VERTICAL ILLUMINATORS, 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 

 b. 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 that they be withdrawn from the prism chamber from below the 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- 



