VERTICAL ILLUMINATORS, METALLURGICAL MICROSCOPES 97 



glass or photographic plate of the camera. Set screws H H serve 

 to adjust and to limit the amount of rotation of K so that when 

 turned as far as it will go in either direction the images formed 

 will be comprised in circular fields uniformly illuminated through- 

 out their entire areas. 



The illuminator prism P is so mounted as to permit its hori- 

 zontal displacement below the objective in order to conform to 

 the requirements of different objectives. Were P immovable, 

 in the majority of cases half the image formed in the eyepiece 

 would be covered with a whitish cloud or fog. The amount of 

 horizontal displacement is also such that P can be slid entirely 

 beyond the aperture of the objective, thus allowing an unob- 

 structed passage of rays from the objective to PI. Transparent 

 objects placed upon the stage can thus be studied by transmitted 

 axial light or by oblique light. 



To prevent undue strains upon the stage when large specimens 

 must be examined a supporting rod L rises from the pillar to 

 which C is attached and passes through a clamp attached to the 

 pillar support of C. Tightening a set screw in the clamp pre- 

 vents any vertical movement on that side of the stage and as a 

 further protection, a set screw is also provided for locking the 

 coarse adjustment in position. The stage may thus be held 

 rigidly in any plane and relatively heavy objects may safely be 

 laid upon it. 



The mirror m is attached to a bar of such length that it can be 

 swung close to the objective and serves to project oblique light 

 upon the surface of the preparation in the study of fissures, slip 

 bands, cavities, etc. 



The radiant R consists of an arc lamp, that shown in the figure 

 being a "hand feed" type. The body of the lamp is raised upon 

 its supporting rod until the center of the crater of the arc lies in 

 the line of the optic axis of the lenses mounted in B and V. 

 Hooke's keys Ni, N2, NS permit adjustment of alignment and 

 illumination while looking into the instrument, NI turning the 

 lamp from side to side, N 3 up or down, and N 2 approaching or 

 drawing apart the carbons. In this lamp both -f- and car- 

 bons are of the same size, but the pitch of the screw threads 



