MICROMETRY MICROMETRIC MICROSCOPES 149 



with dividers; their distance apart is then determined by means of 

 a millimeter scale and the value obtained divided by the actual 

 value of the stage scale corresponding to the enlarged image. The 

 quotient gives the magnification of the microscope. 



It is evident that any note-book record of magnifying power 

 of the various possible combinations of oculars and objectives 

 must be accompanied by a record of the tube-length employed in 

 the measurements. For this reason in determinations of mag- 

 nification it is best to use the tube-length for which the objectives 

 and oculars have been corrected. 1 It is also evident that the 

 paper BB and the reflecting mirror M must be so placed that 

 the axis cb ba is normal to BB and to the optic axis of the mi- 

 croscope. If for any reason the drawing paper must be inclined 

 and is not level, in adjusting the mirror to obtain an axis normal 

 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. 



Method 4. Measurements obtained by means of micrometer 

 oculars. Micrometer oculars are usually of one or two types 

 of construction: (a) those whose scale graduations consist of 



1 For a very comprehensive table of the tube lengths for which objectives are 

 corrected by different manufacturers, see Gage, The Microscope, p. 18. 



