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ELEMENTARY CHEMICAL MICROSCOPY 



to facilitate adjustments, and the axis upon which the mirror 

 tips is graduated into degrees. When the paper lies horizontally 

 with respect to the optic axis of the microscope, the mirror should 

 be set at 45 degrees, providing that the mirror bar is long enough 

 to prevent interferences due to a reflected image of the stage; 

 if not, then the mirror must be tipped to an angle nearer to the 

 horizontal and the drawing paper inclined until the central rays 

 become normal to it. The amount of inclination of the drawing 

 surface must be twice as many degrees as the mirror is tipped 

 below 45. 



Camera lucidas serve not only for drawing but are most useful 

 in micrometry, 1 in reading thermometers when melting, boiling 

 or subliming points are determined, or 

 in reading scales of small voltmeters or 

 ammeters when observations are being 

 made, for upon looking into the micro- 

 scope both the preparation and the 

 scale of the instrument may be seen. 



The Leitz Drawing Eyepiece, shown 

 in section in Fig. 65, consists of a neg- 

 ative eyepiece whose lenses are so 

 mounted as to permit the insertion of 

 a reflecting prism P just above the eye 

 lens extending to the optic axis of the 

 ocular. Light rays (as indicated by 

 the dotted line) from the drawing paper 

 enter the prism, are twice totally re- 

 flected from the inclined surfaces of 

 the prism and enter the eye together 

 with the image-forming rays of the microscope. The eye therefore 

 perceives the image of the object under the microscope appar- 

 ently projected upon the drawing paper. Neutral tinted glasses 

 N serve to reduce the light intensity from the drawing paper 

 and to thus facilitate following the tracings of the pencil point. 

 The screw S serves to clamp the device in place while in use. 



1 See Coghill and Bonardi: Approximate Quantitative Microscopy of Pulverized 

 Ores, Tech. Paper 211 (1919), Bureau of Mines. 



FIG. 65. Drawing Eyepiece. 

 (E. Leitz.) 



