130 



MAGNIFICATION AND MICROMETRY [ CH. IV 



187. Valuation of the Ocular Micrometer. This is the 

 value of the divisions of the ocular micrometer for the purposes of 

 micrometry, and is entirely relative, depending on the magnifica- 

 tion of the real image formed by the objective, consequently it 

 changes with every change in the magnification of the real image, 

 and must be especially determined for every optical combination 

 (i. e. , objective and ocular), and for every change in the length of 

 the tube of the microscope. That is, it is necessary to determine 

 the ocular micrometer valuation for every condition modifying the 

 real image of the microscope ( 177). 



Any Huygenian ocular (Fig. 117) may, however, be used as a micrometer 

 ocular by placing the ocular micrometer at the level of the ocular diaphragm, 

 where the real image is formed. If there is a slit in the side of the ocular, 

 and the ocular micrometer is mounted in some way it may be introduced 

 through the opening at the side. When no side opening exists the mounting 

 of the eye-lens may be unscrewed and the ocular micrometer, if on a cover- 

 glass can be laid on the upper side of the ocular diaphragm. 



FIG. 117. Sectional view of a Huygenian ocular. 



Axis. Optic axis of the ocular. D. Diaphragm of 

 the ocular. E. L. Eye-Lens. F. L. Field-Lens. 



E. P. Eye-point. In micrometry the ocular microm- 

 eter with a Huygenian ocular must be placed at the level 

 of the diaphragm where the real image is formed. In a 

 positive ocular it would be placed below the ocular lenses. 



188. Obtaining the Ocular Micrometer Valuation for 

 an Ocular Micrometer with fixed Lines. Use the stage 

 micrometer as object. Light the field well and look into the micro- 

 scope. The lines of the ocular micrometer should be very sharply 

 defined. If they are not, raise or lower the eye-lens to make them 

 so; that is, focus as with the simple magnifier. 



When the lines of the ocular micrometer are distinct, focus the 

 microscope ( 81, 84) for the stage micrometer. The image of 

 the stage micrometer appears to be directly under or upon the ocular 

 micrometer. 



Make the lines of the two micrometers parallel by rotating the 



