130 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.) 
