174 



ELEMENTARY CHEMICAL MICROSCOPY 



the axis cb ba 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. hi. 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. 



