144 



PKACTICAL PHOTO-MICKOGKAPHY 



guidance in using this apparatus and in making the necessary 

 adjustments ; they sufficiently clearly indicate its purpose, and 

 the objects to which it may be applied (Fig. 54). 



Let B mark the position of the source of light, and J 

 represent the iris-diaphragm. Adjust the position of the lens 

 L 1 within the tube until an image of the iris-diaphragm is formed 

 at E, i.e. about midway above the upper face of prism P. The 

 latter may be turned about an axis at right angles to the plane 

 of the paper and also displaced along AB. 



The pencil of rays proceeding from P, after passing through 

 the objective 0, illuminates the object, whence that portion 



FIG. 54. Diagram of Path of Rays in Vertical Illuminator. (Leitz.) 



which goes to form the final image in the eye-piece or on the 

 ground-glass plate is reflected back through the objective, past 

 the prism P, and through prism P 1 . 4- 



The movable mount of prism P affords a means of so direct- 

 ing the rays through the objective that any light which is 

 reflected by the lens L may be intercepted by prism P. The 

 light does not then pass through the prism P 1 , and no disturbing 

 glare is created. The width of the pencil emerging from the 

 prism P 1 , which is of considerable importance, may be regulated 

 by displacing lens L 1 , which to this end is contained in a 

 movable mount, the distance of L 1 from P being greater with 

 high than with low powers. To direct the correctly illuminated 

 portion of the object into the field of view it is only necessary to 

 turn P 1 about its axis. 



Having completed the adjustment of the illumination, the 



