174 
ELEMENTARY CHEMICAL MICROSCOPY 
the axis ch ha is normal to BB and to the optic axis of the micro¬ 
scope. If for any reason the drawing paper must be inclined 
and is not level, in adjusting the mirror to obtain an axis normal 
Fig. III. Method of Indicating Magnification. 
to the paper, it should be recalled that when light is reflected, 
the angle between the incident and deflected ray is equal to twice 
the angle of inclination of the mirror. Hence, in order that the 
axial rays shall fall normal to the drawing surface, the mirror 
of the camera must be set at 45 degrees. But if so placed, only 
about one-half the field of the microscope can be sketched. 
In order to increase the available field, the mirror must be 
tipped at an angle less than 45 degrees with the horizontal. 
This, however, causes distortion, unless the drawing surface 
is inclined. The amount of inclination is in accordance with 
the law of reflection stated above, that is, that the drawing paper 
must form an angle with the horizontal twice as great as the 
angular amount the mirror is depressed below 45 degrees. 
Having the records of the magnifying power of the various 
possible optical combinations, in order to obtain the dimensions 
of an object, it is only necessary to measure the image obtained 
with the camera lucida under identical conditions and divide 
this value by the magnification. 
