VERTICAL ILLUMINATORS, METALLURGICAL MICROSCOPES 85 



Parallel light should fall upon the lens L. This is obtained by 

 employing a suitable lens between the illuminator and radiant. 

 The Leitz Company supply a very conveniently mounted lens 

 for this purpose. A metal screen A, 

 Fig. 36, is attached to a stand B. 

 Mounted in the screen is a lens in front 

 of which is an iris diaphragm D. The 

 stand and radiant are placed at such 

 distances from L as to project a small 

 beam of approximately parallel light 

 upon L. The milled head a serves as 

 a fine adjustment up and down of the 

 lens and diaphragm. When either day- 

 light illumination, direct sunlight, or a 

 radiant at a distance are to be used, the 

 mirrors R2 and RI are brought into 

 service, the light from the chosen source 

 being received upon Ro, reflected upon 

 RI, and thence through the lens and 

 diaphragm opening. When a radiant 

 close to A is used the mirror RI is 

 raised until it stands in a vertical posi- 

 tion, thus giving an unobstructed passage through the center 

 of A. 



Correct illumination of the surface of an object m is obtained 

 as described above by trying the lens L at different distances 

 from P and by tipping P or k until the most satisfactory angle 

 of inclination is obtained. It may also be necessary to slide S 

 slightly to the right or left of the indicator /. It is usually best 

 to start with a diaphragm opening yielding a beam of light 

 which will not more than half fill the aperture of the lens L. 



Tassin Vertical Illuminator. One of the greatest annoyances 

 encountered in the work with ordinary vertical illuminators is 

 the necessity of readjusting the height of the radiant whenever 

 a change of objective is made or objects of different thicknesses 

 are studied, since refocusing is essential and this necessarily 

 alters the position of the disk or prism with reference to the axis 



FIG. 36. Condensing Lens 

 and Iris Diaphragm for 

 Use with Leitz Vertical 

 Illuminator. 



