VERTICAL ILLUiVlINATORS, METALLURGICAL MICROSCOPES 99 
the prism mounting fits between the diverging arms of the Y. 
Turning the screw therefore moves the prism in or out of the tube 
h. When an objective is in place and the prism illuminator is 
to be employed the knob p in the tube V is moved to the left as 
far as it will go. The indicator i is then moved on its scale until 
the sharp-pointed end of the leg of the Y rests on the scale divi¬ 
sion marked with the equivalent focal length of the objective 
which is in service. 
When the plane glass illuminator is employed the knob p on 
the tube V should be moved to the right as far as it will go. 
Before the vertical illuminators may be changed it is necessary 
that they be withdrawn from the prism chamber from below the 
objective, and moved to the left as far as their sliding tubes will 
permit. The lever to which their tubes are attached may then 
be pushed back or pulled forward as the case may be, until the 
spring catch for holding them in position snaps in place. 
The prism illuminator is so adjusted as to yield slightly oblique 
illumination; this causes fine structures to stand out sharply 
and yields photographs having strong hard contrasts; for softer 
effects and for use with high powers the plane glass illuminator 
should be used. 
The fine adjustment of the microscope is through the milled 
head / or by means of the Hooke’s key N. 
The lever u serves to throw the fine adjustment out of service 
when the microscope is not in use or during transportation. 
The fine adjustment does not move the stage, hence in focus¬ 
ing, the alignment of radiant and illuminators is disturbed. 
The rack and pinion at H serves to focus the photographic 
objective when the set-up in Fig. 43 is employed, or serves as 
a convenient method of moving the front board of the camera 
for changing oculars in the tube C. A mirror in the camera 
box is attached to the lever /. When this lever is pulled forward 
the mirror stands at 45° to the axis of C and thus projects an 
image upon the ground glass k. A large reading glass U enlarges 
the image and aids in studying the field, and in focusing the 
image. Pushing the lever / back, swings the mirror against the 
ground glass k out of line of the light ray from C and thus per- 
