construction of a disk illuminator is shown in Fig. 35. The 

 incident rays I, I strike a glass or mica disk G and are reflected 



