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 
Jens 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 
R], 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 ^ 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 t. 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 
and Iris Diaphragm for 
Use with Leitz Vertical 
Illuminator. 
